BR112017007287B1 - SIGNALING LAYERS FOR SCALABLE ENCODING OF HIGH-ORDER AMBISONIC AUDIO DATA - Google Patents

Abstract

SIGNALING LAYERS FOR SCALABLE ENCODING OF HIGHER ORDER AMBISONIC AUDIO DATA. In general, techniques are described for signaling layers for scalable encoding of higher order ambisonic audio data. A device comprising a memory and a processor can be configured to perform the techniques. The memory can be configured to store the bit stream. The processor may be configured to obtain, from the bitstream, an indication of a specified number of layers in the bitstream, and obtain the specified layers of the bitstream based on the indication of the number of layers.

Description

[0001] This application claims the benefit of the following: Provisional application US number 62/062,584, entitled “SCALABLE CODING OF HIGHER ORDER AMBISONIC AUDIO DATA,” filed on October 10, 2014; Provisional application US number 62/084,461 entitled “SCALABLE CODING OF HIGHER ORDER AMBISONIC AUDIO DATA,” filed November 25, 2014; Provisional application US number 62/087,209, entitled “SCALABLE CODING OF HIGHER ORDER AMBISONIC AUDIO DATA,” filed on December 2014; Provisional application US number 62/088,445, entitled “SCALABLE CODING OF HIGHER ORDER AMBISONIC AUDIO DATA,” filed on December 5, 2014; US provisional application number 62/145,960 entitled “SCALABLE CODING OF HIGHER ORDER AMBISONIC AUDIO DATA,” filed April 10, 2015; US provisional application number 62/175,185, entitled “SCALABLE CODING OF HIGHER ORDER AMBISONIC AUDIO DATA,” filed on June 12, 2015; US Provisional Application Number 62/187,799, entitled “REDUCING CORRELATION BETWEEN HIGHER ORDER AMBISONIC (HOA) BACKGROUND CHANNELS,” filed July 1, 2015; and US Provisional Application No. 62/209,764 entitled “TRANSPORTING CODED SCALABLE AUDIO DATA,” filed on August 25, 2015; The full contents of each are incorporated herein by reference.

TECHNICAL FIELD

[0002] The present disclosure relates to audio data, and more specifically, scalable encoding of higher order ambisonic audio data.

BACKGROUND

[0003] A higher-order ambisonics (HOA) signal (often represented by a plurality of spherical harmonic coefficients (SHC) or other hierarchical elements) is a three-dimensional representation of a sound field. The HOA or SHC representation can represent the sound field in a way that is independent of the local speaker geometry used for reproduction of a multi-channel audio signal supplied from the SHC signal. The SHC signal can also facilitate backward compatibility as the SHC signal can be provided for well-known and highly adopted multi-channel formats such as a 5.1 channel audio format or a 7.1 channel audio format. The SHC representation can therefore allow a better representation of a sound field that also accommodates backward compatibility.

SUMMARY

[0004] In general, techniques are described for scalable encoding of higher order ambisonic audio data. Higher order ambisonic audio data may comprise at least one higher order ambisonic coefficient (HOA) corresponding to a spherical harmonic basis function having an order greater than one. The techniques can provide scalable encoding of the HOA coefficients by encoding the HOA coefficients illustrating multiple layers, such as a base layer and one or more enhancement layers. The base layer may allow reproduction of a sound field represented by HOA coefficients which may be enhanced by one or more enhancement layers. In other words, the enhancement layers (in combination with the base layer) can provide additional resolution that allows for a fuller (or more accurate) reproduction of the sound field compared to the base layer alone.

[0005] In one aspect, a device is configured to decode a bit stream representative of a higher order ambisonic audio signal. The device comprises a memory configured to store the bitstream, and one or more processors configured to obtain, from the bitstream, an indication of a specified number of layers in the bitstream, and obtain the layers of the bitstream based on the indication of the number of layers.

[0006] In another aspect, a method of decoding a bitstream representative of a higher-order ambisonic audio signal, the method comprising obtaining, from the bitstream, an indication of a specified number of layers in the bitstream, and obtaining the layers of the bitstream based on the indication of the number of layers.

[0007] In another aspect, an apparatus is configured to decode a bit stream representative of a higher order ambisonic audio signal. The apparatus comprises means for storing the bit stream, means for obtaining from the bit stream an indication of a specified number of layers in the bit stream, and means for obtaining the layers of the bit stream based on the indication of the number of layers.

[0008] In another aspect, a non-transient computer-readable storage medium having stored thereon instructions which, when executed, cause one or more processors to obtain, from the bitstream, an indication of a specified number of layers in the bitstream, and obtain the layers of the bitstream based on the indication of the number of layers.

[0009] In another aspect, a device is configured to encode a higher order ambisonic audio signal to generate a bit stream. The device comprises memory configured to store the bit stream, and one or more processors configured to specify an indication of a number of layers in the bit stream, and transmit the bit stream including the indicated number of layers.

[00010] In another aspect, a method of generating a bitstream representative of a higher order ambisonic audio signal, the method comprises specifying an indication of a number of layers in the bitstream, and transmitting the bitstream that includes the indicated number of layers.

[00011] In another aspect, a device is configured to decode a bit stream representative of a higher order ambisonic audio signal. The device comprises a memory configured to store the bit stream, and one or more processors configured to obtain, from the bit stream, an indication of a specified number of channels in one or more layers in the bit stream, and obtain the specified channels in one or more layers in the bit stream based on the indication of the number of channels.

[00012] In another aspect, a method of decoding a bitstream representative of a higher-order ambisonic audio signal, the method comprising obtaining, from the bitstream, an indication of a specified number of channels in one or more layers in the bitstream, and obtaining the specified channels in one or more layers in the bitstream based on the indication of the number of channels.

[00013] In another aspect, a device is configured to decode a bit stream representative of a higher order ambisonic audio signal. The device comprises means for obtaining, from the bit stream, an indication of a number of channels specified in one or more layers of the bit stream, and means for obtaining the channels specified in one or more layers in the bit stream based on the indication of the number of channels.

[00014] In another aspect, a non-transient computer-readable storage medium having stored therein instructions which, when executed, cause one or more processors to obtain, from a bitstream representative of a higher-order ambisonic audio signal, an indication of a specified number of channels in one or more layers of the bitstream, and obtain the specified channels in one or more layers of the bitstream based on the indication of the number of channels.

[00015] In another aspect, a device is configured to encode a higher order ambisonic audio signal to generate a bit stream. The device comprises one or more processors configured to specify, in the bitstream, an indication of a specified number of channels in one or more layers of the bitstream, and to specify the indicated number of channels in one or more layers of the bitstream, and a memory configured to store the bitstream.

[00016] In another aspect, a method of encoding a higher order ambisonic audio signal to generate a bitstream, the method comprises specifying, in the bitstream, an indication of a specified number of channels in one or more layers of the bitstream, and specifying the indicated number of channels in one or more layers of the bitstream.

[00017] Details of one or more aspects of the techniques are set out in the attached drawings and description below. Other features, objectives, and advantages of the techniques will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[00018] Figure 1 is a diagram illustrating basic spherical harmonic functions of various orders and suborders.

[00019] Figure 2 is a diagram illustrating a system that can perform various aspects of the techniques described in this disclosure.

[00020] Figure 3 is a block diagram illustrating, in more detail, an example of the audio encoding device shown in the example of Figure 2 that can perform various aspects of the techniques described in this disclosure.

[00021] Figure 4 is a block diagram illustrating the audio decoding device of Figure 2 in more detail.

[00022] Figure 5 is a diagram illustrating, in more detail, the bitstream generation unit of Figure 3 when configured to perform a first of the potential versions of the scalable audio coding techniques described in that disclosure.

[00023] Figure 6 is a diagram illustrating, in more detail, the extraction unit of Figure 4 when configured to perform the first of the potential versions of the scalable audio decoding techniques described in this disclosure.

[00024] Figures 7A-7D are flowcharts illustrating example operation of the audio encoding device in generating an encoded two-layer representation of higher order ambisonic coefficients (HOA).

[00025] Figures 8A and 8B are flowcharts illustrating example operation of the audio coding device in generating a coded three-layer representation of the HOA coefficients.

[00026] Figures 9A and 9B are flowcharts illustrating example operation of the audio coding device in generating a coded four-layer representation of the HOA coefficients.

[00027] Figure 10 is a diagram illustrating an example of an HOA configuration object specified in the bitstream in accordance with various aspects of the techniques.

[00028] Figure 11 is a diagram illustrating sideband information generated by the bitstream generation unit for the first and second layers.

[00029] Figures 12A and 12B are diagrams illustrating sideband information generated in accordance with the audio encoding aspects of the techniques described in this disclosure.

[00030] Figures 13A and 13B are diagrams illustrating sideband information generated in accordance with the scalable encoding aspects of the techniques described in this disclosure.

[00031] Figures 14A and 14B are flowcharts illustrating exemplary audio encoding device operations in performing various aspects of the techniques described in this disclosure.

[00032] Figures 15A and 15B are flowcharts illustrating exemplary audio decoding device operations in performing various aspects of the techniques described in this disclosure.

[00033] Figure 16 is a diagram illustrating scalable audio encoding as performed by the bitstream generation unit shown in the example of Figure 16 in accordance with various aspects of the techniques described in this disclosure.

[00034] Figure 17 is a conceptual diagram of an example where the syntax elements indicate that there are two layers with four coded environment HOA coefficients specified in a base layer and two coded foreground signals are specified in the enhancement layer.

[00035] Figure 18 is a diagram illustrating, in more detail, the bitstream generation unit of Figure 3 when configured to perform a second of the potential versions of the scalable audio coding techniques described in that disclosure.

[00036] Figure 19 is a diagram illustrating, in more detail, the extraction unit of Figure 3 when configured to perform the second of the potential versions of the scalable audio decoding techniques described in this disclosure.

[00037] Fig. 20 is a diagram illustrating a second use case by which the bitstream generation unit of Fig. 18 and the extraction unit of Fig. 19 can perform the second of the potential version of the techniques described in this disclosure.

[00038] Figure 21 is a conceptual diagram of an example where the syntax elements indicate that there are three layers with two encoded environment HOA coefficients specified in a base layer, two encoded foreground signals are specified in a first enhancement layer, and two encoded foreground signals are specified in a second enhancement layer.

[00039] Figure 22 is a diagram illustrating, in more detail, the bitstream generation unit of Figure 3 when configured to perform a third of the potential versions of the scalable audio coding techniques described in that disclosure.

[00040] Figure 23 is a diagram illustrating, in more detail, the extraction unit of Figure 4 when configured to perform the third of the potential versions of the scalable audio decoding techniques described in this disclosure.

[00041] Fig. 24 is a diagram illustrating a third use case whereby an audio encoding device can specify multiple layers in a multilayer bit stream in accordance with the techniques described in this disclosure.

[00042] Figure 25 is a conceptual diagram of an example where the syntax elements indicate that there are three layers with two encoded foreground signals specified in an ase layer, two encoded foreground signals are specified in a first enhancement layer, and two encoded foreground signals are specified in a second enhancement layer.

[00043] Fig. 26 is a diagram illustrating a third use case whereby an audio encoding device can specify multiple layers in a multilayer bit stream in accordance with the techniques described in this disclosure.

[00044] Figures 27 and 28 are block diagrams illustrating a scalable bitstream generation unit and a scalable bitstream extraction unit that can be configured to perform various aspects of the techniques described in this disclosure.

[00045] Figure 29 is a conceptual diagram representing an encoder that can be configured to operate in accordance with various aspects of the techniques described in this disclosure.

[00046] Figure 30 is a diagram illustrating the encoder shown in the example of figure 27 in more detail.

[00047] Figure 31 is a block diagram illustrating an audio decoder that can be configured to operate in accordance with various aspects of the techniques described in this disclosure.

DETAILED DESCRIPTION

[00048] The evolution of surround sound has made many output formats available for entertainment today. Examples of such consumer surround sound formats are mostly 'channel' based in that they implicitly specify feeds to speakers at certain geometric coordinates. Consumer surround sound formats include the popular 5.1 format (which includes the following six channels: front left (FL), front right (FR), center or front center, rear left or surround left, rear right or surround right, and low frequency effects (LFE)), the 7.1 format boosting, various formats that include tall speakers such as the 7.1.4 format, and the 22.2 format (for example, for use with the UHD television standard). Non-consumer formats can cover any number of speakers (in symmetrical and non-symmetrical geometries) often referred to as 'surround arrays'. An example of such an array includes 32 speakers positioned at coordinates at the corners of a truncated icosahedron.

[00049] Input to a future MPEG encoder is optionally one of three possible formats: (i) traditional channel-based audio (as discussed above), which is to be played through speakers at pre-specified positions; (ii) object-based audio, which involves discrete pulse-code-modulation (PCM) data for single audio objects with associated metadata containing their location coordinates (among other information); and (iii) scene-based audio, which involves representing the sound field using spherical harmonic base function coefficients (also called “spherical harmonic coefficients” or SHC, “higher order Ambisonics” or HOA, and “HOA coefficients”). The future MPEG encoder can be described in more detail in a document entitled “Call for proposals for 3D audio,” by the International Organization for Standardization/International Electrotechnical Commission (ISO)/(IEC) JTC1/SC19/WG11/N13411, released in January 2013 in Geneva, Switzerland, and available at http://mpeg.chiariglione.org/sites/default/files/files/stan dards/aprtes/doc s/w134.11.zip.

[00050] There are several surround-sound channel-based formats on the market. They range, for example, from the 5.1 home theater system (which has been the most successful in terms of making inroads into living rooms beyond stereo) to the 22.2 system developed by NHK (Nippon Hoso Kyokai or Japan Broadcasting Corporation). Content creators (eg Hollywood studies) would like to produce the soundtrack for a movie once, not spend effort remixing it for each speaker setup. Recently, standards development organizations are considering ways in which to provide encoding for a standardized bitstream and subsequent decoding that is adaptive and agnostic to speaker geometry (and number) and acoustic conditions at the playback location (involving a renderer).

[00051] To provide such flexibility for content creators, a hierarchical set of elements can be used to represent a sound field. The hierarchical set of elements can refer to a set of elements in which the elements are ordered such that a basic set of lower-order elements provides a complete representation of the modeled sound field. As the set is extended to include higher-order elements, the representation becomes more detailed, increasing resolution.

[00052] An example of a hierarchical set of elements is a set of spherical harmonic coefficients (SHC). The following expression demonstrates a description or representation of a sound field using SHC:

[00053] The expression shows that the pressure pi at any point {rr, θr, ϕr} of the sound field, at time t, can be represented exclusively by the SCH, . Here, is the speed of sound (~343 m/s), {rr, θr, ϕr} is a reference point (or observation point), jn(.) is the spherical Bessel function of order n, and are the spherical harmonic base functions of order n and suborder m. It can be recognized that the term in square brackets is a frequency-domain representation of the signal (i.e., s(ω,rr, θr, ϕr)) that can be approximated by various time-frequency transforms, such as the discrete Fourier transform (DFT), the discrete cosine transform (DCT), or a wavelet transform. Other examples of hierarchical sets include wavelet transform coefficient sets and other multiresolution basis function coefficient sets.

[00054] Figure 1 is a diagram illustrating spherical harmonic base functions from zero order (n = 0) to fourth order (n = 4). As can be seen, for each order there is an expansion of suborders m which are shown but not explicitly mentioned in the example in figure 1 for the sake of ease of illustration.

[00055] The SHC can be physically acquired (e.g., recorded by various microphone array configurations, or alternatively, can be derived from object-based or channel-based descriptions of the sound field. The SHC represents scene-based audio, where the SCH can be input into an audio encoder to obtain encoded SHC that can promote more efficient transmissions or storage. For example, a fourth-order representation involving coefficients (1+4)2 (25, and hence fourth-order) can be used.

[00056] As noted above, the SHC can be derived from a microphone recording using a microphone array. Several examples of how SHC can be derived from microphone arrays are described in Poletti, M., “Three-dimensional surround sound systems based on spherical harmonics,” J. Audio Eng. Soc., Vol. 53, no. 11, November 200, pages 1004-1025.

[00057] To illustrate how SHCs can be derived from an object-based description, consider the following A$W equation. The coefficients for the sound field corresponding to an individual audio object can be expressed as: Where i is the spherical Hankel function (of the second type) of order n, and {rs, θs, Φs} is the location of the object. Knowing the g(®) object source energy as a function of frequency (e.g. using time frequency analysis techniques such as performing a fast Fourier transform on the PCM stream) allows us to convert each object from PCM and corresponding location to SHC Furthermore, it can be shown (since the above is a linear decomposition and <(k) orthogonal) that the coefficients for each object are additive. In this way, a variety of PCM objects can be represented by the coefficients (for example, as a sum of the coefficient vectors for the individual objects). Essentially, the coefficients contain information about the sound field (the pressure as a function of 3D coordinates) and the above represents the transformation from individual objects into a representation of the overall sound field in the vicinity of the observation point {rr, θr, Φr}. The remaining figures are described below in the context of object-based and SHC-based audio coding.

[00058] Figure 2 is a diagram illustrating a system 10 that can perform various aspects of the techniques described in this disclosure. As shown in the example of Figure 2, the system 10 includes a content creation device 12 and a content consumer device 14. While described in the context of content creation device 12 and content consumer device 14, the techniques may be implemented in any control in which SHCs (which may also be referred to as HOA coefficients) or any other hierarchical representation of a sound field are encoded to form a bitstream representative of the audio data. Furthermore, content creation device 12 may represent any form of computing device capable of implementing the techniques described in this disclosure, including a handset (or cell phone), a tablet computer, a smart phone, or a desktop computer to provide a few examples. Similarly, content consuming device 14 can represent any form of computing device capable of implementing the techniques described in this disclosure, including a handset (or cell phone), a tablet computer, a smart phone, a frequency converter, or a desktop computer to provide a few examples.

[00059] Content creator device 12 may be operated by a movie studio or other entity that may generate multi-channel audio content for consumption by operators of content consumer devices, such as content consumer device 14. In some examples, content creator device 12 may be operated by an individual user who would like to compress HOA coefficients 11. Often, the content creator generates audio content in combination with video content. The content consumer device 14 is operable by an individual. Content consumer device 14 may include an audio playback system 16, which may refer to any form of audio playback system capable of providing SHC for playback as multi-channel audio content.

[00060] The content creator device 12 includes an audio editing system 18. The content creator device 12 obtains live recordings 7 in various formats (including directly as HOA coefficients) and audio objects 9, which the content creator device 12 can edit using audio editing system 18. A microphone 5 can capture the live recordings 7. The content creator can, during the editing process, provide HOA coefficients 11 from audio objects audio 9 by listening to the supplied speaker feeds in an attempt to identify various aspects of the sound field that require further editing. Content creator device 12 can then edit HOA coefficients 11 (potentially indirectly through manipulation of objects other than audio objects 9 from which source HOA coefficients can be derived in the manner described above). The content creator device 12 may employ the audio editing system 18 to generate the HOA coefficients 11. The audio editing system 18 represents any system capable of editing audio data and transmitting the audio data as one or more source spherical harmonic coefficients.

[00061] When the editing process is complete, the content creator device 12 can generate a bitstream 21 based on the HOA coefficients 11. That is, the content creator device 12 includes an audio encoding device 20 representing a device configured to encode or otherwise compress HOA coefficients 11 in accordance with various aspects of the techniques described in this disclosure for generating the bitstream 21. The audio encoding device 20 can generate the stream of bits 21 for transmission, as an example, over a transmission channel, which may be a wired or wireless channel, a data storage device or the like. The bitstream 21 may represent an encoded version of the HOA coefficients 11 and may include a primary bitstream and another secondary bitstream, which may be referred to as secondary channel information.

[00062] Although shown in Figure 2 as being directly transmitted to the content consumer device 14, the content creator device 12 can transmit the bitstream 21 to an intermediary device positioned between the content creator device 12 and the content consumer device 14. The intermediary device can store the bitstream 21 for later delivery to the content consumer device 14, which can request the bitstream. The intermediary device may comprise a file server, a network server, a desktop computer, a laptop computer, a tablet computer, a mobile phone, a smart phone, or any other device capable of storing the bit stream 21 for later retrieval by an audio decoder. The intermediary device may reside in a content delivery network capable of streaming the bitstream 21 (and possibly in combination with transmitting a corresponding video data bitstream) to subscribers, such as the content consumer device 14, requesting the bitstream.

[00063] Alternatively, the content creator device 12 can store the bit stream 21 to a storage medium, such as a compact disc, a digital video disc, a high definition video disc or other storage media, most of which are capable of being read by a computer and therefore can be referred to as computer readable storage media or non-transient computer readable storage media. In this context, delivery channel can refer to the channels through which content stored for the media is transmitted (and can include retail stores and other store-based delivery mechanism). In any event, the techniques of this disclosure must therefore not be limited in this respect to the example of Figure 2.

[00064] As further shown in the example of Figure 2, the content consumer device 14 includes the audio playback system 16. The audio playback system 16 can represent any audio playback system capable of playing multi-channel audio data. The audio playback system 16 may include a number of different renderers 22. The renderers 22 may individually provide a different form of rendering, where the different forms of rendering may include one or more of several ways of performing base-vector amplitude gyro (VBAP), and/or one or more of several ways of performing sound field synthesis. As used herein, "A and/or B" means "A or B", or both "A and B".

[00065] The audio reproduction system 16 may further include an audio decoding device 24. The audio decoding device 24 may represent a device configured to decode HOA coefficients 11' from the bit stream 21, where the HOA coefficients 11' may be similar to the HOA coefficients 11, but different due to loss operations (for example, quantization) and/or transmission through the transmission channel. The audio reproduction system 16 can, after decoding the bit stream 21 obtain the HOA coefficients q11' and supply the HOA coefficients 11' to transmit feeds from loudspeakers 25. The loudspeaker feeds 25 can drive one or more loudspeakers (which are not shown in the example of figure 2 for the sake of ease of illustration).

[00066] To select the appropriate renderer or, in some cases, generate an appropriate renderer, the audio reproduction system 16 may obtain speaker information 13 indicative of a number of speakers and/or spatial geometry of the speakers. In some instances, audio playback system 16 may obtain speaker information 13 by using a reference microphone and driving the speakers in such a way as to dynamically determine speaker information 13. In other instances or in combination with dynamically determining speaker information 13, audio playback system 16 may induce a user to interface with audio playback system 16 and enter speaker information 13.

[00067] The audio playback system 16 may then select one of the audio renderers 22 based on the speaker information 13. In some instances, the audio playback system 16 may, when none of the audio renderers 22 fall within some threshold similarity measurement (in terms of the speaker geometry) for the speaker geometry specified in the speaker information 13, generate one of the audio renderers 22 based on the information from speaker 13. The audio playback system 16 may, in some instances, generate one of the audio renderers 22 based on information from speaker 13 without first attempting to select an existing renderer from the audio renderers 22. One or more speakers 3 may then play back the rendered speaker feeds 25. In other words, the speakers 3 may be configured to reproduce a sound field based on higher order ambisonic audio data.

[00068] Figure 3 is a block diagram illustrating in more detail, an example of the audio encoding device 20 shown in the example of Figure 2 that can perform various aspects of the techniques described in this disclosure. The audio encoding device 20 includes a content analysis unit 26, a vector based decomposition unit 27 and a directional based decomposition unit 28.

[00069] Although described briefly below, more information pertaining to the vector-based decomposition unit 27 and the various aspects of compressing HOA coefficients is available in International Patent Application Publication no. WO 2014/194099, entitled “INTERPOLATION FOR DECOMPOSED REPRESENTATIONS OF A SOUND FIELD,” filed May 29, 2014. In addition, more details on various aspects of compressing HOA coefficients according to the MPEG-H audio standard, including a discussion of vector-based decomposition summarized below, can be found in: ISO/IEC document DIS 23008-3, entitled “Information technology - High efficiency coding and media delivery in heterogeneous environments - part 3: 3D audio,” per ISO/IEC JTC 1/SC 29/WG 11, dated 2014-07-25 (available at: http://mpeg.chiariglione.org/standards/mpeg-h/3d-audio/dis-mpeg-h-3d-audio, hereinafter referred to as “Phase 1 of the MPEG-H 3D Audio Standard”); Document ISO/IEC DIS 23008-3:2015/PDAM 3, entitled “Information technology - high efficiency coding and media delivery in heterogeneous environments - part 3: 3D audio, AMENDMENT 3: MPEG-H 3D audio phase 2”, by ISO/IEC JTC 1/SC 29/WG 11, dated 2014-07-25 (available at: http://mpeg.chiariglione.org/ standards/mpeg-h/3d-audio/text-isoiec-23008-3201xpdam-3-mpeg-h-3d-audio-phase-2, hereinafter referred to as “Phase II of the MPEG-H 3D Audio Standard”); and Jurgen Herre, et al., entitled “MPEG-H 3D Audio - The New standard for Coding of immersive spatial audio,” dated August 2015 and published in vol. 9, no. 5 of the IEEE Journal of Selected topics in Signal Processing.

[00070] The content analysis unit 26 represents a unit configured to analyze the content of the HOA coefficients 11 to identify whether the HOA coefficients 11 represent content generated from a live recording or an audio object. The content analysis unit 26 can determine whether the HOA coefficients 11 were generated from a recording of an actual sound field or an artificial audio object. In some instances, when the framed HOA coefficients 11 were generated from a recording, the content analysis unit 26 passes the HOA coefficients 11 to the vector-based decomposition unit 27. In some instances, when the framed HOA coefficients 11 were generated from a synthetic audio object, the content analysis unit 26 passes the HOA coefficients 11 to the directional-based synthesis unit 28. represent a unit configured to perform a directional based synthesis of the HOA 11 coefficients to generate a directional 21 based bit stream.

[00071] As shown in the example of figure 3, the vector-based decomposition unit 27 may include a linear invertible transform (LIT) unit 30, a parameter calculation unit 32, a reordering unit 34, a foreground selection unit 36, an energy compensation unit 38, a decorrelation unit 60 (shown as "decoration unit 60"), a gain control unit 62, an encoder unit 60 40, a bitstream generation unit 42, a sound field analysis unit 44, a coefficient reduction unit 46, a background selection (BG) unit 48, a spatiotemporal interpolation unit 50, and a quantization unit 52.

[00072] The linear invertible transform (LIT) unit 30 receives the HOA coefficients 11 in the form of HOA channels, each channel representing a block or frame of a coefficient associated with a given order, suborder of the spherical base functions (which can be indicated as HOA[k], where k can indicate the current frame or block of samples). The matrix of HOA 11 coefficients can have dimensions D: M x (N+1)2.

[00073] The LIT unit 30 may represent a unit configured to perform a form of analysis referred to as singular value decomposition. Although described with respect to SVD, the techniques described in this disclosure can be performed with respect to any similar transformation or decomposition that provides linearly uncorrelated, energy-packed output sets. Also, references to "sets" in this disclosure are generally intended to refer to non-zero sets unless specifically mentioned to the contrary and are not intended to refer to the classical mathematical definition of sets which includes the so-called "empty set". An alternative transformation might comprise a principal component analysis, which is often referred to as “PCA. ” Depending on the context, PCA may be referred to by a number of different names, such as discrete Karhunen-Loeve transform, Hotelling transform, proper orthogonal decomposition (POD) and eigenvalue decomposition (EVC) to name a few examples. Properties of such operations that are useful to one of the potential underlying purposes of compressing audio data may include one or more of 'energy compression' and 'decorrelation' of multichannel audio data.

[00074] In any event, assuming the LIT unit 30 performs a singular value decomposition (which, again, may be referred to as “SVD”) for purposes of example, the LIT unit 30 may transform the HOA coefficients 11 into two or more sets of transformed HOA coefficients. The “sets” of transformed HOA coefficients can include vectors of transformed HOA coefficients. In the example in Figure 3, the LIT unit 30 can SVD against the HOA coefficients 11 to generate a matrix named V, an S matrix, and a U matrix. SVD, in linear algebra, can represent a factorization of a real or complete y-by-z matrix X (where X can represent multichannel audio data, like the HOA coefficients 11) in the following form: X = USV* U can represent a real or complex y-by-y matrix, where the y-columns of U are known as the left-singular vectors of multichannel audio data. S can represent a y-by-z rectangular diagonal matrix with non-negative real numbers on the diagonal, where the diagonal values of S are known as the singular values of the multichannel audio data. V* (which can indicate a conjugate transpose of V) can represent a real or complex unitary z-by-z matrix, where the z-columns of V* are known as the right-singular vectors of multichannel audio data.

[00075] In some examples, the matrix V* in the SVD mathematical expression referenced above is indicated as the conjugate transpose of the matrix V to reflect that SVD can be applied to matrices comprising complex numbers. When applied to matrices comprising only real numbers, the complex conjugate of the matrix V (or, in other words, the matrix V*) can be considered to be the transpose of the matrix V. Below it is assumed, for ease of illustration, that the HOA 11 coefficients comprise real numbers with the result that the matrix V is transmitted through SVD rather than the matrix V*. Furthermore, although referred to as the V matrix in this disclosure, reference to the V matrix should be understood to refer to the transpose of the V matrix where appropriate. Although assumed to be the V matrix, the techniques can be applied in a similar way to HOA 11 coefficients having complex coefficients, where the output of the SVD is the V* matrix. Therefore, techniques should not be limited in this respect to only providing SVD mapping to generate a V-matrix, but may include SVD mapping to HOA 11 coefficients having complex components to generate a V* matrix.

[00076] In this way, the LIT unit 30 can perform SVD with respect to the coefficients 11 to transmit US[k] 33 vectors (which can represent a combined version of the S vectors and U vectors) having dimensions D: M x (N+1)2, and V[k] 35 vectors having dimensions D: (N+1)2 x (N+1)2. Individual vector elements in the matrix US[k] can also be named XPS(k) while individual vectors in the matrix V[k] can also be named v(k).

[00077] An analysis of the U, S and V matrices can reveal that the matrices carry or represent spatial and temporal characteristics of the underlying sound field represented above by X. Each of the N vectors in U (of samples of length M) can represent separate audio signals, normalized as a function of time (for the time period represented by M samples), which are orthogonal to each other and which have been decoupled from any spatial characteristics (which can also be referred to as directional information). The spatial features, representing shape and spatial position (r, theta, phi) can instead be represented by i° individual vectors, v(i)(k), in matrix V (each of length (N+1)2).

[00078] The individual elements of each of vectors v(i)(k) can represent an HOA coefficient describing the format (including width) and position of the sound field for an associated audio object. The two vectors in the U matrix and V matrix are normalized so that their root mean square energies equal unity. The energies of the audio signals in U are thus represented by the diagonal elements in S. Multiplying U and S to form US[k] (with individual vector elements XPS(k)), thus representing the audio signal with energies. The ability of SVD decomposition to decouple audio time signals (in U), their energies (in S), and their spatial characteristics (in V) may support various aspects of the techniques described in this disclosure. Furthermore, the model of synthesizing the underlying HOA[k] coefficients, X, by a vector multiplication of US[k] and V[k] gives rise to the term “vector-based decomposition,” which is used throughout this document.

[00079] Although described as being performed directly with respect to the HOA 11 coefficients, the LIT unit 30 can apply the linear invertible transform on derivatives of the HOA 11 coefficients. For example, the LIT unit 30 can apply SVD with respect to an energy spectral density matrix derived from the HOA 11 coefficients. can potentially reduce the computational complexity of running SVD in terms of one or more of the processor cycles and storage space, while achieving the same source audio coding efficiency as if SVD were applied directly to the HOA coefficients.

[00080] The parameter calculation unit 32 represents a unit configured to calculate various parameters, such as a correlation parameter (R), parameters of directional properties (θ, 9, r) and an energy property (e). Each of the parameters for the current frame can be specified as R[k], θ[k], 9[k], r[k] and e[k]. The parameter calculation unit 32 can perform an energy analysis and/or correlation (or so-called cross-correlation) against the US[k] vectors 33 to identify the parameters. The parameter calculation unit 32 can also determine the parameters for the previous frame, where the previous frame parameters can be indicated R[k-1], 0[k-1], 9[k-1], r[-1k] and e[k-1], based on the previous frame of vector US[k-1] and vectors V[k-1]. The parameter calculation unit 32 can transmit the current parameters 37 and the previous parameters 39 to the reordering unit 34.

[00081] The parameters calculated by the parameter calculation unit 32 can be used by the reordering unit 34 to reorder the audio objects to represent their natural evaluation or continuation over time. The reordering unit 34 can compare each of the parameters 37 from the first US[k] vectors 33 in the curve direction against each of the parameters 39 to the second US[k] 33 vectors. transmit a 33' reordered matrix US[k] (which can WD be written mathematically as ) and a 35' reordered matrix V[k] (which can be denoted VM) mathematically as for a foreground sound (or predominant sound - PS) selection unit 36 ("foreground selection unit 36") and an energy compensation unit 38.

[00082] The sound field analysis unit 44 may represent a unit configured to perform a sound field analysis with respect to the HOA coefficients 11 in order to potentially obtain a target bit rate 41. The sound field analysis unit 44 may, based on the analysis and/or a received target bit rate 41, determine the total number of psychoacoustic encoder instantiations (which may be a function of the total number of ambient or background channels (BGTO) T) and the number of foreground channels or, in other words, predominant channels. The total number of psychoacoustic encoder instantiations can be stated as numHOATransportChannels.

[00083] The sound field analysis unit 44 can also determine, again to potentially obtain the target bit rate 41, the total number of foreground channels (nFG) 45, the minimum order of the background sound field (or, in other words, ambient) NBG or, alternatively, MinAmbHOAorder), the corresponding number of effective channels representative of the minimum order of background sound field (nBGa = (MinAmbHOAorder + 1)2 and indexes (i) of additional HOA BG channels to send (which can be collectively indicated as background channel information 43 in the example of figure 3). The background channel information 42 can also be referred to as ambient channel information 43. Each of the channels remaining from numHOATransportChannels - nBGa, can be an "additional ambient/background channel", an "active vector based predominant channel", an "active directional based predominant signal" or "to so inactive.” In one aspect, channel types may be indicated (such as a “ChannelType”) syntax element by two bits (eg, 00: directional-based signal; 01: vector-based predominant signal; 10: additional ambient signal; 11: signal inactive). The total number of background or ambient signals, nBGa, can be given by (MinAmbHOAorder +1)2 + the number of times index 10 (in the above example) appears as a channel type in the bitstream for that frame.

[00084] The sound field analysis unit 44 can select the number of background (or, in other words, ambient) channels and the number of foreground (or, in other words, predominant) channels based on the target bitrate 41, selecting more background and/or foreground channels when the target bitrate 41 is relatively higher (for example, when the target bitrate 41 is equal to or greater than 512 Kbps). In one aspect, the numHOATransportChannels can be set to 8 while the MinAmbHOAorder can be set to 1 in the header section of the bitstream. In this scenario, in each frame, four channels can be dedicated to representing the background or ambient portion of the sound field while the other 4 channels can, on a frame-by-frame basis, vary in channel type - for example, used as an additional background/ambient channel or a predominant/foreground channel. The predominant/foreground signals can be one of vector-based or directional-based signals, as described above.

[00085] In some instances, the total number of predominant vector-based signals for a frame can be given by the number of times the ChannelType index is 01 in that frame's bitstream. In the above aspect, for each additional ambient/background channel (eg corresponding to a ChannelType of 10), corresponding information of which of the possible HOA coefficients (beyond the first four) can be represented on that channel. The information, for fourth-order HOA content, may be an index to indicate HOA coefficients 5-25. The first four HOA coefficients 1-4 can be sent at all times when minAmbHOAorder is set to 1, hence the audio coding device may only need to indicate one of the additional ambient HOA coefficients having an index of 525. The information could thus be sent using a 5-bit syntax element (for 4th-order content), which could be denoted as “CodedAmbCoeffIdx.” In any event, the sound field analysis unit 44 transmits the background channel information 43 and the HOA coefficients 11 to the background selection (BG) unit 36, the background channel information 43 to the coefficient reduction unit 46 and the bitstream generation unit 42 and the nFG 45 to a foreground selection unit 36.

[00086] The background selection unit 48 may represent a unit configured to determine ambient or background HOA coefficients 47 based on the background channel information (for example, the background sound field (NBG) and the number (nBGa) and indices (i) of additional HOA BG channels to send). For example, when NBG equals one, background selection unit 48 can select HOA coefficients 11 for each sample of the audio frame having an order equal to or less than one. The background selection unit 48 can, in this example, then select the HOA coefficients 11 having an index identified by one of the indices (i) as HOA BG coefficients, where the nBGa is provided to the bitstream generation unit 42 to be specified in the bitstream 21 so as to allow the audio decoding device, such as the audio decoding device 24 shown in the example of figures 2 and 4, to analyze the background HOA coefficients 4 7 from bitstream 21. The background selection unit 48 may then transmit the ambient HOA coefficients 47 to the power compensation unit 38. The ambient HOA coefficients 47 may have dimensions D: M x [(NBG+1)2 + nBGa]. The ambient HOA coefficients 47 may also be referred to as "ambient HOA coefficients 47," where each of the ambient HOA coefficients 47 corresponds to a separate ambient HOA channel 47 to be encoded by the psychoacoustic audio encoder unit 40.

[00087] The foreground selection unit 36 may represent a unit configured to select the reordered US[k] matrix 33' and the reordered V[k] matrix 35' representing foreground or distinct components of the nFG-based sound field 45 (which may represent one or more indices identifying the foreground vectors). The foreground selection unit 36 may transmit nFG signals 49 (which may be indicated as a reordered US[k]1,...,nFG 49, FG1,...,Nfg[K] 49, or 49) to the psychoacoustic audio encoder unit 40, where the nFG signals 49 can have dimensions D:M x nFG and each represent mono-audio objects. The foreground selection unit 36 may also transmit the reordered matrix V[k] 35' (or v(1...nFG)(k) 35') corresponding to foreground components of the sound field to the space-time interpolation unit 50, where a subset of the reordered matrix V[k] 35' corresponding to the foreground components may be designated as foreground matrix V[k] 51k (which may be mathematically indicated as ) having dimensions D: (N=1)2 x nFG.

[00088] The power compensation unit 38 can represent a unit configured to perform power compensation with respect to the ambient HOA coefficients 47 to compensate for energy loss due to the removal of several of the HOA channels by the background selection unit 48. The power compensation unit 38 can perform power analysis with respect to one or more of the reordered US[k] matrix 33', reordered V[k] matrix 35', nFG signals 49, the vectors Foreground V[k] 51k and ambient HOA coefficients 47 and then perform energy compensation based on energy analysis to generate energy compensated ambient HOA coefficients 47'. The energy compensation unit 38 may transmit the energy compensated ambient HOA coefficients 47' to the decorrelation unit 60.

[00089] The decorrelation unit 60 may represent a unit configured to implement various aspects of the techniques described in this disclosure to reduce or eliminate correlation between the energy compensated ambient HOA coefficients 47' to form one or more decorrelated ambient HOA audio signals 67. The decorrelation unit 40' may transmit the decorrelated HOA audio signals 67 to the gain control unit 62. The gain control unit 62 may represent a unit configured to perform automatic gain control (which may be abbreviated as "AGC") against the decorrelated ambient HOA audio signals 67 to obtain gain controlled ambient HOA audio signals 67'. After applying the gain control, the automatic gain control unit 62 may provide the gain controlled ambient HOA audio signals 67' to the psychoacoustic audio encoder unit 40.

[00090] The decorrelation unit 60 included in the audio coding device 20 may represent single or multiple instances of a unit configured to apply one or more decorrelation transforms to the energy compensated ambient HOA coefficients 47' to obtain the decorrelated HOA audio signals 67. In some examples, the decorrelation unit 40' may apply a UHJ matrix to the energy compensated ambient HOA coefficients 47'. In several instances of this disclosure, the UHJ matrix may also be referred to as a "phase-based transform". The application of the phase-based transform may also be referred to here as “phase shift decorrelation. ”

[00091] Ambisonic UHJ format is a development of the Ambisonic surround sound system designed to be compatible with both mono and stereo media. The UHJ format includes a hierarchy of systems in which the recorded sound field will be reproduced with a degree of accuracy that varies depending on the available channels. In several instances, UHJ is also referred to as “Format-C”. The initials indicate some of the fonts built into the system: U for Universal (UD-4); H of Matrix H; and J of System 45J.

[00092] UHJ is a hierarchical system of encoding and decoding directional sound information in Ambisonics technology. Depending on the number of channels available, a system can carry more or less information. UHJ is fully stereo- and mono-compatible. Up to four channels (L, R, T, Q ) can be used.

[00093] In one form, UHJ 2-channel (L, R), horizontal (or “planar”) surround information can be carried over normal stereo signal channels - CD, FM or digital radio, etc. - which can be retrieved using a UHJ decoder at the listening end. Summing the two channels can provide a compatible mono signal, which can be a more accurate representation of the two-channel version than summing a conventional “mono panpotted” source. If a third channel (T) is available, the third channel can be used to provide improved location accuracy for the planar surround effect when decoded through a 3-channel UHJ decoder. The third channel may not be required to have full audio bandwidth for this purpose, leading to the possibility of so-called '2^channel' systems where the third channel is limited in bandwidth. In an example, the threshold might be 5 kHz. The third channel can be broadcast over FM radio, for example, by means of quadrature phase modulation. The addition of a fourth (Q) channel to the UHJ system can allow encoding of full height surround sound, sometimes referred to as Periphony, with an identical level of accuracy as 4-channel B-Format.

[00094] 2-channel UHJ is a commonly used format for distributing Ambisonic recordings. 2-channel UHJ recordings can be transmitted through all normal stereo channels and any normal 2-channel media can be used unchanged. UHJ supports stereo in that, without decoding, the listener can perceive a stereo image, but one that is significantly wider than conventional stereo (eg, called “Super Stereo”). The left and right channels can also be summed for a very high degree of mono-compatibility. Relayed through a UHJ decoder, surround capability can be revealed.

[00095] An example mathematical representation of the decorrelation unit 60 applying the UHJ matrix (or phase-based transform) is as follows: UHJ encoding: T = imag(htlbert( (-0.1432 * W) + (0.6512 * X) ))-(0.7071 * Y); Q = 0.9772 * Z; S = (0.9397 * W) + (0.1856 * X); D = imag(hilbert( (-0.3420 * W) + (0.5099 * X))) + (0.6555 * Y); Converting S and D to Left and Right: Left = (S+D)q2 Right = (S-D)/2

[00096] In accordance with some implementations of the above calculations, assumptions regarding the above calculations may include the following: HOA background channel are 1st order Ambisonics, normalized FuMa, in channel numbering order Ambisonics W(a00), X(a11), Y(a11-), Z(a10).

[00097] In the calculations listed above, the decorrelation unit 40' can perform a scalar multiplication of several matrices by constant values. For example, to obtain the signal S, the decorrelation unit 60 can perform scalar multiplication of a matrix W by the constant value of 0.9397 (for example, by scalar multiplication), and of a matrix X by the constant value of 0.1856. As also illustrated in the calculations listed above, the decorrelation unit 60 can apply a Hilbert transform (indicated by the function "Hilbert()" in the above UHJ encoding) in obtaining each of the D and T signals.

[00098] Another example mathematical representation of the decorrelation unit 60 applying the UHJ matrix (or phase-based transform) is as follows: UHJ encoding: S = (0.9396926 * W) + (0.151520536509082 * X); D = imag(hilbert( (-0.3420201 * W) + (0.416299273350443 * X) )) + (0.535173990363608 * Y); T = 0.940604061228740 * (imag(hilbert( (-0.1432 * W) + (0.531702573500135 * X))) - (0.577350269189626 * Y)); Q = Zj Conversion of S and D into left and right: Left = (S+D)/2; Right = (S-D)/2;

[00099] In some example implementations of the above calculations, assumptions regarding the above calculations may include the following: HOA background channel are 1st-order Ambisonics, N3D (or “full three-D”) normalized, in Ambisonics channel numbering order W(a00), X(a11), Y(a11-), Z(a10). Although described here with respect to N3D normalization, it will be recognized that the example calculations can also be applied to HOA background channels that are SN3D normalized (or semi-Schmidt normalized). N3D and SN3D normalization can differ in terms of the scaling factors used. An example representation of N3D normalization, in relation to SN3D normalization, is expressed below:

[000100] An example of weighting coefficients used in SN3D normalization is expressed below:

[000101] In the calculations listed above, the decorrelation unit 60 can perform a scalar multiplication of multiple matrices by constant values. For example, to obtain the signal S, the decorrelation unit 60 can perform scalar multiplication of a matrix W by the constant value of 0.9396926 (for example, by scalar multiplication) and of a matrix X by the constant value of 0.151520536509082. As also illustrated in the calculations listed above, the decorrelation unit 60 can apply a Hilbert transform (indicated by the "Hilbert()" function in the above UHJ encoding or phase shift decorrelation) in obtaining each of the D and T signals.

[000102] The decorrelation unit 60 can perform the calculations listed above, so that the resulting S and D signals represent left and right audio signals (or in other words stereo audio signals). In some of these scenarios, the decorrelation unit 60 may transmit the T and Q signals as part of the decorrelated ambient HOA audio signals 67, however a decoding device receiving the bit stream 21 may not process the T and Q signals when rendering to a stereo speaker geometry (or, in other words, stereo speaker configuration). In examples, the 47' ambient HOA coefficients may represent a sound field to be rendered in a mono-audio production system. The decorrelation unit 60 can transmit the S and D signals as part of the decorrelated ambient HOA audio signals 67, and a decoding device receiving the bit stream 21 can combine (or "mix") the S and D signals to form an audio signal to be rendered and/or transmitted in mono-audio format.

[000103] In these examples, the decoding device and/or playback device can recover the mono-audio signal in various ways. An example is by mixing left and right signals (represented by S and D signals). Another example is by applying a UHJ matrix (or phase-based transform) to decode a W signal. By producing a natural left signal and a natural right signal in the form of the S and D signals by applying the UHJ matrix (or phase-based transform), the decorrelation unit 60 can implement techniques of this disclosure to provide potential advantages and/or potential improvements over the technique applying other decorrelation transforms (such as a mode matrix described in the MPEG-H standard).

[000104] In various examples, the decorrelation unit 60 may apply different decorrelation transforms based on a bit rate of the received energy compensated ambient HOA coefficients 47'. For example, the decorrelation unit 60 can apply the UHJ matrix (or phase-based transform) described above in scenarios where the energy-compensated ambient HOA coefficients 47' represent a four-channel input. More specifically, based on the energy-compensated ambient HOA coefficients 47' representing a four-channel input, the decorrelation unit 60 may apply a 4 x 4 UHJ matrix (or phase-based transform). For example, the 4 x 4 matrix can be orthogonal to the four-channel input of energy-compensated ambient HOA coefficients 47'. In other words, in instances where the energy compensated ambient HOA coefficients 47' represent a smaller number of channels (e.g., four), the decorrelation unit 60 can apply the UHJ matrix as the selected decorrelation transform, to decorrelate the background signals from the energy compensated ambient HOA signals 47' to obtain the decorrelated ambient HOA audio signals 67.

[000105] According to this example, if the energy-compensated ambient HOA coefficients 47' represent a larger number of channels (for example, nine), the decorrelation unit 60 can apply a decorrelation transform other than the UHJ matrix (or phase-based transform). For example, in a scenario where the energy-compensated ambient HOA coefficients 47' represent a nine-channel input, the decorrelation unit 60 may apply a mode matrix (e.g., as described in Phase I of the MPEG-H 3D audio standard referenced above), to de-correlate the energy-compensated ambient HOA coefficients 47'. In instances where the energy compensated ambient HOA coefficients 47' represent a nine channel input, the decorrelation unit 60 may apply a 9 x 9 mode matrix to obtain the decorrelated ambient HOA audio signals 67.

[000106] In turn, various components of the audio coding device 20 (such as the psychoacoustic audio coder 40) can perceptually encode the decorrelated ambient HOA audio signals 67 according to AAC or USAC. The decorrelation unit 60 can apply the phase shift decorrelation transform (e.g. the UHJ matrix or phase based transform in the case of a four channel input), to potentially optimize the AAC/USAC coding for HOA. In instances where the energy-compensated ambient HOA coefficients 47' (and thus, the decorrelated ambient HOA audio signals 67) represent audio data to be rendered in a stereo playback system, the decorrelation unit 60 can apply the techniques of this disclosure to improve or optimize the compression, based on AAC and USAC being relatively oriented (or optimized for) stereo audio data.

[000107] It will be understood that the decorrelation unit 60 can apply the techniques described here in situations where the energy compensated ambient HOA coefficients 47' include foreground channels, as well as in situations where the energy compensated ambient HOA coefficients 47' do not include any foreground channels. As an example, the decorrelation unit 40' may apply the techniques and/or calculations described above, in a scenario where the energy-compensated ambient HOA coefficients 47' include zero (0) foreground channels and four (4) background channels (e.g., a lower/lower bitrate scenario).

[000108] In some examples, the decorrelation unit 60 may cause the bitstream generation unit 42 to signal, as part of the vector-based bitstream 21, one or more syntax elements that indicate that the decorrelation unit 60 has applied a decorrelation transform to the energy-compensated ambient HOA coefficients 47'. By providing such an indication to a decoding device, the decorrelation unit 60 can allow the decoding device to perform reciprocal decorrelation transforms on audio data in the HOA domain. In some examples, the decorrelation unit 60 may cause the bitstream generation unit 42 to signal syntax elements that indicate which decorrelation transform has been applied, such as the UHJ matrix (or other phase-based transform) or the mode matrix.

[000109] The decorrelation unit 60 may apply a phase-based transform to the energy-compensated ambient HOA coefficient 47'. The phase-based transform for the first HOA OMIN coefficient sequences of CAMB (k-1) is defined by With the d coefficients as defined in Table 1, the signal frames S(k-2) and M(K-2) being defined by And A+90(k-2) and B+90(k-2) are the +90 degrees A and B phase-shifted signal frames defined by The phase-based transform for the first sequences of HOA coefficient OMIN of CP,AMB(k-1) is defined accordingly. The described transform can introduce a delay of one frame.

[000110] Above, the xAMB,LOW,1 (k-1) to xAMB,LOW,4 (k-2) can correspond to the decorrelated ambient HOA audio signals 67. In the above equation, the variable CAMB,1(k) indicates the HOA coefficients for the kth frame corresponding to the spherical base functions having an (order:suborder) of (0:0), which can also be referred to as the “W” component or channel. The variable CAMB,2 (k) indicates the HOA coefficients for the kth frame corresponding to the spherical basis functions having an (order: suborder) of (1:-1), which may also be referred to as the 'Y' channel or component. The variable CAMB,3 (k) indicates the HOA coefficients for the kth frame corresponding to the spherical basis functions having an (order: suborder) of (1:0), which may also be referred to as the 'Z' channel or component. The variable CAMB,4 (k) indicates the HOA coefficients for the kth frame corresponding to the spherical basis functions having an (order: suborder) of (1:1), which may also be referred to as the 'X' channel or component. The CAMB,1(k) through CAMB,3(k) can correspond to 47' ambient HOA coefficients.

[000111] Table 11 below illustrates an example of coefficients that the decorrelation unit 40 can use to perform a phase-based transform. Table 1 Coefficients for phase-based transform

[000112] In some examples, various components of the audio encoding device 20 (such as the bitstream generation unit 42) may be configured to transmit only first-order HOA representations for lower target bitrates (for example, a target bitrate of 128K or 256K). According to some of these examples, the audio coding device 20 (or components thereof, such as the bit stream generation unit 42) can be configured to discard higher order HOA coefficients (e.g. coefficients with an order greater than the first order, or in other words, N>1). However, in instances where the audio encoding device 20 determines the target bitrate to be relatively high, the audio encoding device 20 (e.g., bitstream generation unit 42) may separate the foreground and background channels, and may assign bits (e.g., in larger amounts) to the foreground channels.

[000113] Although described as being applied to the energy compensated ambient HOA coefficients 47', the audio coding device 20 may not apply decorrelation to the energy compensated ambient HOA coefficients 47'. Instead, the energy compensation unit 38 can supply the energy compensated ambient HOA coefficients 47' directly to the gain control unit 62, which can perform automatic gain control with respect to the energy compensated ambient HOA coefficients 47'. As such, the decorrelation unit 60 is shown as a dashed line to indicate that the decorrelation unit may not always perform decorrelation or be included in the audio decoding device 20.

[000114] The space-time interpolation unit 50 can represent a unit configured to receive the foreground vectors V[k] 51k for the kth frame and the foreground vectors V[k-1] 51k-1 for the previous frame (hence the k-1 notation) and perform space-time interpolation to generate interpolated foreground vectors V[k]. The space-time interpolation unit 50 can recombine the nFG signals 49 with the foreground V[k] vectors 51k to recover reordered foreground HOA coefficients. The space-time interpolation unit 50 can then divide the reordered foreground HOA coefficients by the interpolated V[k] vectors to generate interpolated nFG signals 49'.

[000115] The space-time interpolation unit 50 can also transmit the foreground V[k] vectors 51k that were used to generate the interpolated foreground V[k] vectors so that an audio decoding device, such as the audio decoding device 24, can generate the interpolated foreground V[k] vectors and thereby recover the foreground V[k] vectors 51k. The foreground V[k] vectors 51k used to generate the interpolated foreground V[k] vectors are denoted as the remaining foreground V[k] vectors 53. To ensure that the same V[k] and V[k-1] are used in the encoder and decoder (to create the interpolated V[k] vectors) quantized/dequantized versions of the vectors can be used in the encoder and decoder. The space-time interpolation unit 50 can transmit the interpolated nFG signals 49' to the gain control unit 62 and the interpolated foreground V[k] vectors 51k to the coefficient reduction unit 46.

[000116] The gain control unit 62 may also represent a unit configured to perform automatic gain control (which may be abbreviated as "AGC") with respect to interpolated nFG signals 49' to obtain gain controlled nFG signals 49''. After applying the gain control, the automatic gain control unit 62 can supply the gain controlled nFG signals 49'' to the psychoacoustic audio encoder unit 40.

[000117] The coefficient reduction unit 46 may represent a unit configured to perform coefficient reduction with respect to the remaining foreground vectors V[k] 53 based on background channel information 43 to transmit reduced foreground V[k] vectors 55 to the quantization unit 52. The reduced foreground V[k] vectors 55 may have dimensions D: [(N+1)2 - (NBG +1)2 - BGTOT] x nFG. Coefficient reduction unit 46 may, in this regard, represent a unit configured to reduce the number of coefficients in the remaining foreground vectors V[k] 53. In other words, coefficient reduction unit 46 may represent a unit configured to eliminate the coefficients in the foreground vectors V[k] (which form the remaining foreground V[k] vectors 53) having little to no directional information. In some instances, the coefficients of distinct or, in other words, foreground vectors V[k] corresponding to a first and zero-order basis functions (which may be denoted as NBG) provide little directional information and therefore can be removed from the foreground V vectors (through a process that may be referred to as "coefficient reduction"). In this example, greater flexibility can be provided not only to identify the coefficients that correspond to NBG, but to identify additional HOA channels (which can be indicated by the TotalOfAddAmbHOAChan variable) from the set of [(NBG + 1)2 +1, (N+1)2].

[000118] The quantization unit 52 may represent a unit configured to perform any form of quantization to compress the reduced foreground V[k] vectors 55 to generate encoded foreground V[k] vectors 57, transmitting the encoded foreground V[k] vectors 57 to the bit stream generation unit 42. In operation, the quantization unit 52 may represent a unit configured to compress a spatial component of the sound field , that is, one or more of the reduced foreground vectors V[k] 55 in this example. The quantization unit 52 can perform any one of the following 12 quantization modes set out in phase I or phase II of the MPEG-H 3D audio coding standard referenced above. The quantization unit 52 can also perform predicted versions of any of the above types of quantization modes, where a difference is determined between an element (or a weight when vector quantization is performed) of the V-vector of a previous frame and the element (or weight when vector quantization is performed) of the V-vector of a current frame is determined. The quantization unit 52 can then quantize the difference between the elements or weights of the current frame and the previous frame instead of the V-vector element value of the current frame itself. The quantization unit 52 may supply the encoded foreground V[k] vectors 57 to the bitstream generation unit 42. The quantization unit 52 may also supply the syntax elements indicative of the quantization mode (e.g., the NbitsQ syntax element) and any other syntax elements used to dequantize or otherwise reconstruct the V-vector.

[000119] The psychoacoustic audio encoder unit 40 included in the audio coding device 20 can represent multiple instances of a psychoacoustic audio encoder, each of which is used to encode a different HOA channel or audio object from each of the energy compensated ambient HOA coefficients 47' and the interpolated nFG signals 49' to generate encoded ambient HOA coefficients 59 and encoded nFG signals 61. The unit psychoacoustic audio encoder 40 may transmit the encoded ambient HOA coefficients 59 and encoded nFG signals 61 to the bit stream generation unit 42.

[000120] The bitstream generation unit 42 included in the audio encoding device 20 represents a unit that formats data to conform to a known format (which may refer to a format known by a decoding device), thereby generating the vector-based bitstream 21. The bitstream 21 can, in other words, represent encoded audio data, having been encoded in the manner described above. The bitstream generating unit 42 may represent a multiplexor in some examples, which may receive the coded foreground V[k] vectors 57, the coded ambient HOA coefficients 59, the coded nFG signals 61, and the background channel information 43. 59, the encoded nFG signals 61, and the background channel information 43. In this way, the bitstream generating unit 42 can thereby specify the vectors 57 in the bitstream 21 to obtain the bitstream 21. The bitstream 21 can include a primary or main bitstream and one or more minor channel bitstreams.

[000121] Although not shown in the example of Figure 3, the audio coding device 20 may also include a bitstream output unit that switches the bitstream output from the audio coding device 20 (e.g., between directional-based bitstream 21 and vector-based bitstream 21) based on whether a current frame should be encoded using directional-based synthesis or vector-based synthesis. The bitstream output unit may perform switching based on the syntax element transmitted by the content analysis unit 26 indicating whether a directional based synthesis has been performed (as a result of detecting that HOA coefficients 11 have been generated from a synthetic audio object) or a vector based synthesis has been performed (as a result of detecting that the HOA coefficients have been registered). The bitstream output unit can specify the correct header syntax to indicate the current switching or encoding used for the current frame along with the respective one of the 21 bitstreams.

[000122] Furthermore, as noted above, the sound field analysis unit 44 can identify BGTOT ambient HOA coefficients 47, which may change on a frame-by-frame basis (although sometimes BGTOT may remain constant or the same across two or more adjacent frames (in time)). Changing BGTOT can result in changes to the coefficients expressed in the reduced foreground V[k] vectors 55. Changing BGTOT can result in background HOA coefficients (which may also be referred to as "ambient HOA coefficients") that change on a frame-by-frame basis (although, again, sometimes BGTOT can remain constant or the same across two or more adjacent frames (in time)). The changes often result in a change of energy to the sound field aspects represented by the addition or removal of additional ambient HOA coefficients and the corresponding removal of coefficients from or addition of coefficients to the reduced foreground V[k] vectors 55.

[000123] As a result, the sound field analysis unit 44 can further determine when the ambient HOA coefficients change from frame to frame and generate a flag or other syntax element indicative of the change in the ambient HOA coefficient in terms of being used to represent the ambient components of the sound field (where the change can also be referred to as a “transition” of the ambient HOA coefficient or as a “transition” of the ambient HOA coefficient). In particular, the coefficient reduction unit 46 may generate the flag (which may be indicated as an AmbCoeffTransition flag or an AmbCoeffIdxTransition flag), providing the flag to the bitstream generation unit 42 so that the flag may be included in the bitstream 21 (possibly as part of secondary channel information).

[000124] The coefficient reduction unit 46 can, in addition to specifying the ambient coefficient transition flag, also modify how the reduced foreground V[k] vectors 55 are generated. In one example, after determining that one of the ambient HOA ambient coefficients is in transition during the current frame, the coefficient reduction unit 46 may specify, a vector coefficient (which may also be referred to as a "vector element" or "element") for each of the V-vectors of the reduced foreground V[k] vectors 55 that corresponds to the ambient HOA coefficient in transition. Again, the ambient HOA coefficient can add to or remove from the BGTOT the total number of background coefficients. Therefore, the resulting change in the total number of background coefficients affects whether or not the ambient HOA coefficient is included in the bitstream, and whether the corresponding element of V-vectors is included for the V-vectors specified in the bitstream in the second and third configuration modes described above. More information regarding how the coefficient reduction unit 46 can specify the reduced foreground V[k] vectors 55 to overcome changes in energy is provided in US application serial number 14/594,533, entitled “TRANSITIONING OF AMBIENT HIGHER_ORDER AMBISONIC COEFFICIENTS,” filed January 12, 2015.

[000125] In this regard, the bitstream generation unit 42 can generate a bitstream 21 in a wide variety of different encoding schemes, which can facilitate flexible bitstream generation to accommodate a large number of different content delivery contexts. One context that seems to be gaining momentum in the audio industry is the delivery (or, in other words, “streaming”) of audio data over networks to an increasing number of different playback devices. Delivering audio content over bandwidth-limited networks to devices having varying degrees of playback capabilities can be difficult, especially in the context of HOA audio data that allows a high degree of 3D audio fidelity during playback at the expense of high bandwidth consumption (relative to object-based or channel-based audio data).

[000126] According to the techniques described in this disclosure, the bit stream generation unit 42” can use one or more scalable layers to allow multiple reconstructions of the HOA coefficients 11. Each of the layers can be hierarchical. For example, a first layer (which might be referred to as a “base layer”) can provide a first reconstruction of HOA coefficients that allows stereo speaker feeds to be rendered. A second layer (which may be referred to as a first "enhancement layer") can, when applied to the first reconstruction of the HOA coefficients, scale the first reconstruction of the HOA coefficient to allow horizontal surround sound speaker feeds (e.g. 5.1 speaker feeds) to be rendered. In other words, the layers are hierarchical so that a first layer, when combined with a second layer, provides a higher resolution representation of the higher order ambisonic audio signal.

[000127] Although described above as allowing scaling from an immediately preceding layer, any layer above another layer can scale the layer below. In other words, the third layer described above can be used to scale the first layer even though the first layer has not been “scaled” by the second layer. The third layer, when applied directly to the first layer, can provide height information and thereby allow irregular speaker feeds corresponding to irregularly arranged speaker geometries to be rendered.

[000128] Bitstream generation unit 42 may, to enable layers to be extracted from bitstream 21, specify an indication of a number of specified layers in the bitstream. The bit stream generation unit 42 can transmit the bit stream 21 which includes the indicated number of layers. The bitstream generation unit 42 is described in more detail with reference to Fig. 5. Several different examples of generating the scalable HOA audio data are described in Figs. 7A-9B below, with an example of the sideband information for each of the above examples in Figs. 10-13B.

[000129] Figure 5 is a diagram illustrating, in more detail, the bit stream generation unit 42 of Figure 3 when configured to perform a first of the potential versions of the scalable audio coding techniques described in that disclosure. In the example of Figure 5, the bitstream generation unit 42 includes a scalable bitstream generation unit 1000 and a non-scalable bitstream generation unit 1002. The scalable bitstream generation unit 1000 represents a unit configured to generate a scalable bitstream 21 comprising two or more layers (although in some cases a scalable bitstream may comprise a single layer for certain audio contexts) having HOAFrames() similar to those shown in and described below with respect to the examples of figures 1113B. The non-scalable bitstream generation unit 1002 may represent a unit configured to generate a non-scalable bitstream 21 that does not provide layering or, in other words, scalability.

[000130] Both the non-scalable bitstream 21 and the scalable bitstream 21 may be referred to as "bitstream 21" since both typically include the same underlying data in terms of the encoded ambient HOA coefficients 59, the encoded nFG signals 61 and the encoded foreground V[k] vectors 57. One difference, however, between the non-scalable bitstream 21 and the scalable bitstream 21 is that the scalable bitstream 21 includes layers, which may be denoted as layers 21A, 21B, etc. The layers 21A may include subsets of the encoded ambient HOA coefficients 59, the encoded nFG signals 61, and the encoded foreground V[k] vectors 57, as described in more detail below.

[000131] Although the scalable and non-scalable bitstream 21 can effectively be different representations of the same bitstream 21, the non-scalable bitstream 21 is indicated as non-scalable bitstream 21' to differentiate the scalable bitstream 121 from the non-scalable bitstream 21'. Furthermore, in some instances, the scalable bitstream 21 may include multiple layers that conform to the non-scalable bitstream 21. For example, the scalable bitstream 21 may include a base layer that conforms to the non-scalable bitstream 21. In such instances, the non-scalable bitstream 21' may represent a sub-bitstream of the scalable bitstream 21, where that non-scalable bitstream 21' 21' can be enhanced with additional layers of the scalable bit stream 21 (which are referred to as enhancement layers).

[000132] The bitstream generation unit 42 can obtain scalability information 1003 indicative of whether to invoke the scalable bitstream generation unit 1000 or the non-scalable bitstream generation unit 1002. In other words, the scalability information 1003 can indicate whether the bitstream generation unit 42 should transmit scalable bitstream 21 or non-scalable bitstream 21 ’. For purposes of illustration, scalability information 1003 is assumed to indicate that bitstream generation unit 42 should invoke scalable bitstream generation unit 1000 to transmit scalable bitstream 21'.

[000133] As further shown in the example of Figure 5, the bit stream generation unit 42 can receive the encoded ambient HOA coefficients 59A-59D, the encoded nFG signals 61A and 61B, and the encoded foreground V[k] vectors 57A and 57B. the coded ambient HOA coefficients 59A may represent coded ambient HOA coefficients associated with a spherical basis function having an order of zero and a suborder of zero. The coded ambient HOA coefficients 59B may represent coded ambient HOA coefficients associated with a spherical basis function having an order of one and a suborder of zero. The coded ambient HOA coefficients 59C may represent coded ambient HOA coefficients associated with a spherical basis function having an order of one and a suborder of negative one. The coded ambient HOA coefficients 59D may represent coded ambient HOA coefficients associated with a spherical basis function having an order of one and a suborder of positive one. The ambient coded HOA coefficients 59A-59D may represent an example of, and as a result may be referred to collectively as the ambient coded HOA coefficients 59 discussed above.

[000134] The encoded nFG signals 61A and 61B can individually represent a US audio object representative of, in this example, the two most predominant foreground aspects of the sound field. V[k] encoded foreground vectors 57A and 57B may represent directional information (which may also specify width in addition to direction) for encoded nFG signals 61A and 61B respectively. The encoded nFG signals 61A and 61B may represent an example of, and as a result may be referred to collectively as, the encoded nFG signals 61 described above. V[k] coded foreground vectors 57A and 57B may represent an example of, and as a result may be collectively referred to as, the V[k] coded foreground vectors 57 described above.

[000135] Once invoked, the scalable bitstream generation unit 1000 can generate the scalable bitstream 21 to include layers 21A and 21B in a manner substantially similar to that described below with respect to FIGS. 7A-9B. The scalable bitstream generation unit 1000 can specify an indication of the number of layers in the scalable bitstream 21 as well as the number of foreground elements and background elements in each of the layers 21A and 21B. The scalable bitstream generation unit 1000 can, as an example, specify a NumberOfLayers syntax element that can specify L number of layers, where the variable L can indicate the number of layers. The scalable bitstream generation unit 1000 can then specify, for each layer (which may be denoted as the variable i = 1 for L), the number Bi of the coded ambient HOA coefficients 59 and the number Fi of the coded nFG signals 61 sent to each layer (which may also or alternatively indicate the number of corresponding coded foreground V[k] vectors 57).

[000136] In the example of Figure 5, the scalable bitstream generation unit 1000 can specify in the scalable bitstream 21 that scalable coding has been allowed and that two layers are included in the scalable bitstream 21, that the first layer 21A includes four coded ambient HOA coefficients 59 and zero coded nFG signals 61, and that the second layer 21A includes zero coded ambient HOA coefficients 59 and zero coded signals nFG encoded w 61. The scalable bitstream generating unit 1000 may also generate the first layer 21A (which may also be referred to as a "base layer 21A") for including the encoded ambient HOA coefficients 59. The scalable bitstream generating unit 1000 may further generate the second layer 21A (which may be referred to as an "enhancement layer 21B") for including the encoded nFG signals 61 and the encoded foreground vectors V[k] 57. The scalable bitstream generation unit 1000 may transmit layers 21A and 21B as scalable bitstream 21. In some examples, the scalable bitstream generation unit 1000 may store the scalable bitstream 21' in a memory (either internal or external from the encoder 20).

[000137] In some instances, the scalable bitstream generation unit 1000 may not specify one or more of any of the indications of the number of layers, the number of foreground components (e.g. number of coded nFG signals 61 and coded foreground V[k] vectors 57) in one or more layers, and the number of background components (e.g. coded ambient HOA coefficients 59) in one or more layers. Components may also be mentioned as channels in this disclosure. Instead, the scalable bitstream generation unit 1000 can compare the number of layers for a current frame with the number of layers for a previous frame (e.g., the most temporally recent previous frame). When the comparison results in no difference (meaning that the number of layers in the current frame is equal to the number of layers in the previous frame), the scalable bitstream generation unit 1000 can compare the number of background and foreground components in each layer in a similar fashion.

[000138] In other words, the scalable bitstream generation unit 1000 can compare the number of background components in one or more layers for the current frame with the number of background components in one or more layers for a previous frame. The scalable bitstream generation unit 1000 can further compare the number of foreground components in one or more layers for the current frame with the number of foreground components in one or more layers for the previous frame.

[000139] When both component-based comparisons result in no difference (meaning that the number of foreground and background components in the previous frame is equal to the number of foreground and background components in the current frame), the scalable bitstream generation unit 1000 may specify an indication (e.g., a HOABaseLayerConfigurationFlag syntax element) in the scalable bitstream 21 that the number of layers in the current frame is equal to the number of layers in the previous frame instead specifying one or more or any of the indications of the number of layers, the number of foreground components (e.g., number of nFG signals coded 61 and foreground V[k] vectors coded 57) in one or more layers, and the number of background components (e.g., ambient HOA coefficients coded 59) in one or more layers. The audio decoding device 24 can then determine that the previous frame indications of the number of layers, background components, and foreground components equal the current frame number indications of the number of layers, background components, and foreground components, as described in more detail below.

[000140] When any of the aforementioned comparisons result in differences, the scalable bitstream generation unit 1000 may specify an indication (e.g., a HOABaseLayerConfigurationFlag syntax element) in the scalable bitstream 21 that the number of layers in the current frame is not equal to the number of layers in the previous frame. The scalable bitstream generation unit 1000 can then specify indications of the number of layers, the number of foreground components (e.g., number of coded nFG signals 61 and coded foreground V[k] vectors 57) in one or more layers, and the number of background components (e.g., coded ambient HOA coefficients 59) in one or more layers, as mentioned above. In this regard, the scalable bitstream generation unit 1000 can specify, in the bitstream, an indication of whether a number of layers of the bitstream has changed in a current frame when compared to a number of layers of the bitstream in a previous frame, and specify the indicated number of layers of the bitstream in the current frame.

[000141] In some examples, instead of not specifying an indication of the number of foreground components and an indication of the number of background components, the scalable bitstream generation unit 1000 may not specify an indication of a number of components (for example, a "NumChannels" syntax element, which can be a set having [i]entries where i is equal to the number of layers) in the scalable bitstream 21. The scalable bitstream generation unit 1000 may not specify such an indication of the number of components (where those components may also be referred to as "channels") rather than specifying the number of foreground and background components since the number of foreground and background components can be derived from the more general number of channels. The derivation of the indication of the number of foreground components and the indication of the number of background channels can, in some examples, proceed according to the following table: Table - syntax of ChannelSideinfoData(i) Where the description of the ChannelType is given as follows: ChannelType: 0: direction-based signal 1: vector-based signal (which can represent a foreground signal) 2: additional ambient HOA coefficient (which can represent an ambient or background signal) 3: empty As a result of signaling the ChannelType according to the SideChannelinfo syntax table above, the number of foreground components per layer can be determined as a function of the number of ChannelType syntax elements set to 1 and the number of background components per layer can be determined as a function of the number of ChannelType syntax elements set to 2.

[000142] The scalable bitstream generation unit 1000 may, in some examples, specify a HOADecoderConfig on a frame-by-frame basis, which provides the configuration information to extract the layers from the bitstream 21. The HOADecoderConfig may be specified as an alternative to or in combination with the above table. The following table can define the syntax for the HOADecoderConfig_FrameByFrame() object in bitstream 21.

[000143] In the table above, the HOABaseLayerPresent syntax element can represent a flag that indicates whether the base layer of scalable bitstream 21 is present. When present, scalable bitstream generation unit 1000 specifies a HOABaseLayerConfigurationFlag syntax element, which may represent a syntax element indicating whether configuration information for the base layer is present in bitstream 21. foreground (that is, the NumFGchannels syntax element in the example) for each of the layers, and a number of background channels (that is, the NumBGchannels syntax element in the example) for each of the layers. When the HOABaseLayerPresent flag indicates that the base layer configuration is not present, the scalable bitstream generation unit 1000 may not provide any additional syntax elements and the audio decoding device 24 may determine that the configuration data for the current frame is the same as that for a previous frame.

[000144] In some examples, the scalable bitstream generation unit 1000 may specify the HOADecoderConfig object in the scalable bitstream 21, but not specify the number of foreground and background channels per layer, where the number of foreground and background channels may be static or determined as described above with respect to the ChannelSideInfo table. The HOADecoderConfig can, in this example, be defined according to the following table.

[000145] As yet another alternative, the syntax tables above for HOADEcoderConfig can be replaced with the following syntax table for HOADEcoderConfig. Note: MinAmbHOAOrder = 30...37 are reserved

[000146] In this regard, the scalable bitstream generation unit 1000 can be configured to, as described above, specify, in the bitstream, an indication of a specified number of channels in one or more layers of the bitstream, and specify the indicated number of channels in one or more layers of the bitstream.

[000147] In addition, the scalable bitstream generation unit 1000 may be configured to specify a syntax element (for example, in the form of a NumLayers syntax element or a codedLayerCh syntax element as described in more detail below) indicative of the number of channels.

[000148] In some examples, the scalable bitstream generation unit 1000 may be configured to specify an indication of a total number of channels specified in the bitstream. The scalable bitstream generation unit 1000 may be configured to, in such instances, specify the indicated total number of channels in one or more layers of the bitstream. In these instances, the scalable bitstream generation unit 1000 may be configured to specify a syntax element (for example, a numHOATransportChannels syntax element as described in more detail below) indicative of the total number of channels.

[000149] In these and other examples, the scalable bitstream generation unit 1000 may be configured to specify an indication of a type of one of the specified channels in one or more layers in the bitstream. In these instances, the scalable bitstream generation unit 1000 may be configured to specify the indicated number of the indicated type of one of the channels in one or more layers of the bitstream. The foreground channel may comprise a US audio object and a corresponding V-vector.

[000150] In these and other examples, the scalable bitstream generation unit 1000 can be configured to specify an indication of a type of one of the channels specified in one or more layers in the bitstream, the indication of the type of one of the channels indicating that one of the channels is a foreground channel. In these instances, the scalable bitstream generation unit 1000 may be configured to specify the foreground channel in one or more layers of the bitstream.

[000151] In these and other examples, the scalable bitstream generation unit 1000 can be configured to specify an indication of a type of one of the channels specified in one or more layers in the bitstream, the indication of the type of one of the channels indicating that one of the channels is a background channel. In these instances, the scalable bitstream generation unit 1000 may be configured to specify the background channel in one or more layers of the bitstream. The background channel may comprise an ambient HOA coefficient.

[000152] In these and other examples, the scalable bitstream generation unit 1000 may be configured to specify a syntax element (eg, a ChannelType syntax element) indicative of the type of one of the channels.

[000153] In these and other examples, the scalable bitstream generation unit 1000 can be configured to specify the indication of the number of channels based on a number of channels remaining in the bitstream after one of the layers is taken (as defined for example by a remainingCh syntax element or a numAvaiableTransportChannels syntax element as described in more detail below.

[000154] Figures 7A-7D are flowcharts illustrating example operation of the audio coding device 20 in generating a coded two-layer representation of the HOA coefficients 11. background of a sound field) represented as energy compensated background HOA coefficients 47A'-47D' (300). The first-order ambisonics background 47A'-47D' may include the HOA coefficients corresponding to spherical basis functions having the following (order, suborder): (0.0), (1.0), (1,-1), (1,1).

[000155] The decorrelation unit 60 can transmit the decorrelated ambient HOA audio signals 67 as the aforementioned Q, T, L and R audio signals. The Q audio signal can provide pitch information. The T audio signal can provide horizontal information (including information to represent channels beyond the sweet spot). The L audio signal provides a stereo left channel. The R audio signal provides a stereo right channel.

[000156] In some examples, the UHJ matrix may comprise at least higher order ambisonic audio data associated with a left audio channel. In other examples, the UHJ matrix may comprise at least higher order ambisonic audio data associated with a right audio channel. In yet other examples, the UHJ matrix may comprise at least higher order ambisonic audio data associated with a location channel. In other examples, the UHJ matrix may comprise at least higher order ambisonic audio data associated with a pitch channel. In other examples, the UHJ matrix may comprise at least higher order ambisonic audio data associated with a sideband for automatic gain correction. In other examples, the UHJ matrix may comprise at least higher order ambisonic audio data associated with a left audio channel, a right audio channel, a location channel, and a height channel and a sideband for automatic gain correction.

[000157] The gain control unit 62 can apply automatic gain control (AGC) to the decorrelated ambient HOA audio signals 67 (302). The gain control unit 62 can pass the adjusted ambient HOA audio signals 67' to the bit stream generation unit 42, which can form the base layer based on the adjusted ambient HOA audio signals 67' and at least part of the sideband channel based on the higher order ambisonic gain control (HOAGCD) data (304).

[000158] The gain control unit 62 may also apply automatic gain control with respect to the interpolated nFg audio signals 491 (which may also be referred to as the "vector-based predominant signals") (306). The gain control unit 62 can transmit the 49'' adjusted nFG audio signals together with the HOAGCD for the 49'' adjusted nFG audio signals to the bitstream generating unit 42. The bitstream generating unit 42 can form the second layer based on the 49'' adjusted nFG audio signals while making part of the sideband information based on the HOADGCD for the 49'' adjusted nFG audio signals and the corresponding coded foreground V[k] vectors 57 (308).

[000159] The first layer (i.e. a base layer) of two or more layers of higher order ambisonic audio data may comprise higher order ambisonic coefficients corresponding to one or more spherical basis functions having an order equal to or less than one. In some examples, the second layer (i.e., an enhancement layer) comprises vector-based predominant audio data.

[000160] In some examples, vector-based predominant audio comprises at least predominant audio data and a V-encoded vector. As described above, the encoded V-vector can be decomposed from the higher order ambisonic audio data by applying a linear invertible transform by the LIT unit 30 of the audio coding device 20. In other examples, the vector-based predominant audio data comprises at least one additional higher order ambisonic channel. In yet other examples, the predominant vector-based audio data comprises at least one automatic gain correction sideband. In other examples, vector-based predominant audio data comprises at least predominant audio data, a V-encoded vector, an additional higher order ambisonic channel, and an automatic gain correction sideband.

[000161] In forming the first layer and second layer, the bit stream generation unit 42 can perform error checking processes that provide error detection, error correction or both error detection and correction. In some examples, the bitstream generation unit 42 may perform an error checking process on the first layer (i.e., the base layer). In another example, the audio encoding device may perform an error checking process on the second layer (i.e., the enhancement layer). In yet another example, the bitstream generation unit 42 can perform an error checking process on the first layer (i.e., the base layer, and, in response to the determination that the first layer is error free, the audio encoding device can perform an error checking process on the second layer (i.e., the enhancement layer). robust that is robust to errors.

[000162] With reference to Fig. 7B below, the gain control unit 62 and the bit stream generation unit 42 perform operations similar to that of the gain control unit 62 and the bit stream generation unit 42 described above with respect to Fig. 7A. however, the decorrelation unit 60 may apply a mode matrix decorrelation, rather than UHJ decorrelation, to the first order ambisonics background 47A'-47D' (301).

[000163] With reference to Fig. 7C below, the gain control unit 62 and the bit stream generation unit 42 can perform operations similar to that of the gain control unit 62 and the bit stream unit 42 described above with respect to the examples of Figs. 7A and 7B. However, in the example of Figure 7C, the decorrelation unit 60 may not apply any transform to the first order ambisonics background 47A'-47D'. In each of the following examples 8A-10B, it is assumed but not illustrated that the decorrelation unit 60 may alternatively not apply decorrelation with respect to one or more of the first order ambisonics background 47A'-47D'.

[000164] With further reference to figure 7D, the decorrelation unit 60 and the bit stream generation unit 42 can perform operations similar to that of the gain control unit 52 and the bit stream generation unit 42 described above with respect to the examples of figures 7A and 7B. However, in the example of Figure 7D, the gain control unit 62 may not apply any gain control to the decorrelated ambient HOA audio signals 67. In each of the following examples 8A-10B, it is assumed, but not illustrated, that the gain control unit 52 may alternatively not apply decorrelation with respect to one or more of the decorrelated ambient HOA audio signals 67.

[000165] In each of the examples of figures 7A-7D, the bitstream generation unit 42 can specify one or more syntax elements in the bitstream 21. Figure 10 is a diagram illustrating an example of a HOA configuration object specified in the bitstream 21. For each of the examples of figures 7A-7D, the bitstream generation unit 42 can set the codedVVecLength syntax element 400 to 1 or 2, which indicates that the 1st order background HOA channels contain the 1st order component of all predominant sounds. The bitstream generation unit 42 may also define the ambienceDecorrelationMethod syntax element 402 such that element 402 signals the use of UHJ decorrelation (e.g., as described above with respect to Fig. 7A), signals the use of matrix mode decorrelation (E.g., as described above with respect to Fig. 7B), or signals that no decorrelation has been used (e.g., as described above with respect to Fig. 7C).

[000166] Figure 11 is a diagram illustrating sideband information 410 generated by the bitstream generation unit 42 for the first and second layers. Sideband information 410 includes sideband base layer information 412 and sideband second layer information 414A and 414B. When only the base layer is provided to the audio decoding device 24, the audio decoding device 20 can only provide the sideband base layer information 412. The sideband base layer information 412 includes the HOAGCD for the base layer. Sideband second layer information 414A includes syntax elements from transport channels 1-4 and corresponding HOAGCD. Sideband second layer information 414B includes two corresponding encoded reduced V[k] vectors 57 corresponding to transport channels 1 and 2 (given transport channels 3 and 4 are empty as indicated by the ChannelType syntax element equaling 112 or 310).

[000167] Figures 8A and 8B are flowcharts illustrating example operation of the audio coding device 20 in generating a coded three-layer representation of the HOA coefficients 11. With reference first to the example of Figure 8A, the decorrelation unit 60 and the gain control unit 62 can perform operations similar to those described above with respect to Figure 7A. However, the bis stream generation unit 42 can form the base layer based on the L audio signal and the R audio signal of the adjusted ambient HOA audio signals 67 instead of all the adjusted ambient HOA audio signals 67 (310). The base layer may, in this regard, provide stereo channels when rendered on the audio decoding device 24. The bitstream generation unit 42 may also generate sideband information for the base layer which includes the HOAGCD.

[000168] The operation of the bis stream generation unit 42 can also differ from that described above with respect to figure 7A in that the bit stream generation unit 42 can form a second layer based on the Q and T audio signals of the adjusted ambient HOA audio signals 67 (312). The second layer in the example of Fig. 8A can provide horizontal channels and 3d audio channels when rendered on the audio decoding device 24. The bitstream generation unit 42 can also generate sideband information for the second layer which includes the HOAGCD. The bit stream generation unit 42 can also form a third layer in a manner substantially similar to that described above with respect to forming the second layer in the example of Fig. 7A.

[000169] The bitstream generation unit 42 can specify the HOA configuration object for the bitstream 21 similar to that described above with respect to Fig. 10 . Furthermore, the bitstream generation unit 42 of the audio encoder 20 sets the MinAmbHoaOrder syntax element 404 to 2 in order to indicate that the first-order HOA background is transmitted.

[000170] The bitstream generation unit 42 can also generate sideband information similar to sideband information 412 shown in the example of Fig. 12A. 12A is a diagram illustrating sideband information 412 generated in accordance with the scalable encoding aspects of the techniques described in this disclosure. Sideband information 412 includes sideband base layer information 416, sideband second layer information 418, and sideband layer third information 420A and 420B. sideband base layer information 416 may provide the HOAGCD for the base layer. Sideband second layer information 418 may provide the HOAGCD for the second layer. The third layer sideband information 420A and 420B may be similar to the sideband information 414A and 414B described above with respect to Figure 11.

[000171] Similar to figure 7A, the bit stream generation device 42 can perform error checking processes. In some examples, the bitstream generation device 42 may perform an error checking process on the first layer (i.e., the base layer). In another example, the bit stream generation device 42 can perform an error checking process on the first layer (i.e., the base layer) and refrain from performing an error checking process on the second layer (i.e., the enhancement layer). In yet another example, the bitstream generation device 42 can perform an error checking process on the first layer (i.e., the base layer) and, in response to the determination that the first layer is error free, the audio encoding device can perform an error checking process on the second layer (i.e., the enhancement layer). In any of the above examples where the audio encoding device performs the error checking process on the first layer (i.e. the base layer), the first layer can be considered a robust layer that is robust to errors.

[000172] Although described as providing three layers, in some examples, the bitstream generation device 42 may specify an indication in the bitstream that there are only two layers and specify a first of the bitstream layers indicative of background components of the higher order ambisonic audio signal that provide stereo channel reproduction, and a second of the layers of the bitstream indicative of the background components of the higher order ambisonic audio signal that provide horizontal multichannel reproduction by three or more speakers arranged in a plane single horizontal. In other words, although shown as providing three layers, the bitstream generation device 42 can only generate two of the three layers in some instances. It should be understood that any subset of the layers can be generated, although not described in detail here.

[000173] With reference to Fig. 8B below, the gain control unit 62 and the bit stream generation unit 42 perform operations similar to that of the gain control unit 62 and the bit stream generation unit 42 described above with respect to Fig. 8A. however, the decorrelation unit 60 may apply a mode matrix decorrelation, rather than the UHJ decorrelation, to the first order ambisonics background 47A' (316). In some examples, the background first-order ambisonics 47A' may include the zero-order ambisonic coefficients 47A'. The gain control unit 62 can apply automatic gain control to the first order ambisonic coefficients corresponding to the spherical harmonic coefficients having a first order, and the decorrelated ambient HOA audio signal 67.

[000174] The bitstream generation unit 42 can form a base layer based on the adjusted ambient HOA audio signal 67 and at least part of the sideband based on the corresponding HOAGCD (310). The ambient HOA audio signal 67 can provide a mono channel when rendered in the audio decoding device 24. The bitstream generation unit 42 can form a second layer based on the adjusted ambient HOA coefficients 47B''-47D'' and at least part of the sideband based on the corresponding HOAGCD (318). The adjusted ambient HOA coefficients 47B'-47D' can provide X, Y and Z channels (or stereo, horizontal and height) when rendered in the audio decoding device 24. The bit stream generation unit 42 can form the third layer and at least part of the sideband information in a similar way to that described above with respect to Fig. 8A. bit stream generation unit 42 may generate sideband information 412 as described in more detail with respect to Fig. 12B (326).

[000175] Figure 12B is a diagram illustrating sideband information 414 generated in accordance with the scalable encoding aspects of the techniques described in this disclosure. Sideband information 414 includes sideband base layer information 416, sideband second layer information 422, and sideband layer third information 424A-424C. Sideband base layer information 416 may provide the HOAGCD for the base layer. The sidband second layer information 422 may provide the HOAGCD for the second layer. The third layer sideband information 424A-424C may be similar to the sideband information 414A (except that the sideband information 414A is specified as the third layer sideband information 424A and 424B) and 414B described above with respect to Figure 11.

[000176] Figures 9A and 9B are flowcharts illustrating example operation of the audio coding device 20 in generating a coded four-layer representation of the HOA coefficients 11. With reference first to the example of Figure 9A, the decorrelation unit 60 and the gain control unit 62 can perform operations similar to those described above with respect to Figure 8A. the bitstream generation unit 42 can form the base layer in a similar way to those described above with respect to the example of Fig. 8A, i.e. based on the L audio signal and the R audio signal of the adjusted ambient HOA audio signals 67 instead of all the adjusted ambient HOA audio signals 67 (310). The base layer can, in that respect, provide stereo channels when rendered on audio decoding device 24 (or, in other words, provide stereo channel playback). The bitstream generation unit 42 can also generate sideband information for the base layer that includes the HOAGCD.

[000177] The operation of the bitstream generation unit 42 may differ from that described above with respect to Figure 8A in that the bitstream generation unit 42 can form a second layer based on the T audio signal (and not the Q audio signal) of the adjusted ambient HOA audio signals 67 (322). The second layer in the example of Figure 9A can provide horizontal channels when rendered on audio decoding device 24 (or, in other words, multichannel playback by three or more speakers in a single horizontal plane). Bitstream generation unit 42 may also generate sideband information for the second layer including the HOAGCD. The bitstream generation unit 42 can also form a third layer based on the Q audio signal of the adjusted ambient HOA audio signals 67 (324). The third layer can provide three-dimensional reproduction by three or more speakers arranged in one or more horizontal planes. The bit stream generation unit 42 can form the fourth layer in a substantially similar manner as described above with respect to forming the third layer in the example of Fig. 8A (326).

[000178] The bit stream generation unit 42 can specify the HOA configuration object for the bit stream 21 similar to that described above with respect to figure 10. In addition, the bit stream generation unit 42 of the audio encoder 20 sets the MinAmbHoaOrder syntax element 404 to 2 in order to indicate that the 1st order HOA background is transmitted.

[000179] The bit stream generation unit 42 can also generate sideband information similar to sideband information 412 shown in the example of Fig. 13A. Figure 13A is a diagram illustrating sideband information 430 generated in accordance with the scalable encoding aspects of the techniques described in this disclosure. Sideband information 430 includes sideband base layer information 416, sideband second layer information 418, sideband third layer information 432, and sideband fourth layer information 434A and 434B. The sideband base layer information 416 may provide the HOAGCD for the base layer. Sideband second layer information 418 may provide the HOAGCD for the second layer. Sideband third layer information 430 may provide the HOAGCD for the third layer. The fourth layer sideband information 434A and 434B may be similar to the sideband information 420A and 420B described above with respect to Fig. 12A.

[000180] Similar to figure 7A, the bit stream generation device 42 can perform error checking processes. In some examples, the bit stream generation device 42 may perform an error checking process on the first layer (i.e., the base layer). In another example, the bit stream generation device 42 can perform an error checking process on the first layer (i.e. the base layer) and refrain from performing an error checking process on the remaining layer (i.e. the enhancement layers). In yet another example, the bitstream generation device 42 can perform an error checking process on the first layer (i.e., the base layer) and, in response to the determination that the first layer is error free, the audio encoding device can perform an error checking process on the second layer (i.e., the enhancement layer). In any of the above examples where the audio encoding device performs the error checking process on the first layer (i.e. the base layer), the first layer can be considered a robust layer that is robust to errors.

[000181] With reference to Fig. 9B below, the gain control unit 62 and the bit stream generation unit 42 perform operations similar to that of the gain control unit 62 and bit stream generation unit 42 described above with respect to Fig. 9A. However, the decorrelation unit 60 may apply a mode matrix decorrelation, rather than the UHJ decorrelation, to the first order ambisonics background 47A' (316). In some examples, the background first-order ambisonics 47A' may include the zero-order ambisonic coefficients 47A'. The gain control unit 62 can apply automatic gain control to the first order ambisonic coefficients corresponding to the spherical harmonic coefficients having a first order, and the decorrelated ambient HOA audio signal 67 (302).

[000182] The bitstream generation unit 42 can form a base layer based on the adjusted ambient HOA audio signal 67 and at least part of the sideband based on the corresponding HOAGCD (310). The ambient HOA audio signal 67 can provide a mono channel when rendered in the audio decoding device 24. The bitstream generation unit 42 can form a second layer based on the adjusted ambient HOA coefficients 47B" and 47C" and at least part of the sideband based on the corresponding HOAGCD (322). The adjusted ambient HOA coefficients 47B” and 47C” can provide X, Y horizontal multichannel reproduction by three or more speakers arranged in a single horizontal plane. The bit stream generation unit 42 can form a third layer based on the adjusted ambient HOA coefficients 47D" and at least part of the sideband based on the corresponding HOAGCD (324). Adjusted 47D” ambient HOA coefficients can provide three-dimensional reproduction by three or more loudspeakers arranged in one or more horizontal planes. The bitstream generation unit 42 can form the fourth layer and at least part of the sideband information in a similar manner to that described above with respect to Fig. 8A (326). Bitstream generation unit 42 may generate sideband information 412 as described in more detail with respect to Fig. 12B.

[000183] Figure 13B is a diagram illustrating sideband information 440 generated in accordance with the scalable encoding aspects of the techniques described in this disclosure. Sideband information 440 includes sideband base layer information 416, sideband second layer information 442, sideband third layer information 444, and sideband fourth layer information 446A-446C. sideband base layer information 416 may provide the HOAGCD for the base layer. Sideband second layer information 442 may provide the HOAGCD for the second layer. The sideband third layer information can provide the HOAGCD for the third layer. The fourth layer sideband information 446A-446C may be similar to the sideband information 424A-424C described above with respect to Fig. 12B.

[000184] Figure 4 is a block diagram illustrating the audio decoding device 24 of Figure 2 in more detail. As shown in the example of Figure 4 the audio decoding device 24 may include an extraction unit 72, a directionality based reconstruction unit 90 and a vector based reconstruction unit 92. WO 2014/194099, entitled “INTERPOLATION FOR DECOMPOSED REPRESENTATIONS OF A SOUND FIELD,” filed May 29, 2014. Additional information can also be found in the above referenced Phase I and Phase II of the 3D MPEG-H Audio Coding Standard and the corresponding article referenced above summarizing Phase I of the 3D MPEG-H Audio Coding Standard.

[000185] The extraction unit 72 can represent a unit configured to receive the bit stream 21 and extract the various encoded versions (for example, a directional-based encoded version or a vector-based encoded version) of the HOA coefficients 11. When a directional based encoding has been performed, the extraction unit 72 can extract the directional based version of the HOA coefficients 11 and the syntax elements associated with the encoded version (which is indicated as directional based information 92 in the example of Figure 4), passing the directional based information 91 to the directional based reconstruction unit 90. on information based on directional 91.

[000186] When the syntax element indicates that hoa 11 coefficients were coded using a vector-based synthesis, 72 extraction unit can extract vectors V [K] vectors 57 (which may include encoded weights 57 and/or indexes 63 or scalar quantized vectors), the coefficients Hoa coded environment 59 and the corresponding audio objects 61 (which may 61 also mentioned as coded NFG signs 61). The audio objects 61 individually correspond to one of the vectors 57. The extraction unit 72 can pass the encoded foreground V[k] vectors 57 to the V-vector reconstruction unit 74 and the encoded ambient HOA coefficients 59 along with the encoded nFG signals 61 to the psychoacoustic decoding unit 80. The extraction unit 72 is described in more detail with respect to the example of Figure 6.

[000187] Figure 6 is a diagram illustrating, in more detail, the extraction unit 72 of Figure 4 when configured to perform the first of the potential versions of the scalable audio decoding techniques described in this disclosure. In the example of Figure 6, the extraction unit 72 includes a mode selection unit 1010, a scalable extraction unit 1012 and a non-scalable extraction unit 1014. The mode selection unit 1010 represents a unit configured to select whether scalable or non-scalable extraction is to be performed with respect to the bit stream 21. The mode selection unit 1010 may include a memory in which the bit stream 21 is stored. The mode selection unit 1010 can determine whether scalable or non-scalable extraction is to be performed based on the indication that scalable coding has been enabled. A HOABaseLayerPresent syntax element can represent the indication of whether scalable encoding was performed when encoding bit stream 21.

[000188] When the HOABaseLayerPresent syntax element indicates that scalable encoding has been enabled, the mode selection unit 1010 may identify the bitstream 21 as the scalable bitstream 21 and transmit the scalable bitstream 21 to the scalable extraction unit 1012. When the HOABaseLayerPresent syntax element indicates that scalable encoding has not been enabled, the mode selection unit 101 0 may identify the bit stream 21 as the non-scalable bit stream 21' and transmit the non-scalable bit stream 21' to the non-scalable extraction unit 1014. The non-scalable extraction unit 1014 represents a unit configured to operate in accordance with phase I of the MPEG-H 3D audio coding standard.

[000189] The scalable extraction unit 1012 may represent a unit configured to extract one or more of the ambient HOA coefficients 59, the encoded nFG signals 61 and the encoded foreground V[k] vectors 57 from one or more layers of the scalable bitstream 21 based on various syntax elements described in more detail below (and shown above in various HOADecoderConfig tables). In the example of figure 6, the scalable extraction unit 1012 can extract, as an example, the four encoded ambient HOA coefficients 59A-59D from the base layer 21A of the scalable bitstream 21. The scalable extraction unit 1012 can also extract, from the enhancement layer 21B of the scalable bitstream 21, the two encoded nFG signals 61A and 61B (as an example) as well as the two encoded foreground V[k] vectors 57A and 57B. the scalable extraction unit 1012 may transmit the ambient HOA coefficients 59, the encoded nFG signals 61 and the encoded foreground V[k] vectors 57 to the vector based decoding unit 92 shown in the example of Figure 4.

[000190] More specifically, the extraction unit 72 of the audio decoding device 24 can extract channels from the L layers as exposed in the syntax table HOADecoderCofnig_FramebyFrame above.

[000191] According to the above HOADecoderCofnig_FramebyFrame syntax table, the mode selection unit 1010 can first obtain the HOABaseLayerPresent syntax element, which can indicate whether scalable audio encoding has been performed. When not enabled as specified by, for example, a zero value for the HOABaseLayerPresent syntax element, the mode selection unit 1010 may determine the MinAmbHoaOrder syntax element and provide the non-scalable bit stream to the non-scalable extraction unit 1014, which performs non-scalable extraction processes similar to those described above. When enabled as specified, for example, by a value for the HOABaseLayerPresent syntax element, the mode selection unit 1010 sets the MinAmbHOAOrder syntax element value to be negative one (-1) and provides the scalable bitstream 21' to the scalable extraction unit 1012.

[000192] The scalable extraction unit 1012 can obtain an indication of whether a number of layers of the bitstream has changed in a current frame when compared to a number of layers of the bitstream in a previous frame. The indication of whether the number of bitstream layers has changed in the current frame as compared to the number of bitstream layers in the previous frame can be indicated as a “HOABaseLayerConfigurationFlag” syntax element in the table above.

[000193] The scalable extraction unit 1012 can obtain an indication of a number of layers of the bit stream in the current frame based on the indication. When the indication indicates that the number of layers of the bitstream has not changed in the current frame when compared to the number of layers of the bitstream in the previous frame, the scalable extraction unit 1012 can determine the number of layers of the bitstream in the current frame as equal to the number of layers of the bitstream in the previous frame according to the portion of the above syntax table that states: } else } NumLayers = NumLayersPrevFrame; Where “NumLayers” can represent a syntax element representing the number of layers of the bitstream in the current frame and “NumLayersPrevFrame” can represent a syntax element representing the number of layers of the bitstream in the previous frame.

[000194] According to the above HOADecoderConfig_FramebyFrame syntax table, the scalable extraction unit 1012 can, when the indication indicates that the number of layers of the bitstream has not changed in the current frame when compared to the number of layers of the bitstream in the previous frame, determine a current foreground indication of a current number of foreground components in one or more of the layers for the current frame to be equal to a previous foreground indication for a previous number of foreground components on one or more of the layers in the previous frame. In other words, the scalable extraction unit 1012 can, when the HOABaseLayerConfigurationFlag equals zero, determine the syntax element HumFGchannels[i] representative of the current foreground indication of the current number of foreground components in one or more of the layers of the current frame to be equal to the syntax element NumFGchannels_PrevFrame[i] that is representative of the previous foreground indication of the previous number of foreground components in one or more layers of the previous frame. Scalable extraction unit 1012 can additionally obtain foreground components from one or more layers in the current frame based on the current foreground indication.

[000195] The scalable extraction unit 1012 may also, when the indication indicates that the number of layers of the bitstream has not changed in the current frame when compared to the number of layers of the bitstream in the previous frame, determine a current background indication of a current number of background components in one or more of the layers for the current frame to be equal to a previous background indication for a previous number of background components in one or more of the layers of the previous frame. In other words, the scalable extraction unit 1012 can, when HOABaseLayerConfigurationFlag equals zero, determine the syntax element NumBGchannels[i] representative of the current background indication of the current number of background components in one or more of the layers of the current frame to be equal to the syntax element NumBGchannels_PrevFrame[i] that is representative of the previous background indication of the previous number of background components in one or more layers of the previous frame. The scalable extraction unit 1012 can further obtain the background components from one or more layers in the current frame based on the current background indication.

[000196] To enable the above techniques that can potentially reduce signaling of various indications of the number of layers, foreground components, and background components, the scalable extraction unit 1012 may define the syntax element NumFGChannels_PrevFrame[i] and the syntax element NumBGchannel_PrevFrame[i] to the indications for the current frame (e.g., the syntax element NumFGchannels[i] and the NumBGchannels[i], it running through all layers i. this is represented in the following syntax:

[000197] When the indication indicates that the number of bitstream layers has changed in the current frame when compared to the number of bitstream layers in the previous frame (for example, when the HOABaseLayerConfigurationFlag is equal to one), the scalable extraction unit 1012 gets the syntax element NumLayerBits as a function of numHOATransportChannels, which is passed to the syntax table having been obtained according to other syntax tables not described in this one)

[000198] The scalable extraction unit 1012 in the bit stream (for example, the NumLayers syntax element), where the indication can be a number of bits indicated by the NumLayerBits syntax element. The NumLayers syntax element can specify the number of layers specified in the bitstream, where the number of layers can be indicated as L above. The scalable extraction unit 1012 may then determine the numAvailableTransportChannels as a function of numHOATransportChannels and the numAvailableTransportChannels as a function of numAvailableTransportChannels.

[000199] The scalable extraction unit 1012 can then iterate through NumLayers 1 through Num-Layers- 1 to determine the number of background HOA channels (Bi) and the number of foreground HOA channels (Fi) specified for i layer. The scalable extraction unit 1012 may not iterate through the last layer number (NumLayer) and only through NumLayer-1 since the last BL layer can be determined when the total number of foreground and background HOA channels sent in the bit stream are known by the scalable extraction unit 1012 (e.g. when the total number of foreground and background HOA channels are signaled as syntax elements).

[000200] In this regard, the scalable extraction unit 1012 can obtain the layers of the bitstream based on the indication of the number of layers. The scalable extraction unit 1012 can, as described above, obtain an indication of a specified number of channels in the bitstream 21 (e.g., numHOATransportChannels), and obtain the layers, by at least in part obtaining the layers of the bitstream 21 based on the indication of the number of layers and the indication of the number of channels.

[000201] When iterating through each layer, the scalable extraction unit 1012 may first determine the number of foreground channels for the 1st layer by obtaining the syntax element NumFGchannels[i]. The scalable extraction unit 1012 can then subtract the NumFGchannels[i] from the numAvailableTransportChannels to update the numAvailableTransportChannels to reflect that the NumFGchannels[i] of the foreground HOA channels 61 (which may also be referred to as the "coded nFG signals 61") have been extracted from the bitstream. In this way, the scalable extraction unit 1012 can obtain an indication of a number of foreground channels specified in the bitstream 21 for at least one of the layers (e.g., numFGchannels) and obtain the foreground channels for at least one of the layers of the bitstream based on the indication of the number of foreground channels.

[000202] Similarly, the scalable extraction unit 1012 can determine the number of background channels for the 1st layer by obtaining the NumBGchannels[i] syntax element. The scalable extraction unit 1012 can then subtract the NumBGchannels[i] from the numAvailableTransportChannels to reflect that the NumBGchannels[i] of the background HOA channels 59 (which may also be referred to as the "encoded ambient HOA coefficients 59") have been extracted from the bit stream. In this way, the scalable extraction unit 1012 can obtain an indication of a number of background channels (e.g., NumBGchannels) specified in the bitstream 21 for at least one of the layers, and obtain the background channels for at least one of the layers of the bitstream based on the indication of the number of background channels.

[000203] Scalable extraction unit 1012 can proceed by getting numAvailableTransportChannelsBits as a function of numAvailableTransports. According to the above syntax table, the scalable extraction unit 1012 can parse the number of bits specified by numAvaiableTransportChannelsBits to determine the numFGchannels[i] and the NumBGchannels[i]. As the numAvailableTransportChannelBits changes (e.g., becomes smaller after each iteration), the number of bits used to represent the NumFGchannels[i] syntax element and the numBGchannels[i] syntax element reduces, thereby providing a variable-length encoding form that potentially reduces overhead in signaling the NumFGchannels[i] syntax element and the NumBGchannels[i] syntax element.

[000204] As noted above, the scalable bitstream generation unit 1000 may specify the NumChannels syntax element in place of the NumFGchannels and NumBGchannels syntax elements. In that case, the scalable extraction unit 1012 can be configured to operate in accordance with the second HOADecoderConfig syntax table shown above.

[000205] In this regard, the scalable extraction unit 1012 can, when the indication indicates that the number of layers of the bitstream has changed in the current frame when compared to the number of layers of the bitstream in the previous frame, obtain an indication of a number of components in one or more of the layers for the current frame based on a number of components in one or more of the layers of the previous frame. The scalable extraction unit 1012 may further obtain an indication of a number of background components in one or more layers for the current frame based on the indication of the number of components. The scalable extraction unit 1012 may also obtain an indication of a number of foreground components in one or more layers for the current frame based on the indication of the number of components.

[000206] Given that the number of layers can change from frame to frame that the indication of the number of foreground and background channels can change from frame to frame, the indication that the number of layers has changed can effectively also indicate that the number of channels has changed. As a result, the indication that the number of layers has changed can result in the scalable extraction unit 1012 obtaining an indication of whether the specified number of channels in one or more layers in the bit stream 21 has changed in a current frame when compared to a specified number of channels in one or more layers in the previous frame's bit stream. As such, the scalable extraction unit 1012 can obtain one of the channels based on an indication of whether the number of channels specified in one or more layers in the bitstream has changed in the current frame.

[000207] Furthermore, the scalable extraction unit 1012 can determine the number of channels specified in one or more layers of the bitstream 21 in the current frame as equal to the number of channels specified in one or more layers of the bitstream 21 in the previous frame when the indication indicates that the number of channels specified in one or more layers of the bitstream 21 has not changed in the current frame when compared to the number of channels specified in one or more layers of the bitstream in the previous frame.

[000208] Furthermore, the scalable extraction unit 1012 can, when the indication indicates that the number of channels specified in one or more layers of the bitstream 21 has not changed in the current frame when compared with the number of channels specified in one or more layers of the bitstream in the previous frame, obtain an indication of a current number of channels in one or more of the layers for the current frame as being equal to a previous number of channels in one or more of the layers of the previous frame.

[000209] To allow for the above techniques that can potentially reduce the signaling of various indications of the number of layers and components (which may also be referred to as "channels" in this disclosure), the scalable extraction unit 1012 may set the syntax element NumChannels_PrevFrame[i] to the indications for the current frame (e.g., the syntax element NumChannels[i], iterating through all layers i. This is represented in the following syntax:

[000210] Alternatively, the above syntax element (NumLayersPrevFrame=NumLayers, etc.) can be omitted and the HOADecoderConfig(numHOATransportChannels) syntax table listed above can be updated as shown in the following table:

[000211] As yet another alternative, the extraction unit 72 can operate according to the third HOADecdor Config listed above. According to the third HOADecoderConfig syntax table listed above, the scalable extraction unit 1012 can be configured to obtain, from the scalable bitstream 21, an indication of a specified number of channels in one or more layers in the bitstream, and to obtain the specified channels in one or more layers in the bitstream based on the indication of the number of channels (which may refer to a background component or a foreground component of the sound field). In these and other instances, scalable extraction unit 1012 may be configured to obtain a syntax element (eg, the codedLayerCh in the above-referenced table) indicative of the number of channels.

[000212] In these and other instances, the scalable extraction unit 1012 may be configured to obtain an indication of a total number of channels specified in the bit stream. The scalable extraction unit 1012 can also be configured to obtain the specified channels in one or more layers based on the indication of the specified number of channels in the one or more layers and the indication of the total number of channels. In these and other instances, scalable extraction unit 1012 may be configured to obtain a syntax element (e.g., the aforementioned NumHOATransportChannels syntax element) indicative of the total number of channels.

[000213] In these and other instances, the scalable extraction unit 1012 may be configured to obtain an indication of a type from one of the specified channels in one or more layers in the bitstream. The scalable extraction unit 1012 can also be configured to obtain one of the channels based on the indication of the number of layers and the indication of the type of one of the channels.

[000214] In these and other instances, the scalable extraction unit 1012 can be configured to obtain an indication of a type of one of the channels specified in one or more layers in the bitstream, the indication of the type of one of the channels indicating that one of the channels is a foreground channel. The scalable extraction unit 1012 can be configured to obtain one of the channels based on the indication of the number of layers and the indication that the type of one of the channels is the foreground channel. In these instances, one of the channels comprises a US audio object and a corresponding V-vector.

[000215] In these and other instances, the scalable extraction unit 1012 can be configured to obtain an indication of a type of one of the channels specified in one or more layers in the bitstream, the indication of the type of one of the channels indicating that one of the channels is a background channel. In such instances, the scalable extraction unit 1012 may also be configured to obtain one of the channels based on the indication of the number of layers and the indication that the type of one of the channels is the background channel. In these instances, one of the channels comprises a background higher-order ambisonic coefficient.

[000216] In these and other instances, the scalable extraction unit 1012 can be configured to obtain a syntax element (for example, the ChannelType syntax element described above with respect to figure 30) indicative of the type of one of the channels.

[000217] In these and other instances, the scalable extraction unit 1012 can be configured to obtain the indication of the number of channels based on a number of channels remaining in the bitstream after obtaining one of the layers. That is, the value of the HOALayerChBits syntax element varies as a function of the remainingCh syntax element as shown in the above syntax table throughout the course of the loop. The scalable extraction unit 1012 can then parse the codedLayerCh syntax element based on the mutating HOALayerChBits syntax element.

[000218] Returning to the example of the four background channels and the two foreground channels, the scalable extraction unit 1012 can receive an indication that the number of layers is two, that is, the base layer 21A and the enhancement layer 21B in the example of figure 6. The scalable extraction unit 1012 can obtain an indication that the number of foreground channels is zero for the base layer 21A (for example, from NumFG channels[0] and two for enhancement layer 21B (eg, from NumFGchannels[1]). The scalable extraction unit 1012 may, in that example, also obtain an indication that the number of background channels is four for base layer 21A (eg, from NumBGchannels[0]) and zero for enhancement layer 21B (eg, from NumBGchannels[1]). Although described with respect to a specific example, any different combination of channels background and foreground can be indicated. The scalable extraction unit 1012 can then extract the four specified background channels 59A-59D from the base layer 21A and the two foreground channels 61A and 61B from the enhancement layer 21B (along with the corresponding V-vector information 57A and 57B from the sideband information).

[000219] While described above with regards to NumFGchannels and NumBGchannels syntax elements, the techniques can also be performed using the ChannelType syntax element from the ChannelSideinfo syntax table above. In this respect, NumFGchannels and NumBGchannels can also represent an indication of a type of one of the channels. In other words, the NumBGchannels can represent an indication that a type of one of the channels is a background channel. The NumFGchannels can represent an indication that a type of one of the channels is a foreground channel.

[000220] As such, if either the ChannelType syntax element or the NumFGchannels syntax element with the NumBGchannels syntax element are used (or potentially both or some subset of either), the scalable bitstream extraction unit 1012 can obtain an indication of a type of one of the specified channels in one or more layers in the bitstream. The scalable bitstream extraction unit 1012 can, when the type indication indicates that one of the channels is a background channel, obtain one of the channels based on the indication of the number of layers and the indication that the type of one of the channels is the background channel. The scalable bitstream extraction unit 1012 can, when the type indication indicates that one of the channels is a foreground channel, obtain one of the channels based on the indication of the number of layers and the indication that the type of one of the channels is the foreground channel.

[000221] The V-vector reconstruction unit 74 may represent a unit configured to reconstruct the V-vectors from the encoded foreground V[k] vectors 57. The V-vector reconstruction unit 74 may operate in a reciprocal mode to that of the quantization unit 52.

[000222] The psychoacoustic decoding unit 80 can operate in a reciprocal mode to the psychoacoustic audio encoder unit 40 shown in the example of figure 3 in order to decode the ambient HOA coded coefficients 59 and the encoded nFG signals 61 and thereby generate ambient HOA audio signals adjusted 67' and the interpolated nFG signals adjusted 49'' (which can also be referred to as objects nFG interpolated audio files adjusted 49'). The psychoacoustic decoding unit 80 can pass the ambient HOA audio signals set 67' and the interpolated nFG signals set 49'' to the inverse gain control unit 86.

[000223] The inverse gain control unit 86 can represent a unit configured to perform an inverse gain control with respect to each of the HOA ambient audio signals adjusted 67' and the interpolated nFG signals adjusted 49'', where this inverse gain control is reciprocal to the gain control performed by the gain control unit 62. The inverse gain control unit 86 can perform inverse gain control according to the corresponding HOAGCD specified in sideband information discussed above with respect to the examples of Figures 11-13B. The inverse gain control unit 86 can transmit decorrelated ambient HOA audio signals 67 to the recursion unit 88 (shown as "recurr unit 88" in the example of Figure 4) and the interpolated nFG audio signals 49'' to the foreground formulation unit 78.

[000224] Recurrence unit 88 can implement techniques of this disclosure to reduce correlation between background channels of decorrelated ambient HOA audio signals 67 to reduce or mitigate noise unmasking. In instances where the recursion unit 88 applies a UHJ matrix (e.g., an inverse UHJ matrix) as the selected recursion transform, the recursion unit 81 can improve compression rates and conserve computing resources by reducing data processing operations.

[000225] In some examples, the scalable bitstream 21 may include one or more syntax elements that indicate that a decorrelation transform has been applied during encoding. The inclusion of such syntax elements in the vector-based bit stream 21 may allow the recursion unit 88 to perform reciprocal decorrelation transforms (e.g., correlation or recurration) on the decorrelated ambient HOA audio signals 67. In some examples, the signal syntax elements may indicate which decorrelation transform has been applied, such as the UHJ matrix or the mode matrix, thereby allowing the recursion unit 88 to select the appropriate recursion transform to apply to the HO audio signals The decorrelated 67.

[000226] The re-relation unit 88 can perform re-relation with respect to the decorrelated ambient HOA audio signals 67 to obtain energy-compensated ambient HOA coefficients 47'. The re-correlation unit 88 may transmit the energy-compensated ambient HOA coefficients 47' to the fading unit 770. Although described as performing de-correlation, in some examples no de-correlation may have been performed. As such, vector based reconstruction unit 92 may not perform or in some examples include a recursion unit 88. The absence of recursion unit 88 in some examples is indicated.

[000227] The space-time interpolation unit 76 can operate in a similar manner to that described above with respect to the space-time interpolation unit 50. The space-time interpolation unit 76 can receive the reduced foreground vectors V[k] 55k and perform space-time interpolation with respect to the foreground vectors V[k] 55k and the foreground vectors V[k-1] reduced 55k-1 to generate 55k interpolated foreground V[k] vectors”. The space-time interpolation unit 76 may send the 55k” interpolated foreground vectors V[k] to the fading unit 770.

[000228] The extraction unit 72 can also transmit a signal 757 indicative of when one of the ambient HOA coefficients is in transition to the fading unit 770, which can then determine which of the SHCBG 47' (where the SHCBG 47' can also be indicated as "ambient HOA channels 47"' or "ambient HOA coefficients 47'") and the elements of the interpolated foreground vectors V[k] 55k” are to be faded or faded. In some examples, the fading unit 770 may operate opposite with respect to each of the ambient HOA coefficients 47' and the elements of the interpolated foreground V[k] vectors 55k”. That is, the fading unit 770 can perform a fade or fade, or both a fade and fade with respect to a corresponding one of the ambient HOA coefficients 47', while performing a fade or fade or both a fade and a fade, with respect to a correspondent of the elements of the interpolated foreground vectors V[k] 55k". Fading unit 770 may transmit 47" adjusted ambient HOA coefficients to 55k" adjusted HOA coefficient formulation unit and 55k" adjusted foreground vectors V[k] to 78 foreground formulation unit. 47' and the interpolated foreground V[k] vector elements 55k”.

[000229] The foreground formulation unit 78 can represent a unit configured to perform matrix multiplication with respect to the adjusted foreground vectors V[k] 55k"' and the interpolated nFG signals 49' to generate the foreground HOA coefficients 65. In this regard, the foreground formulation unit 78 can combine the audio objects 49' (which is another way in which to indicate the interpolated nFG signals 49') with the 55k”’ vectors to reconstruct the foreground or, in other words, predominant aspects of the HOA 11’ coefficients. The foreground formulation unit 78 may perform a matrix multiplication of the interpolated nFG signals 49' by the adjusted foreground vectors V[k] 55k"'.

[000230] The HOA coefficient formulation unit 82 can represent a unit configured to combine the foreground HOA coefficients 65 with the ambient HOA coefficients 47” in order to obtain the HOA coefficients 11'. The main notation reflects that the HOA 11' coefficients may be similar to, but not the same as, the HOA 11 coefficients. Differences between the HOA 11 and 11' coefficients may result from loss due to transmission through a lossy transmission media, quantization, or other lossy operations.

[000231] Figures 14A and 14B are flowcharts illustrating example operations of audio encoding device 20 in performing various aspects of the techniques described in this disclosure. Referring first to the example of Fig. 14A, the audio coding device 20 can obtain channels for a current frame of HOA coefficients 11 in the manner described above (eg, a linear decomposition, interpolation, etc.) (500). The channels may comprise coded ambient HOA coefficients 59, coded nFG signals 61 (and corresponding sideband in the form of coded foreground V-vectors 57) or both coded ambient HOA coefficient 59 and coded nFG signals 61 (and corresponding sideband in the form of coded foreground V-vectors 57).

[000232] The bitstream generation unit 42 of the audio coding device 20 can then specify an indication of a number of layers in the scalable bitstream 21 in the manner described above (502). The bitstream generation unit 42 may specify a subset of the channels in the current layer of the scalable bitstream 21 (504). The bit stream generation unit 42 may maintain a counter for the current layer, where the counter provides an indication of the current layer. After specifying the channels in the current layer, the bitstream generation unit 42 can increment the counter.

[000233] The bitstream generation unit 42 can then determine whether the current layer (eg, the counter) is greater than the number of layers specified in the bitstream (506). When the current layer is not greater than the number of layers ("NO" 506), the bitstream generation unit 42 may specify a different subset of the channels in the current layer (which changed when the counter was incremented) (504). The bit stream generation unit 42 can continue in this way until the current layer is greater than the number of layers ("SIM" 506). When the current layer is greater than the number of layers ("YES" 506), the bitstream generation unit can proceed to the next frame with the current frame becoming the previous frame and obtain the channels for the now current frame from the scalable bitstream 21 (500). The process can continue until reaching the last frame of HOA 11 coefficients (500-506). As noted above, in some examples, the indication of the number of layers may not be explicitly stated, but implicitly specified in the scalable bitstream 21 (for example, when the number of layers has not changed from the previous frame to the current frame).

[000234] With reference below to the example of Figure 14B, the audio coding device 20 can obtain channels for a current frame of HOA coefficients 11 in the manner described above (eg, a linear decomposition, interpolation, etc.) (510). The channels may comprise coded ambient HOA coefficients 59, coded nFG signals 61 (and corresponding sideband in the form of coded foreground V-vectors 57) or both coded ambient HOA coefficient 59 and coded nFG signals 61 (and corresponding sideband in the form of coded foreground V-vectors 57).

[000235] The bitstream generation unit 42 of the audio coding device 20 can then specify an indication of a number of channels in a layer of the scalable bitstream 21 in the manner described above (512). The bitstream generation unit 42 can specify corresponding channels in the current layer of the scalable bitstream 20 (514).

[000236] The bitstream generation unit 42 can then determine whether the current layer (eg counter) is greater than a number of layers (516). That is, in the example of Figure 14B, the number of layers can be static or fixed (rather than specified in the scalable bitstream 21), while the number of channels per layer can be specified, unlike the example in Figure 14A where the number of channels can be static or fixed and unsignaled. The bit stream generation unit 42 can still keep the counter indicative of the current layer.

[000237] When the current layer (as indicated by the counter) is not greater than the number of layers (“NO” 516), the bitstream generation unit 42 may specify another indication of the number of channels in another layer of the scalable bitstream 21 for the now current layer (which has changed due to the increment of the counter) (512). The bitstream generation unit 42 may also specify the corresponding number of channels in the additional bitstream layer 21 (514). The bit stream generation unit 42 can continue in this way until the current layer is greater than the number of layers ("SIM" 516). When the current layer is greater than the number of layers ("YES" 516), the bitstream generation unit can proceed to the next frame with the current frame becoming the previous frame and obtain the channels for the now current frame from the scalable bitstream 21 (510). The process can continue until reaching the last frame of HOA 11 coefficients (510-516).

[000238] As noted above, in some examples, the indication of the number of channels may not be explicitly stated, but implicitly specified in scalable bitstream 21 (for example, when the number of layers has not changed from the previous frame to the current frame). Furthermore, although described as separate processes, the techniques described with respect to Figures 14A and 14B can be performed in combination in the manner described above.

[000239] Figures 15A and 15B are flowcharts illustrating example operations of audio decoding device 24 in performing various aspects of the techniques described in this disclosure. Referring first to the example of Fig. 15A, the audio decoding device 24 can obtain a current frame from the scalable bit stream 21 (520). The current frame can include one or more layers, each of which can include one or more channels. The channels may comprise coded ambient HOA coefficients 59, coded nFG signals 61 (and corresponding sideband in the form of coded foreground V-vectors 57) or both coded ambient HOA coefficients 59 and coded nFG signals 61 (and corresponding sideband in the form of coded foreground V-vectors 57).

[000240] The extraction unit 72 of the audio decoding device 24 can then obtain an indication of a number of layers in the current frame from the scalable bitstream 21 in the manner described above (522). The extraction unit 72 can obtain a subset of the channels in the current layer of the scalable bit stream 21 (524). The extraction unit 72 may maintain a counter for the current layer, where the counter provides an indication of the current layer. After specifying the channels in the current layer, the extraction unit 72 can increment the counter.

[000241] The extraction unit 72 can then determine if the current layer (eg the counter) is greater than the number of layers specified in the bit stream (526). When the current layer is not greater than the number of layers ("No" 526), the extraction unit 72 can obtain a different subset of the channels in the current layer (which changed when the counter was incremented) (524). The extraction unit 72 can continue in this way until the current layer is greater than the number of layers ("SIM" 526). When the current layer is greater than the number of layers ("SIM" 526), the extraction unit 72 can proceed to the next frame with the current frame becoming the previous frame and obtain the new current frame from the scalable bit stream 21 (520). The process can continue until reaching the last frame of scalable bit stream 21 (520-526). As noted above, in some examples, the indication of the number of layers may not be explicitly stated, but implicitly specified in the scalable bitstream 21 (for example, when the number of layers has not changed from the previous frame to the current frame).

[000242] With reference below to the example of Fig. 15B, the audio decoding device 24 can obtain a current frame from the scalable bitstream 21 (530). The current frame can include one or more layers, each of which can include one or more channels. The channels may comprise coded ambient HOA coefficients 59, coded nFG signals 61 (and corresponding sideband in the form of coded foreground V-vectors 57) or both coded ambient HOA coefficient 59 and coded nFG signals 61 (and corresponding sideband in the form of coded foreground V-vectors 57).

[000243] The extraction unit 72 of the audio decoding device 24 can then obtain an indication of a number of channels in a layer of the scalable bit stream 21 in the manner described above (532). The bitstream generation unit 42 can obtain the corresponding number of channels from the current layer of the scalable bitstream 21 (534).

[000244] The extraction unit 72 can then determine if the current layer (eg the counter) is greater than a number of layers (536). That is, in the example of Figure 15B, the number of layers can be static or fixed (rather than specified in the scalable bit stream 21), while the number of channels per layer can be specified, unlike the example in Figure 15A, where the number of channels can be static or fixed and unsignaled. The extraction unit 72 can still keep the counter indicative of the current layer.

[000245] When the current layer (as indicated by the counter) is not greater than the number of layers ("NO" 536), the extraction unit 72 can obtain another indication of the number of channels in another layer of the scalable bit stream 21 for the now current layer (which has changed due to the increment of the counter) (532). The extraction unit 72 may also specify the corresponding number of channels in the further layer of the bit stream 21 (514). The extraction unit 72 can continue in this way until the current layer is greater than the number of layers ("SIM" 516). When the current layer is greater than the number of layers ("YES" 516), the bitstream generation unit can proceed to the next frame with the current frame becoming the previous frame and obtain the channels for the now current frame from the scalable bitstream 21 (510). The process can continue until reaching the last frame of HOA 11 coefficients (510-516).

[000246] As noted above, in some examples, the indication of the number of channels may not be explicitly stated, but implicitly specified in scalable bitstream 21 (for example, when the number of layers has not changed from the previous frame to the current frame). Furthermore, although described as separate processes, the techniques described with respect to Figures 15A and 15B can be performed in combination in the manner described above.

[000247] Figure 16 is a diagram illustrating scalable audio coding as performed by the bitstream generation unit 42 shown in the example of Figure 16 in accordance with various aspects of the techniques described in this disclosure. In the example of Figure 16, a HOA audio encoder, such as the audio coding device 20 shown in the examples of Figures 2 and 3, can encode 11 HOA coefficients (which may also be referred to as a "11 HOA signal"). The HOA 11 signal can comprise 24 channels, each channel having 1024 samples. As noted above, each channel includes 1024 samples, which can refer to 1024 HOA coefficients corresponding to one of the spherical basis functions. The audio coding device 20 can, as described above with respect to the bit stream generation unit 42 shown in the example of figure 5, perform various operations to obtain the encoded ambient HOA coefficients 59 (which may also be referred to as the "background HOA channels 59") from the HOA signal 11.

[000248] As further shown in the example of figure 16, the audio coding device 20 obtains the background HOA channels 59 as the first frame channels of the HOA signal 11. The background HOA channels 50 are indicated as where 1:4 reflects that the first four channels of the HOA 11 signal have been selected to represent the background components of the sound field. This channel selection can be flagged as B=4 in a syntax element. The scalable bitstream generation unit 1000 of the audio encoding device 20 can then specify the HOA background channels 59 in the base layer 21A (which may be referred to as a first layer of two or more layers).

[000249] The scalable bitstream generation unit 1000 can generate the base layer 21A to include the background channels 59 and gain information as specified according to the following equation:

[000250] As further shown in the example of figure 16, the audio coding device 20 can obtain F foreground HOA channels, which can be expressed as the US audio objects and the corresponding V-vector. It is assumed for illustration purposes F=2. The audio coding device 20 can therefore select the first and second US audio objects 61 (which may also be referred to as the "encoded nFG signals 61") and the first and second V-vectors 57 (which may also be referred to as the "foreground V[l] vectors 57"), where the selection is indicated in the example of Figure 5 as US1:2 and V1:2, respectively. Scalable bitstream generation unit 1000 may then generate second layer 21B of scalable bitstream 21 to include first and second US audio objects 61 and first and second V-vectors 57.

[000251] The scalable bitstream generation unit 1000 can also generate the enhancement layer 21B to include the foreground HOA channels 61 and gain information along with the V-vectors 57 as specified according to the following equation:

[000252] To obtain the HOA coefficients 11' from the scalable bit stream 21', the audio decoding device 24 shown in the examples of figure 2 and 3 can invoke the extraction unit 72 shown in more detail in the example of figure 6. The extraction unit 72 that can extract the encoded ambient HOA coefficients 59A-59D, the encoded nFG signals 61A and 61B, and 6. The extraction unit 72 may then transmit the coded ambient HOA coefficients 59A-59D, the coded nFG signals 61A and 61B, and the coded foreground V[k] vectors 57A and 57B to the vector-based decoding unit 92.

[000253] The vector-based decoding unit 92 can then multiply the US audio objects 61 by the V-vectors 57 according to the following equations: The first equation gives the mathematical expression of the generic operation with respect to F. The second equation gives the mathematical expression in the example where F is assumed equal to two. The result of this multiplication is indicated as the foreground HOA signal 1020. The vector-based decoding unit 92 then selects the highest channels (given that the four lowest coefficients have already been selected as the background HOA channels 59), where these highest channels are denoted rj FG,1:2 "5:25" as The vector based decoding unit 92 in other words obtains the foreground HOA channels 65 from the foreground HOA signal 1020.

[000254] As a result, the techniques can facilitate variable layering (as opposed to requiring a static number of layers) to accommodate a large number of encoding contexts and potentially provide much more flexibility in specifying the background and foreground components of the sound field. The techniques can provide many other use cases, as described with reference to Figures 17-26. These various use cases can be executed separately or together on a given audio stream. Furthermore, the flexibility in specifying these components in scalable audio coding techniques can allow for many more use cases. In other words, the techniques should not be limited to the use cases described below, but can include any way in which background and foreground components can be signaled in one or more layers of a scalable bitstream.

[000255] Figure 17 is a conceptual diagram of an example where the syntax elements indicate that there are two layers with four encoded ambient HOA coefficients specified in a base layer and two encoded nFG signals are specified in the enhancement layer. The example of Figure 17 shows the HOA frame how the scalable bitstream generation unit 1000 shown in the example of Figure 5 can segment the frame to form the base layer including sideband HOA gain correction data for the encoded ambient HOA coefficients 59A-59D. The scalable bitstream generation unit 1000 may also segment the HOA frame from an enhancement layer 21 that includes the two encoded foreground V[k] vectors 57 and the HOA gain correction data for the encoded ambient nFG signals 61.

[000256] As further shown in the example of Figure 17, the psychoacoustic audio coding unit 40 is shown as divided into separate instances of psychoacoustic audio coder 40A, which may be referred to as base layer temporal encoders 40A, and psychoacoustic audio encoders 40B, which may be referred to as enhancement layer temporal encoders 40B. Base layer temporal encoders 40A represent four instantiations of psychoacoustic audio encoders that process the four base layer components. The enhancement layer temporal encoders 40B represent two instantiations of psychoacoustic audio encoders that process the two enhancement layer components.

[000257] Figure 18 is a diagram illustrating, in more detail, the bitstream generation unit 42 of Figure 3 when configured to perform a second of the potential versions of the scalable audio coding techniques described in that disclosure. In this example, the bitstream generation unit 42 is substantially similar to the bitstream generation unit 42 described above with respect to the example of Figure 5. However, the bitstream generation unit 42 performs the second version of scalable coding techniques to specify three layers 21A-21C instead of two layers 21A and 21B. The scalable bitstream generating unit 1000 can specify indications that two ambient HOA coefficients coded and zero encoded nFG signals are specified in the base layer 21A, indications that ambient HOA coefficients coded zero and two encoded nFG signals are specified in a first enhancement layer 21B, and indications that ambient HOA coefficients encoded zero and two encoded nFG signals 61 are specified in a second enhancement layer 21C. the scalable bitstream generation unit 1000 can then specify the two encoded ambient HOA coefficients 59A and 59B in the base layer 21A, the two encoded nFG signals 61A and 61B with the two encoded foreground V[k] vectors 57A and 57B in the first enhancement layer 21B, and the two encoded nFG signals 61C and 61D with the two V[k] vectors coded foreground images 57C and 57D on the second enhancement layer 21C. The scalable bitstream generation unit 1000 can then transmit these layers as scalable bitstream 21.

[000258] Figure 19 is a diagram illustrating, in more detail, the extraction unit 72 of Figure 3 when configured to perform the second of the potential versions of the scalable audio decoding techniques described in this disclosure. In this example, the bitstream extraction unit 72 is substantially similar to the bitstream extraction unit 72 described above with respect to the example of Figure 6. However, the bitstream extraction unit 72 performs the second version of scalable coding techniques with respect to three layers 21A-21C instead of two layers 21A and 21B. the scalable bitstream extraction unit 1012 can obtain indications that two coded ambient HOA coefficients and zero coded nFG signals are specified in the base layer 21A, indications that zero coded ambient HOA coefficients and two coded nFG signals are specified in a first enhancement layer 21B, and indications that zero coded ambient HOA coefficients and two encoded nFG signals are specified in a second enhancement layer 21C. The scalable bitstream extraction unit 1012 can then obtain the two encoded ambient HOA coefficients 59A and 59B from the base layer 21A, the two encoded nFG signals 61A and 61B with the two encoded foreground V[k] vectors 57A and 57B from the first enhancement layer 21B, and the two encoded nFG signals 61C and 61D with the two vectors Coded foreground V[k] 57C and 57D from the second enhancement layer 21C. the scalable bitstream extraction unit 1012 can transmit the 59 coded ambient HOA coefficients, the 61 coded nFG signals, and the 57 coded foreground V[k] vectors 57 coded nFG signals 61 coded nFG signals 61 61 coded nFG signals to the vector based decoding unit 92.

[000259] Figure 20 is a diagram illustrating a second use case by which the bitstream generation unit of Figure 18 and the extraction unit of Figure 19 can perform the second of potential versions of the techniques described in this disclosure. For example, the bitstream and generation unit 42 shown in the example of Figure 18 may specify the syntax element NumLayer (which is shown as "NumberOfLayers" for ease of understanding) to indicate the number of layers specified in the scalable bitstream 21 is three. The bitstream generation unit 42 can further specify that the number of background channels specified in the first layer 21A (which is also referred to as the "base layer") is two while the number of foreground channels specified in the first layer 21B is zero (i.e., B1=2, F1=0 in the example of Fig. 20). The bit stream generation unit 42 can further specify that the number of background channels specified in the second layer 21B (which is also referred to as the "enhancement layer") is zero while the number of foreground channels specified in the second layer 21B is two (i.e. B2=0, F2=2 in the example of Figure 20). The bitstream generation unit 42 can further specify that the number of background channels specified in the second layer 21C (which is also referred to as the "enhancement layer") is zero while the number of foreground channels specified in the second layer 21C is two (i.e., B3=0, F3=2 in the example of Fig. 20). However, the audio encoding device 20 may not necessarily signal the foreground and background channel information of the third layer when the total number of foreground and background channels are already known in the decoder (e.g., through additional syntax elements such as totalNumBGchannels and totalNumFGchannels).

[000260] Bitstream generation unit 42 can specify these Bi and Fi values as NumBGchannels[i] and NumFGchannels[i]. For the example above, the audio encoding device 20 can specify the NumBGchannels syntax element as {2, 0, 0} and the NumFGchannels syntax element as {0, 2, 2}. Bitstream generation unit 42 may also specify background HOA audio channels 59, foreground HOA channels 61 and V-vectors 57 in scalable bitstream 21.

[000261] The audio decoding device 24 shown in the examples of figures 2 and 4 can operate in a reciprocal mode to that of the audio coding device 20 to parse these syntax elements from the bitstream (for example, as set out in the HOADecoderConfig syntax table), as described above with respect to the bitstream extraction unit 72 of figure 19. The audio decoding device 24 can also parse the corresponding background HOA audio channels 1002 and foreground HOA channels 1010 from bitstream 21 according to the parsed syntax elements, again as described above with respect to bitstream extraction unit 72 of Fig. 19.

[000262] Figure 21 is a conceptual diagram of an example where the syntax elements indicate that there are three layers with two encoded ambient HOA coefficients specified in a base layer, two encoded nFG signals are specified in a first enhancement layer, and two encoded nFG signals are specified in a second enhancement layer. The example of Figure 21 shows the HOA frame how the scalable bitstream generation unit 1000 shown in the example of Figure 18 can segment the frame to form the base layer including sideband HOA gain correction data for the encoded ambient HOA coefficients 59A and 59B. The 1000 Scalpable Bit Flow Generation Unit can also segment the Hoa Frame of a 21B Improvement Layer that includes the two coded forefront V [K] vectors and HAA gain correction data for coded NFG signals 61 and one 21C improvement layer that includes the two additional coded V [k] vectors 57 and the hoa gain correction data FG coded environment 61.

[000263] As further shown in the example of figure 21, the psychoacoustic audio coding unit 40 is shown as divided into separate instances of psychoacoustic audio coder 40A, which may be referred to as base layer temporal encoders 40A, and psychoacoustic audio encoders 40B, which may be referred to as enhancement layer temporal encoders 40B. Base layer temporal encoders 40A represent two instances of psychoacoustic audio encoders that process the four base layer components. The enhancement layer temporal encoders 40B represent four instantiations of psychoacoustic audio encoders that process the two enhancement layer components.

[000264] Figure 22 is a diagram illustrating, in more detail, the bitstream generation unit 42 of Figure 3 when configured to perform a third of the potential versions of the scalable audio coding techniques described in that disclosure. In this example, the bitstream generation unit 42 is substantially similar to the bitstream generation unit 42 described above with respect to the example of Figure 18. However, the bitstream generation unit 42 performs the third version of scalable coding techniques to specify three layers 21A-21C instead of two layers 21A and 21B. Furthermore, the scalable bitstream generation unit 1000 can specify indications that ambient HOA coefficients encoded in zero and two encoded nFG signals are specified in the base layer 21A, indications that ambient HOA coefficients encoded in zero and two encoded nFG signals are specified in a first enhancement layer 21B, and indications that ambient HOA coefficients encoded in zero and two encoded nFG signals are specified in a second enhancement layer 21C. The scalable bitstream generation unit 1000 can then specify the two encoded nFG signals 61A and 61B with the two encoded foreground V[k] vectors 57A and 57B in the base layer 21A, and two encoded nFG signals 61C and 61D with the corresponding encoded foreground V[k] vectors 57C and 57D in the first enhancement layer 21B, and the two n signals FG encoded 61E and 61F with the two corresponding V[k] encoded vectors 57E and 57F in the second enhancement layer 21C. The scalable bitstream generation unit 1000 can then transmit these layers as scalable bitstream 21.

[000265] Figure 23 is a diagram illustrating in more detail, the extraction unit 72 of Figure 4 when configured to perform the third of the potential versions of the scalable audio decoding techniques described in this disclosure. In this example, the bitstream extraction unit 72 is substantially similar to the bitstream extraction unit 72 described above with respect to the example of Fig. 19 . However, the bitstream extraction unit 72 performs the third version of scalable coding techniques with respect to three layers 21A-21C instead of two layers 21A and 21B. Furthermore, the scalable bitstream extraction unit 1012 can obtain indications that ambient HOA coefficients encoded in zero and two encoded nFG signals are specified in the base layer 21A, indications that ambient HOA coefficients encoded in zero and two encoded nFG signals are specified in a first enhancement layer 21B, and indications that ambient HOA coefficients encoded in zero and two encoded nFG signals are specified in a second enhancement layer 21C. the scalable bitstream extraction unit 1012 can then obtain the two coded nFG signals 61A and 61B with the two corresponding coded foreground vectors V[k] 57A and 57N from the base layer 21A, the two coded nFG signals 61C and 61D with the two corresponding coded foreground V[k] vectors 57C and 57D from the first enhancement layer 2 1B, and the two encoded nFG signals 61E and 61F with the two corresponding encoded V[k] vectors 57E and 57F from the second enhancement layer 21C. The scalable bitstream extraction unit 1012 may transmit the encoded nFG signals 61 and the encoded foreground V[k] vectors 57 to the vector based decoding unit 92.

[000266] Figure 24 is a diagram illustrating a third use case whereby an audio encoding device can specify multiple layers in a multilayer bit stream in accordance with the techniques described in this disclosure. For example, the bitstream generation unit 42 of Fig. 22 may specify the NumLayer syntax element (which is shown as "NumberOfLayers" for ease of understanding) to indicate that the number of layers specified in the bitstream 21 is three. The bitstream generation unit 42 can further specify that the number of background channels specified in the first layer (which is also referred to as the "base layer") is zero while the number of foreground channels specified in the first layer is two (i.e., B1 = 0, F1 = 2 in the example of Fig. 24 ). In other words, the base layer not only provides transport of ambient HOA coefficients, but can allow specification of predominant, or in other words, foreground, HOA audio signals.

[000267] These two foreground audio channels are indicated as the 61A/B encoded nFG signals and the 57A/B encoded foreground V[k] vectors and can be mathematically represented by the following equation: O indicates the two foreground audio channels, which can be represented by the first and second audio objects (US1 and US2) along with the corresponding V-vectors (V1 and V2).

[000268] The bitstream generation device 42 can further specify that the number of background channels specified in the second layer (which is also referred to as the "enhancement layer") is zero while the number of foreground channels specified in the second layer is two (i.e., B2 = 0, F2 = 2) in the example of Fig. 24 ). These two foreground audio channels are denoted as the 61C/D encoded nFG signals and the 57C/D encoded foreground V[k] vectors and can be mathematically represented by the following equation: O indicates the two foreground audio channels, which can be represented by the third and fourth audio objects (US3 and US4) along with the corresponding V-vectors (V3 and V4).

[000269] Furthermore, the bitstream generation unit 42 can specify that the number of background channels specified in the third layer (which is also referred to as the "enhancement layer") is zero while the number of foreground channels specified in the third layer is two (i.e., B3 = 0, F3 = 2 in the example of Figure 24). These two foreground audio channels are denoted as foreground audio channels 1024 and can be mathematically represented by the following equation: O indicates the two foreground audio channels 1024, which can be represented by the fifth and sixth audio objects (US5 and US6) along with the corresponding V-vectors (V5 and V6). However, the bitstream generation unit 42 may not necessarily signal such third-layer foreground and background channel information when the total number of foreground and background channels are already known in the decoder (e.g., via additional syntax elements such as totalNumBGchannels and totalNumFGchannels). The bitstream generation unit 42 may, however, not signal the third-layer background and foreground channel information when the total number of foreground and background channels are already known in the decoder (e.g., via additional syntax elements such as totalNumBGchannels and totalNumFGchannels).

[000270] Bitstream generation unit 42 can specify these Bi and Fi values as NumBGchannels[i] and NumFGchannels[i]. For the example above, the audio encoding device 20 can specify the NumBGchannels syntax element as {0, 0, 0} and the NumFGchannels syntax element as {2, 2, 2}. Audio encoding device 20 may also specify foreground HOA channels 1020-1024 in bitstream 21.

[000271] The audio decoding device 24 shown in the examples of figures 2 and 4 can operate in a reciprocal mode to that of the audio coding device 20 to parse, as described above with respect to the bitstream extraction unit 72 of figure 23, these syntax elements from the bitstream (for example, as set out in the HOADecoderConfig syntax table above). The audio decoding device 24 can also parse, again as described above with respect to the bitstream extraction unit 72 of Fig. 23, the corresponding foreground HOA audio channels 10201024 from the bitstream 21 according to the parsed syntax elements and reconstruct HOA coefficients 1026 by summing the foreground HOA audio channels 1020-1024.

[000272] Figure 25 is a conceptual diagram of an example where the syntax elements indicate that there are three layers with two encoded nFG signals specified in a base layer, two encoded nFG signals are specified in a first enhancement layer, and two encoded nFG signals are specified in a second enhancement layer. Example ad Figure 25 shows the HOA frame how the scalable bitstream generation unit 1000 shown in the example of Figure 22 can segment the frame to form the base layer by including sideband HOA gain correction data for encoded nFG signals 61A and 61B and two foreground encoded V[k] vectors 57. The scalable bitstream generation unit 1000 can also segment the HOA frame to form a layer of enhancement 21B that includes the two encoded foreground V[k] vectors 57 and the HOA gain correction data for the encoded ambient Nfg signals 61 and an enhancement layer 21C that includes the additional two encoded foreground V[k] vectors 57 and the HOA gain correction data for the encoded ambient nFG signals 61.

[000273] As further shown in the example of Fig. 25, the psychoacoustic audio coding unit 40 is shown as divided into separate instances of psychoacoustic audio coder 40A, which may be referred to as base layer temporal encoders 40A, and psychoacoustic audio encoders 40B, which may be referred to as enhancement layer temporal encoders 40B. Base layer temporal encoders 40A represent two instances of psychoacoustic audio encoders that process the four base layer components. The enhancement layer temporal encoders 40B represent four instantiations of psychoacoustic audio encoders that process the two enhancement layer components.

[000274] Figure 26 is a diagram illustrating a third use case whereby an audio encoding device can specify multiple layers in a multilayer bit stream in accordance with the techniques described in this disclosure. For example, the audio encoding device 20 shown in the example of figures 2 and 3 may specify the syntax element NumLayer (which is shown as "NumberOfLayers" for ease of understanding) to indicate that the number of layers specified in the bit stream 21 is four. The audio encoding device 20 can further specify that the number of background channels specified in the first layer (which is also referred to as the "base layer" is one while the number of foreground channels specified in the first layer is zero (i.e., B1 = 1, F1 = 0 in the example of Fig. 26 ).

[000275] The audio coding device 20 can further specify that the number of background channels specified in the second layer (which is also referred to as a "first layer of enhancement") is one while the number of background channels specified in the second layer is zero (i.e. B2 = 1, F2 = 0 in the example of Figure 26). The audio encoding device 20 can also specify that the number of background channels specified in the third layer (which is also referred to as a "second enhancement layer") is one while the number of foreground channels specified in the third layer is zero (i.e., B3=1, F3=0 in the example of Fig. 26). Furthermore, the audio encoding device 20 can specify that the number of background channels specified in the fourth layer (which is also referred to as the "enhancement layer") is one while the number of foreground channels specified in the third layer is zero (i.e., B4=1, F4=0 in the example of Fig. 26). However, the audio encoding device 20 may not necessarily signal the background and foreground channel information of the fourth layer when the total number of foreground and background channels are already known in the decoder (e.g., through additional syntax elements such as numBGchannels and totalNumFGchannels).

[000276] Audio encoding device 20 can specify these Bi and Fi values as NumBGchannels[i] and NumFGchannels[i]. For the example above, the audio encoding device 20 can specify the NumBGchannels syntax element as {1, 1, 1, 1} and the NumFGchnanels syntax element as {0, 0, 0, 0}. Audio encoding device 20 may also specify background HOA audio channels 1030 in bitstream 21. In this regard, techniques may allow enhancement layers to specify ambience, or in other words background HOA channels 1030, which may have been decorrelated prior to being specified in base and enhancement layers of bitstream 21 as described above with respect to the examples of Figures 7A-9B. However, again, the techniques disclosed in this disclosure are not necessarily limited to decorrelation and may not provide syntax elements or any other indications in the relevant bitstream for decorrelation as described above.

[000277] The audio decoding device 24 shown in the examples of figures 2 and 4 can operate in a reciprocal mode to that of the audio coding device 20 to parse these syntax elements from the bit stream (for example, as exposed in the HOADecoderConfig syntax table). The audio decoding device 24 can also parse the corresponding background HOA audio channels 1030 from the bitstream 21 according to the parsed syntax elements.

[000278] As noted above, in some instances, the scalable bitstream 21 may include multiple layers that conform to the non-scalable bitstream 21. For example, the scalable bitstream 21 may include a base layer that conforms to the non-scalable bitstream 21. In these instances, the non-scalable bitstream 21 may represent a sub-bitstream of the scalable bitstream 21, where that sub-stream Unscalable bitstream 21 can be enhanced with additional layers of scalable bitstream 21 (which are referred to as enhancement layers).

[000279] Figures 27 and 28 are block diagrams illustrating a scalable bitstream generation unit 42 and a scalable bitstream extraction unit 72 that can be configured to perform various aspects of the techniques described in this disclosure. In the example of Fig. 27, the scalable bitstream generation unit 42 can represent an example of the bitstream generation unit 42 described above with respect to the example of Fig. 3. The scalable bitstream generation unit 42 can transmit a base layer 21 that conforms (in terms of syntax and ability to be decoded by audio decoders that do not support scalable coding) to a non-scalable bitstream 21. The bitstream generation unit The scalable bitstream generation unit 42 may operate in modes described above with respect to any of the above bitstream generation units 42 except that the scalable bitstream generation unit 42 does not include a non-scalable bitstream generation unit 1002. Rather, the scalable bitstream generation unit 42 transmits a base layer 21 that conforms to a non-scalable bitstream and as such does not require a separate non-scalable bitstream generation unit 1 000. In the example of Fig. 28, the scalable bitstream extraction unit 72 can operate reciprocally with the scalable bitstream generation unit 42.

[000280] Figure 29 is a conceptual diagram representing an encoder 900 that can be configured to operate in accordance with various aspects of the techniques described in this disclosure. Encoder 900 may represent another example of audio coding device 20. Encoder 900 may include a spatial decomposition unit 902, a decorrelation unit 904, and a temporal coding unit 906. The spatial decomposition unit 902 may represent a unit configured to transmit the vector-based predominant sounds (in the form of the aforementioned audio objects), the corresponding V-vectors associated with those vector-based predominant sounds, and horizontal ambient HOA coefficients 903. The spatial decomposition unit 902 may differ from a directional-based decomposition in that the V-vectors describe both the direction and the corresponding object width of the audio objects as each audio object moves over time in the sound field.

[000281] The spatial decomposition unit 902 may include units 30-38 and 44-52 of the vector-based synthesis unit 27 shown in the example of Figure 3 and operates, in general, in the manner described above with respect to unit 30-38 and 44-52. The spatial decomposition unit 902 may differ from the vector-based synthesis unit 27 in that the spatial decomposition unit 902 may not perform psychoacoustic coding or otherwise include a psychoacoustic encoder unit 40 and may not include a bitstream generation unit 42. Furthermore, in the context of scalable audio coding, the spatial decomposition unit 902 may pass through horizontal ambient HOA coefficients 903 (meaning, in some examples, that these coefficients s Horizontal HOA may not be modified or otherwise adjusted and are analyzed from HOA 901 coefficients).

[000282] Horizontal ambient HOA coefficients 903 may refer to any of the HOA coefficients 901 (which may also be referred to as HOA audio data 901) that describe a horizontal component of the sound field. For example, horizontal ambient HOA coefficients 903 can include HOA coefficients associated with a spherical basis function having an order of zero and a suborder of zero, higher order ambisonic coefficients corresponding to a spherical basis function having an order of one and a suborder of negative one, and third higher order ambisonic coefficients corresponding to a spherical basis function having an order of one and a suborder of one.

[000283] The decorrelation unit 904 represents a unit configured to perform decorrelation with respect to a first layer of two or more layers of the higher order ambisonic audio data 903 (where ambient HOA coefficients 903 are an example of such HOA audio data) to obtain a decorrelated representation 905 of the first layer of two or more layers of the higher order ambisonic audio data. Base layer 903 may be similar to any of the first layers, base layers, or subbase layers described above with respect to Figures 21-26. The decorrelation unit 904 can perform decorrelation using the aforementioned UHJ matrix or the mode matrix. Decorrelation unit 904 may also perform decorrelation using a transformation, such as rotation, in a similar manner to that described in US application no. Serial 14/192,829, entitled “TRANSFORMING SPHERICAL HARMONIC COEFFICIENTS,” filed February 27, 2014, except that rotation is performed to obtain a decorrelated representation of the first layer rather than reducing the number of coefficients.

[000284] In other words, the decorrelation unit 904 can perform a rotation of the sound field to align energy of the ambient HOA coefficients 903 along three different horizontal axes separated by 120 degrees (such as 0 degrees azimuthal / 0 degrees elevation, 120 degrees azimuthal / 0 degrees elevation, and 240 degrees azimuth / 0 degrees elevation). By aligning these energies with the three horizontal axes, the decorrelation unit 904 can attempt to decorrelate the energies with each other so that the decorrelation unit 904 can utilize a spatial transformation to effectively render three channels of decorrelation audio 905. The decorrelation unit 904 can apply this spatial transformation in order to compute the spatial audio signals 905 at azimuth angles of 0 degrees, 120 degrees, and 240 degrees.

[000285] Although described with reference to azimuth angles of 0 degrees, 120 degrees, and 240 degrees, the techniques can be applied with respect to any three azimuthal angles that evenly or nearly evenly divide the 360 degrees of azimuth of the circle. For example, the techniques can also be performed against a transform that computes the 905 spatial audio signals at azimuth angles of 60 degrees, 180 degrees, and 300 degrees. Furthermore, although described with respect to three ambient HOA coefficients 901, the techniques may be performed more generally with respect to any horizontal HOA coefficients, including those as described above and any other horizontal HOA coefficients, such as those associated with a spherical basis function having an order of two and suborder of two, a spherical basis function having an order of two and a suborder of negative two, ..., a spherical basis function having an order of X and a suborder of X, and a spherical basis function having an order of X and a suborder of negative X, where X can represent any number including 3, 4, 5, 6, etc.

[000286] As the number of horizontal HOA coefficients increases, the number of uniform or nearly uniform portions of the 360 degree circle may increase. For example, when the number of horizontal HOA coefficients increases to five, the decorrelation unit 904 can segment the circle into five even divisions (eg, of approximately 72 degrees each). The number of horizontal HOA coefficients of X can, as another example, result in X uniform divisions with each division being 360 degrees / X degrees.

[000287] The decorrelation unit 904 may, in order to identify the rotation information indicative of the amount by which to rotate the sound field represented by the horizontal ambient HOA coefficients 903, perform a sound field analysis, content feature analysis, and/or spatial analysis. Based on one or more of these analyses, the decorrelation unit 904 can identify the rotation information (or other transformation information of which the rotation information is an example) as a number of degrees by which to horizontally rotate the sound field, and rotate the sound field, effectively obtaining a rotated representation (which is an example of the more general transformed representation) of the base layer of higher order ambisonic audio data.

[000288] The decorrelation unit 904 can then apply a spatial transform to the rotated representation of the base layer 903 (which may also be referred to as a first layer 903 of two or more layers) of the higher order ambisonic audio data. The spatial transform can convert the rotated representation of the base layer of the two or more layers of the higher order ambisonic audio data from a spherical harmonic domain to a spatial domain to obtain a decorrelated representation of the first layer of the two or more layers of the higher order ambisonic audio data. The decorrelation representation of the first layer may include spatial audio signals 905 rendered at the three corresponding azimuth angles of 0 degrees, 120 degrees and 240 degrees as mentioned above. The decorrelation unit 904 can then pass the horizontal ambient spatial audio signals 905 to the temporal coding unit 906.

[000289] Temporal decoding unit 906 may represent a unit configured to perform psychoacoustic audio encoding. Temporal coding unit 906 can represent an AAC encoder or a unified speech and audio (USAC) encoder to provide two examples. Temporal audio coding units such as temporal coding unit 906 can normally operate against decorrelated audio data such as the 6 channels of a 5.1 speaker setup, those 6 channels having been rendered to decorrelated channels. However, the horizontal ambient HOA coefficients 903 are additive in nature and thus correlate to some extent. Supplying these horizontal ambient HOA coefficients 903 directly to the temporal coding unit 906 without first performing some form of decorrelation can result in unmasking spatial noise in which sounds appear in places they were not intended. These perceptual artifacts, such as spatial noise unmasking, can be reduced by performing the transform-based (or, more specifically, rotation-based in the example in Figure 29) decorrelation described above.

[000290] Figure 30 is a diagram illustrating the encoder 900 shown in the example of Figure 27 in more detail. In the example of Fig. 30, encoder 900 may represent a base layer encoder 900 that encodes the first order HOA horizontal only base layer 903 and does not show spatial decomposition unit 902 since such unit 902 does not, in this passing example, perform significant operations other than providing the base layer 903 to a sound field analysis unit 910 and a two-dimensional (2D) rotation unit of decorrelation unit 904.

[000291] That is, the decorrelation unit 904 includes the sound field analysis unit 910 and the 2D rotation unit 912. The sound field analysis unit 910 represents a unit configured to perform the above-described sound field analysis in more detail to obtain a rotation angle parameter 911. The rotation angle parameter 911 represents an example of transformation information in the form of rotation information. The 2D rotation unit 912 represents a unit configured to perform a horizontal rotation about the Z-axis of the sound field based on the rotation angle parameter 911. This rotation is two-dimensional in that the rotation only involves a single rotation axis and does not include any, in this example, elevation rotation. 2D rotation unit 912 may obtain reverse rotation information 913 (by inverting, as an example, rotation angle parameter 911 to obtain reverse rotation angle parameter 913), which may be an example of more general reverse transformation information. The 2D rotation unit 912 may provide the reverse rotation angle parameter 913 so that the encoder 900 can specify the reverse rotation angle parameter 913 in the bit stream.

[000292] In other words, the 2D rotation unit 912 can, based on the sound field analysis, rotate the 2D sound field so that the predominant energy is potentially arriving from one of the spatial sampling points used in the 2D spatial transform module (0°, 120°, 240°). The 2D rotation unit 912 can, as an example, apply the following rotation matrix: - 1 0 0 In some instances, the 2D rotation unit 912 may, to avoid frame artifacts, apply a smoothing (interpolation) function to ensure a smooth transition of the time varying rotation angle. Such a smoothing function may comprise a linear smoothing function. However, other smoothing functions including non-linear smoothing functions can be used. The 2D rotation unit 912 can, for example, use a groove smoothing function.

[000293] To illustrate, when the sound field analysis unit module 910 indicates that the dominant sound field direction is at 70° azimuth in an analysis frame, the 2D rotation unit 912 can smoothly rotate the sound field by Φ = -70° so that the dominant direction is now 0°. As another possibility, the 2D rotation unit 912 can rotate the sound field by Φ = 50°, so that the dominant direction is now 120°. The 2D rotation unit 912 can then signal the applied rotation angle 913 as an additional sideband parameter in the bitstream, so that a decoder can apply the correct reverse rotation operation.

[000294] As further shown in the example of Figure 30, the decorrelation unit 904 also includes a 2D spatial transformation unit 914. The 2D spatial transformation unit 914 represents a unit configured to convert the rotated representation of the base layer from the spherical harmonic domain to the spatial domain, effectively rendering the rotated base layer 915 for the three azimuth angles (for example, 0, 120 and 240 ). The 2D spatial transformation unit 914 may multiply the rotated base layer coefficients 915 with the following transformation matrix, which assumes HOA coefficient order '00+', '11-', '11+' and N3D normalization: The above matrix computes the 905 spatial audio signals at azimuth angles 0°, 120° and 240°, so that the 360° circle is uniformly divided into 3 portions. As noted above, other separations are possible as long as each portion covers 120 degrees, for example computing the spatial signals at 60°, 180° and 300°.

[000295] Thus, the art can provide a device 900 configured to perform scalable higher order ambisonic audio data encoding. Device 900 may be configured to perform decorrelation with respect to a first layer 903 of two or more layers of the higher order ambisonic audio data to obtain a decorrelated representation 905 of the first layer of the two or more layers of the higher order ambisonic audio data.

[000296] In these and other instances, the first layer 903 of two or more layers of the higher order ambisonic audio data comprises ambient higher order ambisonic coefficients corresponding to one or more spherical basic functions having an order equal to or less than one. In these and other instances the first layer 903 of two or more layers of the higher order ambisonic audio data comprises ambient higher order ambisonic coefficients corresponding only to basic spherical functions descriptive of horizontal aspects of the sound field. In these and other instances, ambient higher order ambisonic coefficients corresponding only to spherical basis functions descriptive of horizontal aspects of the sound field may comprise first ambient higher order ambisonic coefficients corresponding to a spherical basis function having an order of zero and a suborder of zero, second higher order ambisonic coefficients corresponding to a spherical basis function having an order of one and a suborder of negative one, and third order ambisonic coefficients of negative one. upper corresponding to a spherical basis function having an order of one and a suborder of one.

[000297] In these and other instances, the device 900 may be configured to perform a transformation (eg, via the 2D rotation unit 912) with respect to the first layer 903 of the higher order ambisonic audio data.

[000298] In these and other instances, the device 900 may be configured to perform a rotation (eg, via the 2D rotation unit 912) with respect to the first layer 903 of the higher order ambisonic audio data.

[000299] In these and other instances, the device 900 can be configured to apply a transformation (e.g., via the 2D rotation unit 912) with respect to the first layer 903 of two or more layers of the higher-order ambisonic audio data to obtain a transformed representation 915 of the first layer of the two or more layers of the higher-order ambisonic audio data, and converting the transformed representation 915 of the first layer of the two or more layers of the audio data higher order ambisonic domain (e.g., by means of the 2D spatial transformation unit 914) from a spherical harmonic domain to a spatial domain to obtain a decorrelated representation 905 of the first layer of the two or more layers of the higher order ambisonic audio data.

[000300] In these and other instances, the device 900 can be configured to apply a rotation with respect to the first layer 903 of the two or more layers of the higher order ambisonic audio data to obtain a 915 rotated representation of the first layer of the two or more layers of the higher order ambisonic audio data, and convert the 915 rotated representation of the first layer of the two or more layers of the higher order ambisonic audio data from a harmonic domain and pheric to a spatial domain to obtain a decorrelated representation 905 of the first layer of the two or more layers of the higher order ambisonic audio data.

[000301] In these and other instances, the device 900 can be configured to obtain transformation information 911, apply a transformation with respect to the first layer 903 of the two or more layers of the higher order ambisonic audio data based on the transformation information 911 to obtain a transformed representation 915 of the first layer of the two or more layers of the higher order ambisonic audio data, and convert the transformed representation 915 of the first layer of the two or more layers of the higher order ambisonic audio data higher order ambisonic audio from a spherical harmonic domain to a spatial domain to obtain a 905 decorrelated representation of the first layer of the two or more layers of the higher order ambisonic audio data.

[000302] In these and other instances, the device 900 can be configured to obtain rotation information 911, and apply a rotation with respect to the first layer 903 of the two or more layers of the higher order ambisonic audio data based on the rotation information 911 to obtain a rotated representation 915 of the first layer of the two or more layers of the higher order ambisonic audio data, and convert the rotated representation 915 of the first layer of the two or more layers of the higher order ambisonic audio data from a spherical harmonic domain to a spatial domain to obtain a 905 decorrelated representation of the first layer of the two or more layers of the higher order ambisonic audio data.

[000303] In these and other instances, the device 900 can be configured to apply a transform with respect to the first layer 903 of the two or more layers of the higher order ambisonic audio data using at least in part a smoothing function to obtain a transformed representation 915 of the first layer of the two or more layers of the higher order ambisonic audio data, and convert the first layer transformed representation 915 of the two or more layers of the higher order ambisonic audio data to from a spherical harmonic domain to a special domain to obtain a 905 decorrelated representation of the first layer of two or more layers of the higher order ambisonic audio data.

[000304] In these and other instances, the device 900 can be configured to apply a rotation with respect to the first layer 903 of the two or more layers of the higher order ambisonic audio data using at least in part a smoothing function to obtain a rotated representation 915 of the first layer of two or more layers of the higher order ambisonic audio data, and convert the rotated representation 915 of the first layer of the two or more layers of the higher order ambisonic audio data from a spherical harmonic domain to a spatial domain to obtain a decorrelated representation of the first layer of the two or more layers of the higher order ambisonic audio data.

[000305] In these and other instances, device 900 can be configured to specify an indication of the smoothing function to be used when applying an inverse transform or an inverse rotation.

[000306] In these and other instances, the device 900 can be further configured to apply a linear invertible transform to the higher-order ambisonic audio data to obtain a V-vector, and specify the V-vector as a second layer of the two or more layers of the higher-order ambisonic audio data, as described above with respect to Figure 3.

[000307] In these and other instances, the device 900 can be further configured to obtain higher order ambisonic coefficients associated with a spherical basis function having an order of one and a suborder of zero, and specify the higher order ambisonic coefficients as a second layer of the two or more layers of the higher order ambisonic audio data.

[000308] In these and other instances, the device 900 may be further configured to perform temporal encoding with respect to the decorrelated representation of the first layer of the two or more layers of the higher order ambisonic audio data.

[000309] Figure 31 is a block diagram illustrating an audio decoder 920 that can be configured to operate in accordance with various aspects of the techniques described in this disclosure. Decoder 920 may represent another example of the audio decoding device 24 shown in the example of Figure 2 in terms of reconstructing the HOA coefficients, reconstructing V-vectors of the enhancement layers, performing temporal audio decoding (as performed by a temporal audio decoding unit 922), etc. However, decoder 920 differs in that decoder 920 operates with respect to scalable encoded higher order ambisonic audio data as specified in the bit stream.

[000310] As shown in the example of Figure 31, the audio decoder 920 includes a temporal decoding unit 922, an inverse 2D spatial transformation unit 924, a base layer rendering unit 928 and an enhancement layer processing unit 930. The temporal decoding unit 922 can be configured to operate in a reciprocal mode to that of the temporal coding unit 906. 2D inverse 924 may represent a unit configured to operate in a reciprocal mode to that of the 2D spatial transformation unit 914.

[000311] In other words, the inverse 2D spatial transformation unit 924 can be configured to apply the matrix below to the spatial audio signals 905 to obtain the 915 horizontally rotated ambient HOA coefficients (which may also be referred to as "the 915 rotated base layer"). The inverse 2D spatial transformation unit 924 can transform the 3 transmitted audio signals 905 back to the HOA domain using the following transformation matrix, which like the matrix above assumes HOA coefficient order '00+', '11-', '11+' and N3D normalization: The matrix above is the inverse of the transformation matrix used in the decoder.

[000312] The 2D rotation unit 926 can be configured to operate in a reciprocal mode to that described above with respect to the 2D rotation unit 912. In this regard, the 2D rotation unit 912 can perform a rotation according to the rotation matrix mentioned above based on the inverse rotation angle parameter 913 instead of the rotation angle parameter 911. In other words, the reverse rotation unit 926 can, with based on the Φ-signed rotation, applied to the following matrix, which again assumes coefficient order '00+', '11-', '11+' and N3D normalization: The inverse 2D rotation unit 926 can use the same smoothing (interpolation) function used in the decoder to ensure a smooth transition to the time varying rotation angle, which can be signaled in the bit stream or configured a priori.

[000313] Base layer rendering unit 928 may represent a unit configured to render the ambient HOA coefficients horizontally only from the base layer to speaker feeds. Enhancement layer processing unit 930 may represent a unit configured to perform additional base layer processing with any incoming enhancement layers (decoded through a separate enhancement layer decoding path that invokes much of the decoding described above with respect to additional ambient HOA coefficients and the V-vectors along with the audio objects corresponding to the V-vectors) to render speaker feeds. The enhancement layer processing unit 930 can effectively augment the base layer to provide a higher resolution representation of the sound field which can provide a more immersive audio experience having sounds that potentially move realistically in the sound field. The base layer can be similar to any of the first layers, base layers or subbase layers described above with respect to Figures 11-13B. the enhancement layers may be similar to any of the second layers, enhancement layers, or enhancement sub-layers described above with respect to Figures 11-13B.

[000314] In this regard, the techniques provide a device 920 configured to perform scalable higher order ambisonic audio data decoding. The device may be configured to obtain a first layer decorrelated representation of two or more layers of the higher order ambisonic audio data (e.g., spatial audio signals 905), the higher order ambisonic audio data descriptive of a sound field. The decorrelated representation of the first layer is decorrelated by performing decorrelation with respect to the first layer of the higher order ambisonic audio data.

[000315] In some instances, the first layer of two or more layers of higher-order ambisonic audio data comprises ambient higher-order ambisonic coefficients corresponding to one or more spherical basis functions having an order equal to or less than one. In these and other instances, the first layer of the two or more layers of higher order ambisonic audio data comprises ambient higher order ambisonic coefficients corresponding only to spherical basis functions descriptive of horizontal aspects of the sound field. In these and other instances, the ambient higher order ambisonic coefficients corresponding only to spherical basis functions descriptive of the horizontal aspects of the sound field comprise first ambient higher order ambisonic coefficients corresponding to a spherical basis function having an order of zero and a suborder of zero, higher order second ambisonic coefficients corresponding to a spherical basis function having an order of one and a suborder of negative one, and higher order third ambisonic coefficients corresponding to a spherical basis function having an order of one and a suborder of one.

[000316] In these and other instances the decorrelated representation of the first layer is decorrelated by performing a transformation with respect to the first layer of the higher order ambisonic audio data as described above with respect to encoder 900.

[000317] In these and other instances, the device 920 may be configured to perform one rotation (eg, per unit inverse 2D rotation 926) with respect to the first layer of the higher order ambisonic audio data.

[000318] In these and other instances, the device 920 can be configured to re-correlate the decorrelated representation of the first layer of two or more layers of the higher order ambisonic audio data to obtain the first layer of two or more layers of the higher order ambisonic audio data as described above, for example, with respect to the inverse 2D spatial transformation unit 924 and inverse 2D rotation unit 926.

[000319] In these and other instances, the device 920 can be configured to convert the decorrelated representation 905 of the first layer of the two or more layers of the higher order ambisonic audio data from a spatial domain to a spherical harmonic domain to obtain a transformed representation 915 of the first layer of the two or more layers of the higher order ambisonic audio data, and apply an inverse transformation (e.g., as described above with respect to the 2D rotation unit in versa 926) with respect to transforming representation 915 of the first layer of the two or more layers of the higher order ambisonic audio data to obtain the first layer of the two or more layers of the higher order ambisonic audio data.

[000320] In these and other instances, the device 920 can be configured to convert the decorrelated representation 905 of the first layer of the two or more layers of the higher-order ambisonic audio data from a spatial domain to a spherical harmonic domain to obtain a transformed representation 915 of the first layer of the two or more layers of the higher-order ambisonic audio data, and apply an inverse rotation with respect to the transformed representation 915 of the first layer of the two or more layers of the higher order ambisonic audio data to obtain the first layer of the two or more layers of higher order ambisonic audio data.

[000321] In these and other instances, the device 920 can be configured to convert the decorrelated representation 905 of the first layer of the two or more layers of the higher order ambisonic audio data from a spatial domain to a spherical harmonic domain to obtain a transformed representation 915 of the first layer of the two or more layers of the higher order ambisonic audio data, obtain transformation information 913, and apply an inverse transformation with respect to the transformed representation 9 15 of the first layer of the two or more layers of the higher order ambisonic audio data based on the transformation information 913 to obtain the first layer of the two or more layers of the higher order ambisonic audio data.

[000322] In these and other instances, the device 920 can be configured to convert the decorrelated representation 905 of the first layer of the two or more layers of the higher order ambisonic audio data from a spatial domain to a spherical harmonic domain to obtain a transformed representation 915 of the first layer of the two or more layers of the higher order ambisonic audio data, obtain rotation information 913, and apply an inverse rotation with respect to the transformed representation 9 15 of the first layer of the two or more layers of the higher order ambisonic audio data based on the rotation information 913 to obtain the first layer of the two or more layers of the higher order ambisonic audio data.

[000323] In these and other instances, the device 920 can be configured to convert the decorrelated representation 905 of the first layer of the two or more layers of the higher-order ambisonic audio data from a spatial domain to a spherical harmonic domain to obtain a transformed representation 915 of the first layer of the two or more layers of the higher-order ambisonic audio data, and apply an inverse transformation with respect to the transformed representation 915 of the first layer of the two or more layers of the higher order ambisonic audio data using, at least in part, a smoothing function to obtain the first layer of the two or more layers of higher order ambisonic audio data.

[000324] In these and other instances, the device 920 can be configured to convert the decorrelated representation 905 of the first layer of two or more layers of the higher order ambisonic audio data from a spatial domain to a spherical harmonic domain to obtain a transformed representation 915 of the first layer of the two or more layers of the higher order ambisonic audio data, and apply an inverse rotation with respect to the representation 915 of the first layer of the two or more layers of the higher order ambisonic audio data using, at least in part, a smoothing function to obtain the first layer of the two or more layers of the higher order ambisonic audio data.

[000325] In these and other instances, device 920 may be further configured to obtain an indication of the smoothing function to be used when applying the inverse transform or inverse rotation.

[000326] In these and other instances, the device 920 can be further configured to obtain a second layer representation of the two or more layers of the higher order ambisonic audio data, where the second layer representation comprises vector-based predominant audio data, the vector-based predominant audio data comprises at least one predominant audio data and a V-encoded vector, and the V-encoded vector is decomposed from the higher order ambisonic audio data through application of a linear invertible transform, as described above with respect to the example in figure 3.

[000327] In these and other instances, the device 920 may be further configured to obtain a second layer representation of the two or more layers of the higher order ambisonic audio data, where the second layer representation comprises higher order ambisonic coefficients associated with a spherical basis function having an order of one and a suborder of zero.

[000328] In this way, the techniques can enable a device to be configured for, or provide an apparatus comprising means to execute, or non-transient computer-readable media having stored therein instructions that, when executed, cause one or more processors to execute the method set forth in the following clauses.

[000329] Clause 1A. A method of encoding a higher order ambisonic audio signal to generate a bit stream, the method comprising specifying an indication of a number of layers in the bit stream, and transmitting the bit stream including the indicated number of layers.

[000330] Clause 2A. The method of clause 1A, further comprising specifying an indication of a number of channels included in the bit stream.

[000331] Clause 3A. The method of clause 1A, wherein the indication of the number of layers comprises an indication of a number of layers in the bitstream for a previous frame, and the method further comprising specifying, in the bitstream, an indication of whether a number of layers of the bitstream has changed for a current frame when compared to the number of layers of the bitstream for the previous frame, and specifying the indicated number of layers of the bitstream in the current frame.

[000332] Clause 4A. Device of clause 3A, wherein the specification of the indicated number of layers comprises, when the indication indicates that the number of layers of the bitstream has not changed in the current frame when compared to the number of layers of the bitstream in the previous frame, specifying the indicated number of layers without specifying, in the bitstream, an indication of a current number of background components in one or more of the layers for the current frame as being equal to a previous number of background components in one or more of the layers of the previous frame.

[000333] Clause 5A. Method of Clause 1A, wherein the layers are hierarchical such that a first layer, when combined with a second layer, provides a higher resolution representation of the higher order ambisonic audio signal.

[000334] Clause 6A. The method of clause 1A, wherein the bitstream layers comprise a base layer and an enhancement layer, and the method further comprises applying a decorrelation transform with respect to one or more channels of the base layer to obtain a decorrelated representation of background components of the higher order ambisonic audio signal.

[000335] Clause 7A. The method of clause 6A, wherein the decorrelation transform comprises a UHJ transform.

[000336] Clause 8A. The method of clause 6A, wherein the decorrelation transform comprises a mode matrix transform.

[000337] In addition, the techniques may allow a device to be configured for, or provide an apparatus comprising means to execute, or a non-transient computer-readable medium having stored therein instructions that, when executed, cause one or more processors to execute the method set forth in the following clauses.

[000338] Clause 1B. A method of encoding a higher order ambisonic audio signal to generate a bitstream, the method comprising specifying, in the bitstream, an indication of a specified number of channels in one or more layers of the bitstream, and specifying the indicated number of channels in one or more layers of the bitstream.

[000339] Clause 2B. The method of clause 1B, further comprising specifying an indication of a total number of specified channels in the bitstream, wherein specifying the indicated number of channels comprises specifying the indicated total number of channels in one or more layers of the bitstream.

[000340] Clause 3B. The method of clause 1B, further comprising specifying an indication of a type of one of the specified channels in one or more layers in the bitstream, and specifying the indicated number of channels comprises specifying the indicated number of the indicated type of one of the channels in one or more layers of the bitstream.

[000341] Clause 4B. The method of clause 1B, further comprising specifying an indication of a type of one of the specified channels in one or more layers in the bitstream, indicating the type of one of the channels indicating that one of the channels is a foreground channel, and wherein specifying the indicated number of channels comprises specifying the foreground channel in one or more layers of the bitstream.

[000342] Clause 5B. The method of clause 1B, further comprising specifying an indication, in the bitstream, of a specified number of layers in the bitstream.

[000343] Clause 6B. The method of clause 1B, further comprising specifying a type indication of one of the specified channels in one or more layers in the bit stream, indicating the type of one of the channels indicating that one of the channels is a background channel, wherein specifying the indicated number of channels comprises specifying the background channel in one or more layers of the bit stream.

[000344] Clause 7B. The method of clause 6B, wherein one of the channels comprises a background higher order ambisonic coefficient.

[000345] Clause 1B. The method of clause 1B, wherein specifying the indication of the number of channels comprises specifying the indication of the number of channels based on a number of channels remaining in the bit stream after one of the layers is specified.

[000346] In this way, the techniques can allow a device to be configured to or provide an apparatus comprising means to execute, or non-transient computer-readable media having stored therein instructions that, when executed, cause one or more processors to execute the method set forth in the following clauses.

[000347] Clause 1C. A method of decoding a bitstream representative of a higher order ambisonic audio signal, the method comprising obtaining, from the bitstream, an indication of a specified number of layers in the bitstream, and obtaining the layers of the bitstream based on the indication of the number of layers.

[000348] Clause 2C. The method of clause 1C, further comprising obtaining an indication of a specified number of channels in the bitstream, and wherein obtaining the layers comprises obtaining the layers of the bitstream based on the indication of the number of layers and indication of the number of channels.

[000349] Clause 3C. The method of clause 1C, further comprising obtaining an indication of a specified number of foreground channels in the bitstream for at least one of the layers, and wherein obtaining the layers comprises obtaining the foreground channels for at least one of the layers of the bitstream based on the indication of the number of foreground channels.

[000350] Clause 4C. The method of clause 1C, further comprising obtaining an indication of a specified number of background channels in the bitstream for at least one of the layers, and wherein obtaining the layers comprises obtaining the background channels for at least one of the layers of the bitstream based on the indication of the number of background channels.

[000351] Clause 5C. The method of clause 1C, wherein indicating the number of layers indicates that the number of layers is two, wherein the two layers comprise a base layer and an enhancement layer, and wherein obtaining the layers comprises obtaining an indication that a number of foreground channels is zero for the base layer and two for the enhancement layer.

[000352] Clause 6C. The method of clause 1C or 5C, wherein the indication of the number of layers indicates that the number of layers is two, wherein the two layers comprise a base layer and an enhancement layer, and the method further comprises obtaining an indication that a number of background channels is four for the base layer and zero for the enhancement layer.

[000353] Clause 7. Method of clause 1C, in which the indication of the number of layers indicates that the number of layers is three, in which the three layers comprise a base layer, a first enhancement layer and a second enhancement layer, and in which the method further comprises obtaining an indication that a number of foreground channels is zero for the base layer, two for the first enhancement layer and two for the third enhancement layer.

[000354] Clause 8C. The method of clause 1C or 7C, wherein indicating the number of layers indicates that the number of layers is three, wherein the three layers comprise a base layer, a first enhancement layer, and a second enhancement layer, and the method further comprising obtaining an indication that a number of background channels is two for the base layer, zero for the first enhancement layer, and zero for the third enhancement layer.

[000355] Clause 9C. The method of clause 1C, wherein indicating the number of layers indicates that the number of layers is three, wherein the three layers comprise a base layer, a first enhancement layer and a second enhancement layer, and the method further comprising obtaining an indication that a number of foreground channels is two for the base layer, two for a first enhancement layer, and two for a third enhancement layer.

[000356] Clause 10C. The method of clause 1C or 9C, wherein indicating the number of layers indicates that the layer number is three, wherein the three layers comprise a base layer, a first enhancement layer, and a second enhancement layer, and the method further comprises obtaining a background syntax element indicating that the number of background channels is zero for the base layer, zero for the first enhancement layer, and zero for the third enhancement layer.

[000357] Clause 11C. The method of clause 1C, wherein the indication of the number of layers comprises an indication of a number of layers in a previous frame of the bitstream, and the method further comprising obtaining an indication of whether a number of layers of the bitstream has changed in a current frame when compared to the number of layers of the bitstream in the previous frame, and obtaining the number of layers of the bitstream in the current frame based on the indication of whether the number of layers of the bitstream has changed in the current frame.

[000358] Clause 12C. The method of clause 11C, further comprising determining the number of layers of the bitstream in the current frame as equal to the number of layers of the bitstream in the previous frame when the indication indicates that the number of layers of the bitstream has not changed in the current frame when compared to the number of layers of the bitstream in the previous frame.

[000359] Clause 13C. The method of clause 11C, wherein the method further comprises, when the indication indicates that the number of layers of the bitstream has not changed in the current frame when compared to the number of layers of the bitstream in the previous frame, obtaining an indication of a current number of components in one or more of the layers for the current frame as being equal to a previous number of components in one or more of the layers of the previous frame.

[000360] Clause 14C. The method of clause 1C, wherein indicating the number of layers indicates that three layers are specified in the bitstream, and wherein obtaining the layers comprises obtaining a first of layers of the bitstream indicative of background components of the higher-order ambisonic audio signal providing stereo channel reproduction, obtaining a second of layers of the bitstream indicative of background components of the higher-order ambisonic audio signal that provide three-dimensional reproduction by three or more speakers disposed in one or more horizontal planes, and obtaining a third of layers of the stream bitrate indicative of foreground components of the higher order ambisonic audio signal.

[000361] Clause 15C. The method of clause 1C, wherein the indication of the number of layers indicates that three layers are specified in the bit stream, and wherein obtaining the layers comprises obtaining a first of layers of the bis stream indicative of background components of the higher order ambisonic audio signal that provide mono channel reproduction, obtaining a second of layers of the bit stream indicative of background components of the higher order ambisonic audio signal that provide three dimensional reproduction by three or more speakers disposed in one or more horizontal planes, and obtaining a third of layers of the bitstream indicative of foreground components of the higher order ambisonic audio signal.

[000362] Clause 16C. The method of clause 1C, wherein the indication of the number of layers indicates that three layers are specified in the bitstream, and wherein obtaining the layers comprises obtaining a first of the layers of the bitstream indicative of background components of the higher order ambisonic audio signal providing stereo channel reproduction, obtaining a second of layers of the bitstream indicative of background components of the higher order ambisonic audio signal that provide multichannel reproduction by three or more speakers arranged in a single horizontal plane, obtaining a third of the layers of the stream bit stream indicative of background components of the higher order ambisonic audio signal providing three-dimensional reproduction by three or more speakers arranged in two or more horizontal planes, and obtain a fourth of the layers of bit stream indicative of foreground components of the higher order ambisonic audio signal.

[000363] Clause 17C. The method of clause 1C, wherein the indication of the number of layers indicates that three layers are specified in the bitstream, and wherein obtaining the layers comprises obtaining a first of layers of the bitstream indicative of background components of the higher order ambisonic audio signal providing mono channel reproduction, obtaining a second of layers of the bitstream indicative of background components of the higher order ambisonic audio signal that provide multichannel reproduction by three or more speakers arranged in a single horizontal plane, and obtaining a third of layers of the higher order ambisonic audio signal. bis indicative of background components of the higher-order ambisonic audio signal providing three-dimensional reproduction by three or more speakers arranged in two or more horizontal planes, and obtaining a fourth of the layers of the bitstream indicative of foreground components of the higher-order ambisonic audio signal.

[000364] Clause 18C. The method of clause 1C, wherein the indication of the number of layers indicates that two layers are specified in the bitstream, and wherein obtaining the layers comprises obtaining a first of layers of the bitstream indicative of background components of the higher-order ambisonic audio signal providing stereo channel reproduction, and obtaining a second of layers of the bitstream indicative of background components of the higher-order ambisonic audio signal that provide horizontal multichannel reproduction by three or more speakers arranged in a single horizontal plane.

[000365] Clause 19C. The method of clause 1C, further comprising obtaining an indication of a specified number of channels in the bitstream, wherein obtaining the layers comprises obtaining the layers of the bitstream based on the indication of the number of layers and the indication of the number of channels.

[000366] Clause 20C. The method of clause 1C, further comprising obtaining an indication of a specified number of foreground channels in the bitstream for at least one of the layers, wherein obtaining the layers comprises obtaining the foreground channels for at least one of the layers of the bitstream based on the indication of the number of foreground channels.

[000367] Clause 21C. The method of clause 1C, further comprising obtaining an indication of a specified number of background channels in the bitstream for at least one of the layers, wherein obtaining the layers comprises obtaining the background channels for at least one of the layers of the bitstream based on the indication of the number of background channels.

[000368] Clause 22C. The method of clause 1C, further comprising parsing an indication of a specified number of foreground channels in the bitstream for at least one of the layers based on a number of channels remaining in the bitstream after at least one of the layers is obtained, wherein obtaining the layers comprises obtaining the foreground channels of at least one of the layers based on the indication of the number of foreground channels.

[000369] Clause 23C. Method of clause 22C, wherein the number of channels remaining in the bitstream after at least one of the layers is obtained is represented by a syntax element.

[000370] Clause 24C. The method of clause 1C, further comprising parsing an indication of a specified number of background channels in the bitstream for at least one of the layers based on a number of channels after at least one of the layers is obtained, wherein obtaining the background channels for at least one of the layers from the bitstream based on the indication of the number of background channels.

[000371] Clause 25C. Method of clause 24C, wherein the number of channels remaining in the bit stream after at least one of the layers is obtained is represented by a syntax element.

[000372] Clause 26C. The method of clause 1C, wherein the bit stream layers comprise a base layer and an enhancement layer, and the method further comprises applying a correlation transform with respect to one or more channels of the base layer to obtain a correlated representation of background components of the higher order ambisonic audio signal.

[000373] Clause 27C. The method of clause 26C, wherein the correlation transform comprises an inverse UHJ transform.

[000374] Clause 28C. The method of clause 26C, wherein the correlation transform comprises an inverse matrix transform.

[000375] Clause 29C. Method of clause 1C, where a number of channels for each of the layers of the bitstream is fixed.

[000376] In addition, the techniques may allow a device to be configured for, or provide an apparatus comprising means to execute, or a non-transient computer-readable medium having stored therein instructions that, when executed, cause one or more processors to execute the method set forth in the following clauses.

[000377] Clause 1D. A method of decoding a bitstream representative of a higher order ambisonic audio signal, the method comprising obtaining, from the bitstream, an indication of a specified number of channels in one or more layers in the bitstream, and obtaining the specified channels in one or more layers in the bitstream based on the indication of the number of channels.

[000378] Clause 2D. The method of clause 1D, further comprising obtaining an indication of a total number of specified channels in the bit stream, and wherein obtaining the channels comprises obtaining the specified channels in one or more layers based on indicating the number of specified channels in one or more layers and indicating the total number of channels.

[000379] Clause 3D. The method of clause 1D, further comprising obtaining an indication of a type of one of the channels specified in one or more layers in the bit stream, and wherein obtaining the channels comprises obtaining one of the channels based on the indication of the number of channels and the indication of the type of one of the channels.

[000380] Clause 4D. The method of clause 1D, further comprising obtaining an indication of a type of one of the channels specified in one or more layers in the bitstream, indicating the type of one of the channels by indicating that one of the channels is a foreground channel, and wherein obtaining the channels comprises obtaining one of the channels based on the indication of the number of channels and indicating that the type of one of the channels is the foreground channel.

[000381] Clause 5D. The method of clause 1D, further comprising obtaining an indication of a specified number of layers in the bitstream, and wherein obtaining the channels comprises obtaining one of the channels based on the indication of the number of channels and the indication of the number of layers.

[000382] Clause 6D. The method of clause 5D, wherein indicating the number of layers comprises an indication of a number of layers in a previous frame of the bit stream, the method further comprising obtaining an indication of whether the number of channels specified in one or more layers in the bit stream has changed in a current frame when compared to a number of channels specified in one or more layers in the bit stream of the previous frame, and wherein obtaining the channels comprises obtaining one of the channels based on an indication of whether the number of channels specified in one or more layers in the bit stream has changed in the current frame .

[000383] Clause 7D. The method of clause 5D, further comprising determining the number of channels specified in one or more layers of the bitstream in the current frame as equal to the number of channels specified in one or more layers of the bitstream in the previous frame when the indication indicates that the number of channels specified in one or more layers of the bitstream in the previous frame has not changed in the current frame when compared to the number of channels specified in one or more layers of the bitstream in the previous frame.

[000384] Clause 8D. The method of clause 5D, wherein one or more processors are further configured to, when the indication indicates that the number of channels specified in one or more layers of the bitstream has not changed in the current frame when compared to the number of channels specified in one or more layers of the bitstream in the previous frame, obtain an indication of a current number of channels in one or more of the layers for the current frame as being equal to a previous number of channels in one or more of the layers of the previous frame.

[000385] Clause 9D. The method of clause 1D, further comprising obtaining an indication of a type of one of the channels specified in one or more layers in the bitstream, indicating the type of one of the channels by indicating that one of the channels is a background channel, wherein obtaining the channels comprises obtaining one of the channels based on the indication of the number of layers, and indicating that the type of one of the channels is the background channel.

[000386] Clause 10D. The method of clause 9D, further comprising obtaining an indication of a type of one of the channels specified in one or more layers in the bit stream, indicating the type of one of the channels by indicating that one of the channels is a background channel, wherein obtaining the channels comprises obtaining one of the channels based on the indication of the number of layers, and indicating that the type of one of the channels is the background channel.

[000387] Clause 11D. The method of clause 9D, wherein one of the channels comprises a background higher order ambisonic coefficient.

[000388] Clause 12D. The method of clause 9D, wherein obtaining the type indication of one of the channels comprises obtaining a syntax element indicative of the type of one of the channels.

[000389] Clause 13D. The method of clause 1D, wherein obtaining the indication of the number of channels comprises obtaining the indication of the number of channels based on a number of channels remaining in the bitstream after one of the layers is obtained.

[000390] Clause 14D. Method of Clause 1D, wherein the layers comprise a base layer.

[000391] Clause 15D. Method of Clause 1D, wherein the layers comprise a base layer and one or more enhancement layers.

[000392] Clause 16D. Clause 1D method, where a number of one or more layers is fixed.

[000393] The above techniques can be performed against any number of different contexts and audio ecosystems. A number of example contexts are described below, although techniques should be limited to example contexts. An example audio ecosystem might include audio content, film studios, music studios, game audio studios, channel-based audio content, encoding engines, game audio trunks, game audio encoding/rendering engines, and delivery systems.

[000394] Movie studios, music studios, and game audio studios can receive audio content. In some examples, audio content may represent the output of an acquisition. Film studios can stream channel-based audio content (for example, in 2.0, 5.1, and 7.1) as well by using a digital audio workstation (DAW). Music studios can stream audio content based on channel (eg in 2.0 and 5.1) as well as by using a DAW. In any case, encoding engines can receive and encode channel-based audio content based on one or more codecs (e.g., AAC, AC3, Dolby True HD, Dolby Digital Plus, and DTS Master Audio) for transmission by delivery systems. Game audio studios can stream one or more streams of game audio, such as through the use of a DAW. Game audio encoding/rendering engines can encode and or render the audio streams into channel-based audio content for transmission by delivery systems. Another example context in which the techniques may be performed comprises an audio ecosystem that may include broadcast recording audio objects, professional audio systems, consumer device capture, HOA audio format, on-device rendering, consumer audio, TV and accessories, and car audio systems.

[000395] Broadcast recording audio objects, professional audio systems, and consumer device capture can all encode their output using HOA audio format. In this way, audio content can be encoded using the HOA audio format into a single representation that can be played back using on-device rendering, consumer audio, TV and accessories, and car audio systems. In other words, the unique representation of the audio content can be played on a generic audio playback system (that is, as opposed to requiring a specific configuration like 5.1, 7.1, etc.)

[000396] Other examples of contexts in which techniques can be performed include an audio ecosystem that may include acquisition elements, and playback elements. Acquisition elements can include wired and/or wireless acquisition devices (eg, Eigen microphones), on-device surround sound capture, and mobile devices (eg, smartphones and tablets). In some examples, wired and/or wireless acquisition devices can be coupled to a mobile device through wired and/or wireless communication channel(s).

[000397] In accordance with one or more techniques of this disclosure, the mobile device can be used to acquire a sound field. For example, the mobile device may acquire a sound field through wired and/or wireless acquisition devices and/or device-based surround sound capture (e.g., a plurality of microphones built into the mobile device). The mobile device can then encode the acquired sound field into HOA coefficients for playback by one or more of the playback elements. For example, a mobile device user can record (acquire a sound field from) a live event (eg, a meeting, conference, play, concert, etc.) and encode the recording into HOA coefficients.

[000398] The mobile device may also use one or more of the playback elements to reproduce the HOA encoded sound field. For example, the mobile device may decode the HOA-encoded sound field and transmit a signal to one or more of the playback elements that causes one or more of the playback elements to recreate the sound field. As an example, the mobile device may utilize wireless and/or wireless communication channels to transmit the signal to one or more speakers (eg, arrays of speakers, sound buses, etc.). As another example, the mobile device may use docking solutions to transmit the signal to one or more docking stations and/or one or more docked speakers (e.g., sound systems in cars and/or smart homes). As another example, the mobile device can use headphone rendering to transmit the signal to a set of headphones, for example, to create realistic binaural sound.

[000399] In some examples, a specific mobile device can both acquire a 3D sound field and reproduce the same 3D sound field at a later time. In some examples, the mobile device may acquire a 3D sound field, encode the 3D sound field into HOA, and transmit the encoded 3D sound field to one or more other devices (e.g., other mobile devices and/or other non-mobile devices) for playback.

[000400] Yet another context in which the techniques can be performed includes an audio ecosystem which may include audio content, game studios, encoded audio content, rendering engines, and delivery systems. In some instances, game studios may include one or more DAWs that can support HOA signal editing. For example, one or more DAWs may include HOA plugins and/or tools that can be configured to operate with (eg work with) one or more game audio systems. In some instances, game studios may stream new stem formats that support HOA. In any case, game studios can stream encoded audio content to rendering engines that can render a sound field for playback by delivery systems.

[000401] The techniques can also be performed with respect to exemplary audio acquisition devices. For example, the techniques can be performed with respect to an Eigen microphone that can include a plurality of microphones that are collectively configured to record a 3D sound field. In some examples, the plurality of Eigen microphone microphones may be located on the surface of a substantially spherical ball having a radius of approximately 4 cm. In some examples, the audio encoding device 20 can be integrated into the Eigen microphone so as to transmit a bit stream 21 directly from the microphone.

[000402] Another exemplary audio acquisition context may include a production truck that can be configured to receive a signal from one or more microphones such as one or more Eigen microphones. The production truck may also include an audio encoder, such as audio encoder 20 in Figure 3.

[000403] The mobile device may also, in some instances, include a plurality of microphones that are collectively configured to record a 3D sound field. In other words, the plurality of microphones can have X,Y,Z diversity. In some examples, the mobile device can include a microphone that can be rotated to provide X,Y,Z diversity with respect to one or more other microphones of the mobile device. The mobile device may also include an audio encoder, such as audio encoder 20 in figure 3.

[000404] An enhanced video audio capture device can be additionally configured to record a 3D sound field. In some examples, the reinforced video capture device may be attached to a helmet of a user engaged in an activity. For example, the reinforced video capture device can be attached to a user's helmet when rafting in clear water. In this way, the enhanced video capture device can capture a 3D sound field that represents the action around the user (eg water lapping behind the user, another rafter speaking in front of the user, etc.).

[000405] The techniques can also be performed with respect to an enhanced accessory mobile device, which can be configured to record a 3D sound field. In some instances, the mobile device may be similar to the mobile devices discussed above, with the addition of one or more accessories. For example, an Eigen microphone can be connected to the aforementioned mobile device to form an enhanced accessory mobile device. In this way, the enhanced mobile device accessory can capture a higher quality version of the 3D sound field than just using integral sound capture components with the enhanced mobile device accessory.

[000406] Example audio playback devices that can perform various aspects of the techniques described in this disclosure are further discussed below. In accordance with one or more techniques of this disclosure, speakers and/or sound buses can be arranged in any arbitrary configuration while still reproducing a 3D sound field. Furthermore, in some examples, headphone playback devices may be coupled to a decoder 24 via a wired or wireless connection. In accordance with one or more techniques of this disclosure, a single generic representation of a sound field can be used to render the sound field in any combination of speakers, sound buses, and headphone playback devices.

[000407] A number of different example audio playback environments may also be suitable for performing various aspects of the techniques described in this disclosure. For example, a 5.1 speaker playback environment, a 2.0 speaker playback environment (e.g., stereo), a 9.1 speaker playback environment with full height speakers, a 22.2 speaker playback environment, a 16.0 speaker playback environment, an automotive speaker playback environment, and a mobile device with headset playback environment may be suitable environments for performing various aspects of the techniques described in this disclosure.

[000408] In accordance with one or more techniques of this disclosure, a single generic representation of a sound field may be used to render the sound field in any of the above playback environments. Additionally, the techniques in this disclosure allow a renderer to render a sound field from a generic representation for playback in playback environments other than the one described above. For example, if design considerations prohibit proper placement of speakers according to a 7.1 speaker playback environment (for example, if it is not possible to place a certain surround speaker), the techniques in this reveal allow for rendering to compensate with the other 6 speakers so that playback can be achieved in a 6.1 speaker playback environment.

[000409] Also, a user can watch a sports game while wearing headphones. According to one or more techniques of this disclosure, the 3D sound field of the sports game can be acquired (for example, one or more Eigen microphones can be placed in and/or around the baseball stadium), HOA coefficients corresponding to the 3D sound field can be obtained and transmitted to a decoder, the decoder can reconstruct the 3D sound field based on the HOA coefficients and transmit the reconstructed 3D sound field to a renderer, the renderer can obtain an indication regarding the type of playback environment (for example, headphones), and render the reconstructed 3D sound field into signals that cause the headphone sources to transmit a representation of the sports game's 3D sound field.

[000410] In each of the various instances described above, it should be understood that the audio encoding device 20 may perform a method or otherwise comprise means for performing each step of the method for which the audio encoding device 20 is configured to perform. In some instances, the medium may comprise one or more processors. In some instances, one or more processors may represent a special purpose processor configured via instructions stored on a non-transient computer-readable storage medium. In other words, various aspects of the techniques in each of the sets of coding examples can provide a non-transient computer-readable storage medium having stored thereon instructions that, when executed, cause one or more processors to perform the method for which the audio coding device 20 has been configured to perform.

[000411] In one or more examples, the functions described may be implemented in hardware, software, firmware or any combination thereof. If implemented in software, functions may be stored in or transmitted through as one or more instructions or code on computer-readable media and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to tangible media such as data storage media. Data storage media can be any available media that can be accessed by one or more computers or one or more processors to retrieve code instructions and/or data structures for implementing the techniques described in this disclosure. A computer program product may include computer readable media.

[000412] Similarly, in each of the various instances described above, it should be understood that the audio decoding device 24 may perform a method or otherwise comprise means for performing each step of the method for which the audio decoding device 24 is configured to perform. In some instances, the medium may comprise one or more processors. In some instances, one or more processors may represent a special purpose processor configured via instruction stored on a non-transient computer readable storage medium. In other words, various aspects of the techniques in each of the sets of coding examples can provide a non-transient computer-readable storage medium having stored therein instructions that, when executed, cause one or more processors to perform the method for which the audio decoding device 24 has been configured to perform.

[000413] As an example, and not limitation, such computer-readable storage media may comprise RAM, ROM EEPROM, CD-ROM or other optical disk storage, magnetic disk storage, or other magnetic storage devices, flash memory, or any other media that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. It should be understood, however, that computer-readable storage media and data storage media do not include connections, carrier waves, signals, or other transient media, but are instead intended for non-transient tangible storage media. Disk and disc, as used here, include compact disc (CD), laser disc, optical disc, digital versatile disc (DD), floppy disk, and Blu-ray disc, where disks typically reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above shall also be included in the scope of computer readable media.

[000414] Instructions can be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application-specific integrated circuits (ASICs), field-programmable logic arrays (FPGAs), or other equivalent discrete or integrated logic circuitry. Accordingly, the term "processor" as used herein may refer to any of the above frameworks or any other framework suitable for implementing the techniques described herein. Furthermore, in some respects, the functionality described here may be provided in dedicated hardware and/or software modules configured to encode and decode, or incorporated into a combined codec. Also, the techniques can be fully implemented in one or more circuits or logic elements.

[000415] The techniques of this disclosure can be implemented in a wide variety of devices or appliances, including a cordless telephone handset, an integrated circuit (IC) or a set of ICs (eg, a chip set). Various components, modules, or units are described in this disclosure to emphasize functional aspects of devices configured to perform the disclosed techniques, but do not necessarily require performance by different hardware units. Rather, as described above, multiple units may be combined into a codec hardware unit or provided by a collection of Inter-Operative hardware units, including one or more processors as described above, in combination with suitable software and/or firmware.

[000416] Various aspects of the techniques have been described. These and other aspects of the techniques are within the scope of the following claims.

Claims (6)

1. Device configured to decode a bit stream representative of a higher order ambisonic audio signal (HOA) that is provided in multiple layers and comprises at least one HOA coefficient that corresponds to a spherical harmonic base function that has an order greater than one, the device characterized in that it comprises: a memory configured to store the bit stream; and one or more processors configured to: - obtain, from the bitstream, an indication of a total number of transport channels (59 A-D, 61 A-B) specified in the bitstream; - determine whether the bit stream is provided in multiple layers; - in response to determining that the bit stream is provided in multiple layers, obtaining from the bit stream (21) an indication of the number of layers (21 A-C) specified in the bit stream; and - obtaining the layers of the bitstream based on indicating the number of layers specified in the bitstream, indicating the number of transport channels specified in the bitstream, and a number of channels representative of a minimum HOA order of a background sound field of the audio signal, wherein obtaining the number of layers specified in the bitstream comprises obtaining an indication of a number of transport channels for each layer based on a syntax element included in the bitstream for the layer, wherein indications of numbers of transport channels for the layers are obtained iteratively while a number of transport channels remaining is greater than one, the number of transport channels remaining being calculated in each iteration to be the total number of transport channels minus the cumulative sum of the numbers of transport channels already obtained, wherein, in each iteration, the indication of the number of transport channels for the respective layer is encoded into the bit stream using a number of bits calculated based on the number of transport channels remaining, wherein the indication of the number of layers specified in the bit stream is obtained by incrementing a counter at each it eration, where if, after the iterations, the number of remaining channels is equal to one, then the number of layers is incremented by one and the number of transport channels for a final layer is set to be equal to one. 2. Device according to claim 1, characterized in that the bit stream is provided in three or more layers. 3. Device according to claim 1, characterized in that it additionally comprises speakers configured to reproduce a sound field based on HOA audio signals. 4. Method for decoding a bit stream representative of a higher order ambisonic audio signal (HOA) that is provided in multiple layers and comprises at least one HOA coefficient that corresponds to a spherical harmonic base function that has an order greater than one, the method characterized in that it comprises: obtaining an indication of a number of transport channels (59 A-D, 61 A-B) specified in the bit stream; determining if the bit stream is provided in multiple layers; in response to determining that the bit stream is provided in multiple layers, obtaining, from the bit stream (21), a number of layers (21 A-C) specified in the bit stream; and obtaining the layers of the bitstream based on indicating the number of layers specified in the bitstream, indicating the number of transport channels specified in the bitstream, and a number of channels representative of a minimum HOA order of a background sound field of the audio signal, wherein obtaining the number of layers specified in the bitstream comprises obtaining an indication of a number of transport channels for each layer based on a syntax element included in the bitstream for the layer, wherein indications of numbers of transport channels for the layers are obtained iteratively while a number of transport channels remaining is greater than one, the number of transport channels remaining being calculated in each iteration to be the total number of transport channels minus the cumulative sum of the numbers of transport channels already obtained, wherein, in each iteration, the indication of the number of transport channels for the respective layer is encoded in the bit stream using a number of bits calculated based on the number of transport channels remaining, wherein the indication of the number of layers specified in the bit stream is obtained by incrementing a counter in each iteration , where if, after the iterations, the number of remaining channels is equal to one, then the number of layers is incremented by one and the number of transport channels for a final layer is set to be equal to one. 5. Method according to claim 4, characterized in that the bit stream is provided in three or more layers. 6. Computer-readable memory characterized by the fact that it comprises instructions stored therein that, when executed, cause one or more processors to perform the steps of the method as defined in claim 4 or 5.