AU2015246158A1 - Advanced stereo coding based on a combination of adaptively selectable left/right or mid/side stereo coding and of parametric stereo coding. - Google Patents

Abstract

The application relates to audio encoder and decoder systems. An embodiment of the encoder system comprises a downmix stage for generating a downmix signal and a residual signal based on a stereo signal. In addition, the encoder system comprises a parameter determining stage for determining parametric stereo parameters such as an inter-channel intensity difference and an inter-channel cross-correlation. Preferably, the parametric stereo parameters are time- and frequency-variant. Moreover, the encoder system comprises a transform stage. The transform stage generates a pseudo left/right stereo signal by performing a transform based on the downmix signal and the residual signal. The pseudo stereo signal is processed by a perceptual stereo encoder. For stereo encoding, left/right encoding or mid/side encoding is selectable. Preferably, the selection between left/right stereo encoding and mid/side stereo encoding is time- and frequency-variant

Description

Advanced stereo coding based on a combination of adaptively selectable left/right or mid/side stereo coding and of parametric stereo coding Related Applications 5 The present application is a divisional application of Australian patent Application No. 2013206557, itself a divisional application of Australian patent Application No. 2010225051; the entire contents of both applications are incorporated herein by reference. Technical Field 10 The application relates to audio coding, in particular to stereo audio coding combining parametric and waveform based coding techniques. Background of the Invention Joint coding of the left (L) and right (IR) channels of a stereo signal enables more efficient coding compared to independent coding of L and R. A common approach 15 for joint stereo coding is mid/side (M/S) coding. Here, a mid (M) signal is formed by adding the L and R signals, e.g. the M signal may have the form M = 1/2(L + R). Also, a side (S) signal is formed by subtracting the two channels L and R, e.g. the S signal may have the form 20 S = 1/2(L - R) . In case of M/S coding, the M and S signals are coded instead of the L and R signals. In the MPEG (Moving Picture Experts Group) AAC (Advanced Audio Coding) standard (see standard document ISO/IEC 13818-7), L/R stereo coding and M/S stereo coding can be chosen in a time-variant and frequency-variant manner. Thus, 25 the stereo encoder can apply L/R coding for some frequency bands of the stereo signal, whereas M/S coding is used for encoding other frequency bands of -2 the stereo signal (frequency variant), Moreover, the encoder can switch over time between LIR and M/S coding (time-variant), In MPEG AAC, the stereo encoding is carried out in the frequency domain, more particularly in the MDCT (modified discrete cosine transfonn) domain, This allows to adaptive choose either L/R or 5 M/S coding in a frequency and also time variant manner. The decision between L/R and MIS stereo encoding may be based by evaluating the side signal: when the energy of the side signal is low, M/S stereo encoding is more efficient and should be used. Alternatively, for deciding between both stereo coding schemes, both coding schemes may be tried out and the selection may be based on the re 10 suiting quantization. efforts, i.e., the observed perceptual entropy. An alternative approach to joint stereo coding is parametric stereo (PS) coding. Here, the stereo signal is conveyed as a mono downmix signal after encoding the downmix signal with a conventional audio encoder such as an AAC encoder. The 15 downimix signal is a superposition of the L and R channels. The mono downmix signal is conveyed in combination with additional time-variant and frequency variant PS parameters, such as the inter-channel (i.e. between L and R) intensity difference (IID) and the inter-channel cross-correlation (ICC). In the decoder, based on the decoded downmix signal and the parametric stereo parameters a ste 20 reo signal is reconstructed that approximates the perceptual stereo image of the original stereo signal. For reconstructing, a decorrelated version of the downmix signal is generated by a decorrelator. Such decorrelator may be realized by an appropriate all-pass filter. PS encoding and decoding is described in the paper "Low-Complexity Parametric Stereo Coding in MPEG-4", IK. Pumhagen, Proc. Of 25 the 7m" Int. Conference on Digital Audio Effects (DAx'04), Naples, Italy, Octo ber 5-8, 2004, pages 163-168, The disclosure of this document is hereby incorpo rated by reference. The MPEG Surround standard (see document ISO/lEC 23003-1) makes use of the 30 concept of PS coding, In an MPEG Surround decoder a plurality of output chan nels is created based on fewer input channels and control parameters. MPEG Surround decoders and encoders are constructed by cascading parametric stereo modules, which in MPEG Surround are referred to as OTT modules (One-To-Two modules) for the decoder and R-OTT modules (Reverse-One-To-Two modules) for the encoder, An OTT module determines two output channels by means of a 5 single input channel (downmix signal) accompanied by PS parameters. An OTT module corresponds to a PS decoder and an R-OTT module corresponds to aPS encoder, Parametric stereo can be realized by using MPEG Surround with a single OTT' module at the decoder side and a single R-OTT module at the encoder side; this is also referred to as "MPEG Surround 2-1-2" mode. The bitstream syntax 10 may differ, but the underlying theory and signal processing are the same. There fore, in the following all the references to PS also include "MPEG Surround 2-1 2" or MPEG Surround based parametric stereo. In a PS encoder (e.g. in a MPEG Surround PS encoder) a residual signal (RES) 15 may be determined and transmitted in addition to the downnix signal. Such resi dual signal indicates the error associated with representing original channels by their downnix and PS parameters. In the decoder the residual signal may be used instead of the decorrelated version of the downiix signal. This allows to better reconstruct the waveforms of the original channels L and R, The use of an addi 20 tional residual signal is e.g. described in the MPEG Surround standard (see docu ment ISO/IEC 23003-1) and in the paper "MPEG Surround - The ISO/MPEG Standard for Efficient and Compatible Multi-Channel Audio Coding, J. Herre et at., Audio Engineering Convention Paper 7084, 122d Convention, May 5-8, 2007. The disclosure of both documents, in particular the remarks to the residual 25 signal therein, is herewith incorporated by reference. PS coding with residual is a mote general approach to joint stereo coding than M/S coding: M/S coding performs a signal rotation when transfonning UR sig nals into MIS signals, Also, PS coding with residual performs a signal rotation 30 when transfonning the U/R signals into downmix and residual signals. However, in the latter case the signal rotation is variable and depends on the PS parameters.

-4 Due to the more general approach of PS coding with residual, PS coding with residual allows a more efficient coding of certain types of signals like a paned mono signal than M/S coding. Thus, the proposed coder allows to efficiently combine parametric stereo coding techniques with wavefonn based stereo coding $ techniques, Often, perceptual stereo encoders, such as an MPEG AAC perceptual stereo en coder, can decide between LR stereo encoding and MIS stereo encoding, where in the latter case a mid/side signal is generated based on the stereo signal. Such o selection may be frequency-variant, i.e. for some frequency bands LiR stereo en coding may be used, whereas for other frequency bands MIS stereo encoding may be used. In a situation where the L and R channels are basically independent signals, such 15 perceptual stereo encoder would typically not use M/S stereo encoding since in this situation such encoding scheme does not offer any coding gain in comparison to L/R stereo encoding. The encoder would fall back to plain L/R stereo encoding, basically processing L and R independently. 20 In the same situation, a PS encoder system would create a downmix signal that contains both the L and R channels, which prevents independent processing of the L and R channels, For PS coding with a residual signal, this can imply less effi cient coding compared to stereo encoding, where L/R stereo encoding or M/S stereo encoding is adaptively selectable. 25 Thus, there are situations where a PS coder outperforms a perceptual stereo coder with adaptive selection between L/R stereo encoding and M/S stereo encoding, whereas in other situations the latter coder outperforms the PS coder. 30 -5 Summary of the invention The present application describes an audio encoder system and an encoding me thod that are based on the idea of combing PS coding using a residual with adap 5 tive UiR or M/S perceptual stereo coding (e.g. AAC perceptual joint stereo coding in the MDCT domain), This allows to combine the advantages of adaptive L!R or M/S stereo coding (e.g. used in MPEG AAC) and the advantages of PS coding with a residual signal (e.g. used in MPEG Surround). Moreover, the application describes a corresponding audio decoder system and a decoding method. 10 A first aspect of the application relates to an encoder system for encoding a stereo signal to a bitstream signal. According to an embodiment of the encoder System, the encoder system comprises a downmix stage for generating a downmix signal and a residual signal based on the stereo signal. The residual signal may cover all 15 or only a part of the used audio frequency range. In addition, the encoder system comprises a parameter detennining stage for determining PS parameters such as an inter-channel intensity difference and an inter-channel cross-correlation. Pre ferably, the PS parameters are frequency-variant. Such downmix stage and the parameter determining stage are typically part of a PS encoder. 20 in addition, the encoder system comprises perceptual encoding means down stream of the downmix stage, wherein two encoding schemes are selectable: - encoding based on a sum of the downmix signal and the residual signal and based on a difference of the dowmnix signal and the residual signal or 25 - encoding based on the downmix signal and based on the residual signal. It should be noted that in case encoding is based on the down-nix signal and the residual signal, the downmix signal and the residual signal may be encoded or signals proportional thereto may be encoded. In case encoding is based on a sum 30 and on a difference, the sum and difference may be encoded or signals propor tional thereto may be encoded.

-6 The selection may be frequency-variant (and time-variant), i.e. for a first frequen cy band it may be selected that the encoding is based on a sun signal and a differ ence signal, whereas for a second frequency band it may be seleded that the en 5 coding is based on the downmix signal and based on the residual signal. Such encoder system has the advantage that is allows to switch between L/R ste reo coding and PS coding with residual (preferably in a frequency-variant man net): If the perceptual encoding means select (for a particular band or for the 10 whole used frequency range) encoding based on downimix and residual signals, the encoding system behaves like a system using standard PS coding with resi dual. However, if the perceptual encoding means select (for a particular band or for the whole used frequency range) encoding based on a sum signal of the downmix signal and the residual signal and based on a difference signal of the ts downmix signal and the residual signal, under certain circumstances the sum and difference operations essentially compensate the prior downmix operation (except for a possibly different gain factor) such that the overall system can actually per form U/R encoding of the overall stereo signal or for a frequency band thereof. E.g. such circumstances occur when the L and R channels of the stereo signal are 20 independent and have the same level as will be explained in detail later on. Preferably, the adaption of the encoding scheme is time and frequency dependent, Thus, preferably some frequency bands of the stereo signal are encoded by a L/R encoding scheme, whereas other frequency bands of the stereo signal are encoded 25 by a PS coding scheme with residual. It should be noted that in case the encoding is based on the downmix signal and based on the residual signal as discussed above, the actual signal which is input to the core encoder may be formed by two serial operations on the downrnix signal 3o and residual signal which are inverse (except for a possibly different gain factor). E,g, a downmix signal and a residual signal are fed to an M/S to L/R transform stage and then the output of the transform stage is fed to a L/R to M/S transform stage, The resulting signal (which is then used for encoding) corresponds to the downmix signal and the residual signal (expect for a possibly different gain fac tor). 5 The following embodiment makes use of this idea, According to an embodiment of the encoder system, the encoder system comprises a downmix stage and a pa rameterldeterining stage as discussed above. Moreover, the encoder system comprises a transform stage (e.g. as part of the encoding 10 means discussed above). The transform stage generates a pseudo LR stereo signal by performing a transform of the downnix signal and the residual signal. The transform stage preferably performs a sum and difference transforn, where the downmix signal and the residual signals are summed to generate one channel of the pseudo stereo signal (possibly, the sum is also multiplied by a factor) and sub 15 tracted from each other to generate the other channel of the pseudo stereo signal (possibly, the difference is also multiplied by a factor). Preferably, a first channel (e.g the pseudo left channel) of the pseudo stereo signal is proportional to the sum of the downmix and residual signals, where a second channel (e.g. the pseudo right channel) is proportional to the difference of the downmix and residual sig 20 nals. Thus, the downmix signal DMX and residual signal RES from the PS encod er may be converted into a pseudo stereo signal Lp, Ry according to the following equations: L,= g(DMX + RES) R,= g(DMX-- RES) 25 In the above equations the gain normalization factor g has e.g. a value of The pseudo stereo signal is preferably processed by a perceptual stereo encoder (e.g. as part of the encoding means). For encoding, Lk stereo encoding or M/S 30 stereo encoding is selectable. The adaptive L/R or M/S perceptual stereo encoder may be an AAC based encoder, Preferably, the selection between L stereo en coding and v/S stereo encoding is frequencyvariant; thus, the selection may vary for different frequency bands as discussed above. Also, the selection between L/R encoding and M/S encoding is preferably time-variant The decision between L 5 encoding and M/S encoding is preferably made by the perceptual stereo encoder. Such perceptual encoder having the option for M/S encoding can internally com pute (pseudo) M and S signals (in the time domain or in selected frequency bands) based on the pseudo stereo UP signal Such pseudo M and S signals correspond 10 to the downmix and residual signals (except for a possibly different gain factor). Hence, if the perceptual stereo encoder selects MIS encoding, it actually encodes the downidx and residual signals (which correspond to the pseudo M and S sig nals) as it would be done in a system using standard PS coding with residual. 15 Moreover, under special circumstances the transform stage essentially compen sates the prior downmnix operation (except -for a possibly different gain factor) such that the overall encoder system can actually perform LK encoding of the overall stereo signal or for a frequency band thereof (if LAR encoding is selected in the perceptual encoder). This is e.g, the case when the L and R channels of the 20 stereo signal are independent and have the same level as will be explained in de tail later on, Thus, for a given frequency band the pseudo stereo signal essentially corresponds or is proportional to the stereo signal, if - for the frequency band - the left and right channels of the stereo signal are essentially independent and have essentially the same level Thus, the encoder system actually allows to switch between AR stereo coding and PS coding with residual, in order to be able to adapt to the properties of the given stereo input signal. Preferably, the adaption of the encoding scheme is time and frequency dependent. Thus, preferably some frequency bands of the stereo signal 30 are encoded by a LAR encoding scheme, whereas other frequency bands of the stereo signal are encoded by a PS coding scheme with residual, It should be noted 9 that M/S coding is basically a special case of PS coding with residual (since the L/R to M/S transform is a special case of the PS downmix operation) and thus the encoder system may also perform overall M/S coding. 5 Said embodiment having the transform stage downstream of the PS encoder and upstream of the L/R or MIS perceptual stereo encoder has the advantage that a conventional PS encoder and a conventional perceptual encoder can be used. Nevertheless, the PS encoder or the perceptual encoder may be adapted due to the special use here, 10 The new concept improves the performance of stereo coding by enabling an effi cient combination of PS coding and joint stereo coding According to an alternative embodiment, the encoding means as discussed above 15 comprise a transform stage for perfordng a sum and difference transform based on the downmix signal and the residual signal for one or more frequency bands (e.g. for the whole used frequency range or only for one frequency range). The transformn may be performed in a frequency domain or in a time domain. The transform stage generates a pseudo left/right stereo signal for the one or more fre 20 quency bands. One channel of the pseudo stereo signal corresponds to the sum and the other channel corresponds to the difference Thus, in case encoding is based on the sum. and difference signals the output of the transform stage may be used for encoding, whereas in case encoding is based 25 on the downrnix signal and the residual signal the signals upstream of the encod ing stage may be used for encoding. Thus, this embodiment does not use two seri al sum and difference transforms on the downmix signal and residual signal, re sulting in the downmix signal and residual signal (except for a possibly different gain factor). 30 - 10 When selecting encoding based on the downmix signal and residual signal, para metric stereo encoding of the stereo signal is selected. When selecting encoding based on the sum and difference (i.e. encoding based on the pseudo stereo signal) L/R encoding of the stereo signal is selected. The transform stage may be a LiR to M/S transform stage as part of a perceptual encoder with adaptive selection between L/R and MIS stereo encoding (possibly the gain factor is different in comparison to a conventional L/R to M/S transform stage). It should be noted that the decision between L/R and M/S stereo encoding 1o should be inverted. Thus, encoding based on the dowanix signal and residual signal is selected (i.e. the encoded signal did not pass the transform stage) when the decision means decide M/S perceptual decoding, and encoding based on the pseudo stereo signal as generated by the transform stage is selected (i.e, the en coded signal passed the transform stage) when the decision means decide L/R 15 perceptual decoding, The encoder system according to any of the embodiments discussed above may comprise an additional SBR (spectral band replication) encoder. SBR is a form of HFR (ligh Frequency Reconstruction). An SBR encoder determines side infor 20 nation for the reconstruction of the higher frequency range of the audio signal in the decoder. Only the lower frequency range is encoded by the perceptual encod er, thereby reducing the bitrate. Preferably, the SBR encoder is connected up stream of the PS encoder. Thus, the SBR encoder may be in the stereo domain and generates SBR parameters for a stereo signal, This will be discussed in detail in 25 connection with the drawings. Preferably, the PS encoder (i.e. the downmix stage and the parameter determining stage) operates in an oversampled frequency domain (also the PS decoder as dis cussed below preferably operates in an oversampled frequency domain), For time 30 to-frequency transform e.g. a complex valued hybrid filter bank having a QMF (quadrature mirror filter) and a Nyquist filter may be used upstream of the PS encoder as described in MPEG Surround standard (see document ISO/IEC 23003-1). This allows for time and frequency adaptive signal processing without audible aliasing artifacts. The adaptive L/R or MI/S encoding, on the other hand, is prefer ably carried out in the critically sampled MDCT domain (e.g. as described in 5 AAQ in order to ensure an efficient quantized signal representation, The conversion between downinix and residual signals and the pseudo L./R stereo signal may be carried out in the tine domain since the PS encoder and the percep tual stereo encoder are typically connected in the time domain anyway. Thus, the 10 transform stage for generating the pseudo L/R signal may operate in the time do main. In other embodiments as discussed in connection with the drawings, the transform stage operates in an oversampled frequency domain or in a critically sampled 15 MDCT domain, A second aspect of the application relates to a decoder system for decoding a bit stream signal as generated by the encoder system discussed above, 20 According to an embodiment of the decoder system, the decoder system compris es perceptual decoding means for decoding based on the bitstream signal. The decoding means are configured to generate by decoding an (internal) first signal and an (internal) second signal and to output a downmix signal and a residual sig nal. The downmix signal and the residual signal is selectively 25 - based on the sum of the first signal and of the second signal and based on the difference of the first signal and of the second signal or - based on the first signal and based on the second signal. 30 As discussed above in connection with the encoder system, also here the selection may be frequency-variant or frequency-invariant.

-12 Moreover, the system comprises an upmix stage for generating the stereo signal based on the downinix signal and the residual signal, with the upmnix operation of the uprnix stage being dependent on the one or more parametric stereo parameters. Analogously to the encoder system, the decoder system allows to actually switch between IJR decoding and PS decoding with residual, preferably in a time and frequency varian manner, 10 According to another embodiment, the decoder system comprises a perceptual stereo decoder (e.g. as part of the decoding means) for decoding the bitstream signal, with the decoder generating a pseudo stereo signal. The perceptual decoder may be an AAC based decoder. For the perceptual stereo decoder, L/R perceptual 15 decoding or MI/S perceptual decoding is selectable in a frequency-variant or fre quency-invariant manner (the actual selection is preferably controlled by the deci sion in the encoder wbich is conveyed as side-information in the bitstream). The decoder selects the decoding scheme based on the encoding scheme used for en coding. The used encoding scheme may be indicated to the decoder by informa 20 tion contained in the received bitstream. Moreover, a transform stage is provided for generating a downmix signal and a residual signal by performing a transform of the pseudo stereo signal. In other words: The pseudo stereo signal as obtained from the perceptual decoder is con 25 verted back to the downmix and residual signals. Such transform is a sum and difference transform: The resulting downmix signal is proportional to the sum of a left channel and a right channel of the pseudo stereo signal. The resulting residual signal is proportional to the difference of the left channel and the right channel of the pseudo stereo signal. Thus, quasi an L/R to M/S transform was carried out. 30 The pseudo stereo signal with the two channels Lb Rp may be converted to the downmix and residual signals according to the following equations; -13 DMX=1.- (L,+ RP) 2g RES = - R 1 ) 2g In the above equations the gain normalization factor g may have e.g. a value of g = 1/2.The residual signal RES used in the decoder may cover the whole used 5 audio frequency range or only a part of the used audio frequency range. The dowmix and residual signals are then processed by an upniix stage of a PS decoder to obtain the final stereo output signal, The upmixing of the downmix and residual signals to the stereo signal is dependent on the received PS parameters. I0 According to ah alternative embodiment, the perceptual decoding means may comprise a sum and difference transform stage for performing a transform based on the first signal and the second signal for one or more frequency bands (e.g. for 15 the whole used frequency range). Thus, the transform stage generates the down mix signal and the residual signal for the case that the downmix signal and the residual signal are based on the sum of the -first signal and of the second signal and based on the difference of the first signal and of the second signaL The trans form stage may operate in the time domain or in a frequency domain. 20 As similarly discussed in connection with the encoder system, the transform stage may be a M/S to L/R transform stage as part of a perceptual dedoder with adaptive selection between L/R and M/S stereo decoding (possibly the gain factor is differ ent in comparison to a conventional M/S to L/R transform stage). It should be 25 noted that the selection between L/R and M/S stereo decoding should be inverted. The decoder system according to any of the preceding embodiments may com prise an additional SBR decoder for decoding the side information from the SBR encoder and generating a high frequency component of the audio signal, Prefera- - 14 bly, the SBR decoder is located downstream of the PS decoder. This will be discussed in detail in connection with drawings. Preferably, the upmix stage operates in an oversampled frequency domain, e.g. a hybrid filter bank as discussed above may be used upstream of the PS decoder. 5 The L/R to MiS transform may be carried out in the time domain since the perceptual decoder and the PS decoder (including the upmix stage) are typically connected in the time domain. In other embodiments as discussed in connection with the drawings, the. L/R to M/S transform is carried out in ai oversampled frequency domain (e.g., QMF), or in a 10 critically sampled frequency domain (e.g., MDCT) A third aspect of the application relates to a method for encoding a stereo signal to a bitstream signal. The method operates analogously to the encoder system discussed above. Thus, the above remarks related to the encoder system are basically also applicable to encoding method. 15 A fourth aspect of the invention relates to a method for decoding a bitstream signal including PS parameters to generate a stereo signal. The method operates in the same way as the decoder system discussed above. Thus, the above remarks related to the decoder system are basically also applicable to decoding method. A fifth aspect of the invention relates to an encoder system configured for encoding a 20 stereo signal to a bitstream signal, the encoder system comprising a downmixing means configured for generating a downmix signal and a residual signal based on the stereo signal; a parameter determining means configured for determining one or more parametric stereo parameters; and perceptual encoding means downstream of the downmixing means, wherein the perceptual encoding means are configured for 25 encoding the downmix signal and the residual signal, and wherein the perceptual encoding means are configured for selecting left/right perceptual encoding, or mid/side perceptual encoding. A sixth aspect of the invention relates to a decoder system configured for decoding a bitstream signal including one or more parametric stereo parameters to a stereo 30 signal, the decoder system comprising: perceptual decoding means configured for decoding based on the bitstreamn signal, wherein the decoding means are configured to generate a downmix signal and a residual signal, wherein the decoding means are -15 configured to selectively perform left/right perceptual decoding or mid/side perceptual decoding; and upmixing means configured for performing an upmix operation for generating the stereo signal based on the downmix signal and the residual signal, with the upmix operation of the upmixing means being dependent on 5 the one or more parametric stereo parameters. A seventh aspect of the invention relates to a method for encoding a stereo signal to a bitstream signal, the method comprising: generating a downmix signal and a residual signal based on the stereo signal; determining one or more parametric stereo parameters; perceptual encoding downstream of generating the downmix signal and 10 the residual signal, wherein left/right perceptual encoding, or mid/side perceptual encoding is selectable. An eighth aspect of the invention relates to a method for decoding a bitstream signal including parametric stereo parameters to a stereo signal, the method comprising: perceptual decoding based on the bitstream signal, wherein perceptual decoding is comprises generating a dowmnix signal and a residual signal by selectively performing left/right perceptual decoding or mid/side perceptual decoding; and generating the stereo signal based on the downinix signal and the residual signal by an upmix operation, with the upmix operation being dependent on the parametric stereo parameters. 20 The invention is explained below by way of illustrative examples with reference to the accompanying drawings, wherein Fig. 1 illustrates an embodiment of an encoder system, where optionally the PS parameters assist the psycho-acoustic control in the perceptual stereo encoder; Fig. 2 illustrates an embodiment of the PS encoder; 25 Fig. 3 illustrates an embodiment of a decoder system; Fig. 4 illustrates a fTither embodiment of the PS encoder including a detector to deactivate PS encoding if IlR encoding is beneficial; Fig. 5 illustrates an embodiment of a conventional PS encoder system having an additional SBR encoder for the downmix; 30 Fig. 6 illustrates an embodiment of an encoder system having an additional SBR encoder for the downmix signal; Fig. 7 illustrates an embodiment of an encoder system having an additional SBR encoder in the stereo domain; Figs. 8a-8d illustrate various time-frequency representations of one of the two output channels at the decoder output; 5 Fig. 9a illustrates an embodiment of the core encoder; Fig. 9b illustrates an embodiment of an encoder that permits switching between coding in a linear predictive domain (typically for mono signals only) and coding in a transform domain (typically for both mono and stereo signals); Fig. 10 illustrates an embodiment of an encoder system; 10 Fig. 11 a illustrates a part of an embodiment of an encoder system; Fig. 11 b illustrates an exemplary implementation of the embodiment in Fig, 11 a; Fig. II c illustrates an alternative to the embodiment in Fig, I Ia; Fig. 12 illustrates an embodiment of an encoder system; Fig. 13 illustrates an embodiment of the stereo coder as part of the encoder system of 15 Fig. 12; Fig. 14 illustrates an embodiment of a decoder system for decoding the bitstream signal as generated by the encoder system of Fig. 6; Fig. 15 illustrates an embodiment of a decoder system for decoding the bitstream signal as generated by the encoder system of Fig. 7; 20 Fig. 16a illustrates a part of an embodiment of a decoder system; Fig. 16b illustrates an exemplary implementation of the embodiment in Fig. 16a; Fig. 16c illustrates an alternative to the embodiment in Fig. 16a; Fig. 17 illustrates an embodiment of an encoder system; and Fig, 18 illustrates an embodiment of a decoder system. 25 Fig. I shows an embodiment of an encoder system which combines PS encoding using a residual with adaptive L/R or M/S perceptual stereo encoding. This embo- -17 diment is merely illustrative for the principles of the present application. It is un derstood that modifications and variations of the embodiment will be apparent to others skilled in the art. The encoder system comprises a PS encoder I receiving a stereo signal L, R, The PS encoder I has a dowumix stage for generating down 5 mix DMX and residual RES signals based on the stereo signal L, R, This opera tion can be described by means of a 2-2 downmix matrix H-' that converts the L and R signals to the dowunix signal DMX and residual signal RES: RES) R Typically, -the matrix H-'is frequency-variant and time-variant, i.e. the elements 10 of the matrix H- vary over frequency and vary -from time slot to time slot. The matrix H may be updated every frame (e.g. every 21 or 42 ms) and may have a frequency resolution of a plurality of bands, e.g. 28, 20, or 10 bands (named "pa rameter bands") on a perceptually oriented (Bark-like) frequency scale. 15 The elements of the matrix H' depend on the time- and frequency-variant PS parameters IID (inter-channel intensity difference; also called CLD - channel lev el difference) and ICC (inter-channel cross-correlation). For determining PS pa rameters 5, e.g. IID and ICC, -the PS encoder 1 comprises a parameter determining stage. An example for computing the matrix elements of the inverse matrix H is 20 given by the following and described in the MPEG Surround specification docu ment ISO/IEC 23003-1, subelause 6.53.2 which is hereby incorporated by refer ence: F c, cos(a+ R) c 1 sin(1x+,) H=I H ca cos(-a+ p3) c, sina(-a + p where 25 c cm and c, I 1+10d 1+10 and where - 18$ p6 arctan tan(a) and a =arccos(p), C2 +g and where p =iCC Moreover, the encoder system comprises a transform stage 2 that converts the s downmix signal DMX and residual signal RES from the PS encoder I into a pseudo stereo signal L,, R,, eg according to the following equations: L,=g(DMvY + RES)

R

, = g(DMX - RES) In the above equations the gain normalization factor g has eg. a value of 10 g ji/i For g= I/2, the two equations for pseudo stereo signal Lp, R, can be rewritten as R 112 ) RfES ) The pseudo stereo signal L,, Rp is then fed to a perceptual stereo encoder 3, which 15 adaptively selects either L/R or M/S stereo encoding. M/S encoding is a forn of joint stereo coding. L/R encoding may be also based on joint encoding aspects, e.g, bits may be allocated jointly for the L and R channels from a common bit reservoir. 20 The selection between L/R or M/S stereo encoding is preferably frequency variant, i.e. some frequency bands may be L/R encoded, whereas other frequency bands may be M/S encoded, An embodiment for implementing the selection be tween L/R or M/S stereo encoding is described in the document "Sum-Difference Stereo Transform Coding", J. D. Johnston et al., IEEE Intemational Conference 25 on Acoustics; Speech, and Signal Processing (ICASSP) 1992, pages 569-572. The discussion of the selection between L/R or M/S stereo encoding therein, in partic ular sections 5.1 and 5.2, is hereby incorporated by reference.

- 19 Based on the pseudo stereo signal L, 1R, the perceptual encoder 3 can internally compute (pseudo) mid/side signals Mp, S., Such signals basically corspond to the downmix signal DMX and residual signal RES (except for a possibly different gain factor). Hence, if the perceptual encoder 3 selects M/S encoding for a fre 5 quency band, the perceptual encoder 3 basically encodes the downMix signal DMX and residual signal RES for that frequency band (except for a possibly dif ferent gain factor) as it also would be done in a conventional perceptual encoder system using conventional PS coding with residual. The PS parameters 5 and the output bitstream 4 of the perceptual encoder 3 are multiplexed into a single bit 10 stream 6 by a multiplexer 7. In addition to PS encoding of the stereo signal, the encoder system in Fig; 1 al lows EAR coding ofthe stereo signal as will be explained in the following: As dis cussed above, the elements of the dowmnix matrix Ff' of the encoder (and also of is the upnix matrix H used in the decoder) depend on the time- and frequency variant PS parameters lID (inter-channel intensity difference; also called CLD channel level difference) and ICC (inter-channel cross-correlation). An example for computing the matrix elements of the upmix matrix H is described above. In case of using residual coding, the right column of the 2'2 upmix matrixH is given 20 as -u 1 ) However, preferably, the right column of the 2-2 matrixH should instead be mod ified to 25 (/ The left colum is preferably computed as given in the MPEG Surround specifica tion.

-20 Modifying the right column of the upmix matrix 11 ensures that for IID= 0 dB and ICC = 0 (i.e. the case where for the respective band the stereo channels L and R are independent and have the same level) the following upinix. natrix H is ob 5 tained for the band: H= 0v/2 hN 1/2 -f/2 ). Please note that the upmix matrix H and also the downmix matrix B ' are typi to cally frequency-variant and time-variant. Thus, the values of the matrices are dif ferent for different time/frequency tiles (a tile corresponds to the intersection of a particular frequency band and a particular time period). In the above case the downmix matrix H is identical to the upmix matrix 1i. Thus, for the band the pseudo stereo signal Lp, Rp can computed by the following equation: 15 RLP 112 1 /2 SDMX 1/2 -. 1/ (FI/2 1/2 4/2 12 (L) =(l 0) L = L fl _2 ji/- ) R 0 1 R R Hence, in this case the PS encoding with residual using the downmix matrix H' followed by the generation of the pseudo L/R signal in the transform stage 2 cor 20 responds to the unity matrix and does not change the stereo signal for the respec tive frequency band at all, ie. LP =L R, R 25 In other words: the transform stage 2 compensates the downmix matrix H- such that the pseudo stereo signal L., R, corresponds to the input stereo signal L, R.

-.21 This allows to encode the original input stereo signal L, R by the perceptual en coder 3 for the particular band: When L/R encoding is selected by the perceptual encoder 3 for encoding the particular band, the encoder system behaves like a L/R perceptual encoder for encoding the band of the stereo input signal L, R. -5 The encoder system in Fig. 1 allows seamless and adaptive switching between L/R coding and PS coding with residual in a frequency- and time-variant manner. The encoder system avoids discontinuities in the waveforn when switching the coding scheme. This prevents artifacts. in order to achieve smooth transitions, 10 linear interpolation may be applied to the elements of the matrix 4 in the encod er and the matrix H in the decoder for samples between two stereo parameter up dates. Fig. 2 shows an embodiment of the PS encoder 1. The PS encoder 1 comprises a 15 downmix stage 8 which generates the downmix signal DMX and residual signal RES based on the stereo signal L, R. Further, the PS encoder 1 comprises a para meter estimating stage 9 for estimating the PS parameters 5 based on the stereo signal L, R, 20 Fig. 3 illustrates an embodiment of a corresponding decoder system configured to decode the bitstream 6 as generated by the encoder system of Fig. L, This embo diment is merely illustrative fox the principles of the present application., It is un derstood that modifications and variations of the embodiment will be apparent to others skilled in the art. The decoder system comprises a demultiplexer 10 for 25 separating the PS parameters 5 and the audio bitstream 4 as generated by the per ceptual encoder 3, The audio bitstream 4 is fed to a perceptual stereo decoder 11, which can selectively decode an L/R encoded bitstream or an M/S encoded audio bitstream, The operation of the decoder 11 is inverse to the operation of the en coder 3. Analogously to the perceptual encoder 3, the perceptual decoder 11 pre 30 ferably allows for a frequency-variant and time-variant decoding scheme. Some -frequency bands -which are L/R encoded by the encoder 3 are L/R decoded by the - 22 decoder 11, whereas other frequency hands which are M/S encoded by the encod er 3 are M/S decoded by the decoder 11. The decoder II outputs the pseudo stereo signal L, R, which was input to the perceptual encoder 3 before. The pseudo ste reo signal L,, R, as obtained from the perceptual decoder 11 is converted back to 5 the downmix signal DMX and residual signal RES by a L/R to M/S transfonn stage 12, The operation of the L/R to MI/S transform stage 12 at the decoder side is inverse to the operation of the transform stage 2 at the encoder side. Preferably, the transform stage 12 determines the downmix signal DMX and residual signal RES according to the following equations: 10 DM I(L,+RP) 2g RES =-(Lp - R) 2g In the above equations, the gain normalization factor g is identical to the gain normalization factor g at the encoder side and has e,g. a value of g E/2 15 The downmix signal DMX and residual signal RES are then processed by the PS decoder 13 to obtain the final L and R output signals. The upmix step in the de coding process for PS coding with a residual can be described by means of the 22 upmix matrix H that converts the downmix signal DMX and residual signal RES back to the L and R channels: 20 = ' L)_ H91 RJ R ES The computation of the elements of the upmix matrix H was already discussed above. The PS encoding and PS decoding process in the PS encoder I and the PS decoder 25 13 is preferably carried out in an oversampled frequency domain. For time-to frequency transform e.g. a complex valued hybrid filter bank having a QMF (qua drature mirror filter) and a Nyquist filter may be used upstream of the PS encoder, such as the filter bank described in MPEG Surround standard (see docunent - 23 ISO/IEC 23003-1). The complex QMF representation of the signal is oversampled with factor 2 since it is complex-valued and not real-valued. This allows for time and frequency adaptive signal processing without audible aliasing artifacts. Such hybrid filter bank typically provides high frequency resolution (narrow band) at 5 low frequencies, While at high frequency, several QMF bands are grouped into a wider band. The paper "Low Complexity Parametric Stereo Coding in MPEG-4", H. Pumuhagen, Proc, of the 7flInt, Conference on Digital Audio Effects (DAFx'04), Naples, Italy, October 5-8, 2004, pages 163-168 describes an embo dinent of a hybrid filter bank (see section 3,2 and Fig, 4). This disclosure is here 10 by incorporated by reference, In this document a 48 kIz sampling rate is as sumed, with the (nominal) bandwidth of a band from a 64 band QMF bank being 375 Hz. The perceptual Bark frequency scale however asks for a bandwidth of approximately 100 Hz for frequencies below 500 Hz. Hence, the first 3 QMF bands may be split into further more narrow subbands by means of a Nyquist filter is bank, The first QMF band may be split into 4 bands (plus two more for negative frequencies), and the 2nd and 3rd QMF bands may be split into two bands each. Preferably, the adaptive L/.R or M/S encoding, on the other hand, is carried out in the critically sampled MDCT domain (e.g. as described in AAC) in order to en 20 sure an efficient quanitized signal representation The conversion of the downmix signal DMX and residual signal RES to the pseudo stereo signal L,, R, in the transform stage 2 may be carried out in the time domain since the PS encoder I and the perceptual encoder 3 may be connected in the time domain anyway. Also in the decoding system, the perceptual stereo decoder 11 and the PS decoder 13 25 are preferably connected in the time domain 'Thus, the conversion of the pseudo stereo signal Lp,, Rp to the dowmix signal DMX and residual signal RES in the transform stage 12 may be also carried out in the time domain, An adaptive L/R or M/S stereo coder such as shown as the encoder 3 in Fig. 1 is 30 typically a perceptual audio coder that incorporates a psychoacoustic model to enable high coding efficiency at low bitrates. An example for such encoder is an -24 AAC encoder, which employs transform coding in a critically sampled MDCT domain in combination with time- and frequency-variant quantization controlled by using a psycho-acoustic model. Also, the time- and frequency-variant decision between L/R and M/S coding is typically controlled with help of perceptual entro 5 py measures that are calculated using a psycho-acoustic model. The perceptual stereo encoder (such as the encoder 3 in Fig, 1) operates on a pseudo L/R stereo signal (see Lp, Rp in Fig, 1). For optimizing the coding efficien cy of the stereo encoder (in particular for making the right decision between L/R. 10 encoding and M/S encoding) it is advantageous to modify the psycho-acoustic control mechanism (including the control mechanism which decides between LIR and M/S stereo encoding and the control mechanism which controls the time- and frequency-variant quantization) in the perceptual stereo encoder in order to ac count for the signal modifications (pseudo L/i to DMX and RES conversion, fol is lowed by PS decoding) that are applied in the decoder when generating the final stereo output signal L, R. These signal modifications can affect binaural masking phenomena that are exploited in the psycho-acoustic control mechanisms. There fore, these psycho-acoustic control mechanisms should preferably be adapted ac cordingly, For this, it can be beneficial if the psycho-acoustic control mechanisms 20 do not have access only to the pseudo L/R signal (see Lp, Rp in Fig. 1) but also to the PS parameters (see 5 in Fig. 1) and/or to the original stereo signal L, R. The access of the psycho-acoustic control mechanisms to the PS parameters and to the stereo signal L, R is indicated in Fig, I by the dashed lines. Based on this informa tion, e.g. the masking threshold(s) may be adapted. 25 An alternative approach to optimize psycho-acoustic control is to augment the encoder system With a detector forming a deactivation stage that is able to effec tively deactivate PS encoding when appropriate, preferably in a time- and fre quency-variant manner. Deactivating PS encoding is e.g. appropriate when L/f :40 stereo coding is expected to be beneficial or when the psycho-acoustic control would have problems to encode the pseudo L/R signal efficiently. PS encoding -25 may be effectively deactivated by setting the downmix matrix FP in such a way that the dowmnix matrix H followed by the transform (see stage 2 in Fig, 1) corresponds to the unity matrix (i.e. to an identity operation) or to the unity matrix times a factor. Eg. PS encoding may be effectively deactivated by forcing the PS 5 parameters IID and/or ICC to IID =0 dB and ICC= 0, In this case the pseudo stereo signal Lp, R, corresponds to the stereo signal L, R as discussed above. Such detector controlling a PS parameter modification is shown in Fig, 4. Here, the detector 20 receives the PS parameters 5 determined by the parameter estimat 10 lig stage 9. When the detector does not deactivate the PS encoding, the detector 20 passes the PS parameters through to the downmix stage 8 and to the multiplex er 7, i.e. in this case the PS parameters 5 correspond to the PS parameters 5' fed to the downmix stage 8. In case the detector detects that PS encoding is disadvanta geous and PS encoding should be deactivated (for one or more frequency bands), 15 the detector modifies the affected PS parameters 5 (e.g. set the PS parameters lID and/or ICC to IID = 0 dB and ICC =0) and feeds the modified PS parameters 5' to downmix stage 8, The detector can optionally also consider the left and right signals L, R for deciding on a PS parameter modification (see dashed lines in Fig. 4). 20 In the following figures, the term QMF (quadrature mirror niter or filter bank) also includes a QMF subband filter bank in combination with a Nyquist filter bank, i.e. a hybrid filter bank structure. Furthermore, all values in the description below may be frequency dependent, e.g. different downmix and upmnix matrices 25 may be extracted for different frequency ranges. Furthermore, the residual coding may only cover part of the used audio frequency range (i the residual signal is only coded for a part of the used audio frequency range). Aspects of downmix as will be outlined below may for some frequency ranges occur in the QMF domain (e.g. according to prior art), while for other frequency ranges only eg. phase as 30 pects will be dealt with in the complex QMF domain, whereas amplitude trans formation is dealt with in the real-valued MDCT domain.

-26 In Fig. 5, a conventional PS encoder system is depicted. Each of the stereo chdn nels L, R, is at first analyzed by a complex QMF 30 with M subbands, e.g. a QMF with M = 64 subbands, The subband signals are used to estimate PS parameters 5 5 and a dowmnix signal DMX in a PS encoder 31. The downmix signal DMX is used to estimate SR (Spectfal Bandwidth Replication) parameters 33 in an SBR enoder 32. The SBR encoder 32 extracts the SBR parameters 33 representing the spectral envelope of the original high band signal, possibly in combination with noise and tonality measures. As opposed to the PS encoder 31, the SBR encoder 10 32 does not affect the signal passed on to the core coder 34. The dowinmix signal DVDX of the PS encoder 31 is synthesized using an inverse QMF 35 with N sub bands. E.g. a complex QMF with N 32 may be used, where only the 32 lowest subbands of the 64 subbands used by the PS encoder 31 and the SBR encoder 32 are synthesized. Thus, by using half the number of subbands for the same frame 15 size, a time domain signal of half the bandwidth compared to the input is ob tained, and passed into the core coder 34. Due to the reduced bandwidth the sam pling rate can be reduced to the half (not shown). The core encoder 34 performs perceptual encoding of the mono input signal to generate a bitstream 36. The PS parameters 5 are embedded in the bitstream 36 by a multiplexer (not shown). 20 Fig. 6 shows a further embodiment of an encoder system which combines PS cod ing using a residual with a stereo core coder 48, with the stereo core coder 48 be ing capable of adaptive L/R or M/S perceptual stereo coding. This embodiment is merely illustrative for the principles of the present application. It is understood 25 that modifications and variations of the embodiment will be apparent to others skilled in the art. The input channels L, R representing the left and right original channels are analyzed by a complex QMF 30, in a similar way as discussed in connection with Fig. 5. In contrast to the PS encoder 31 in Fig. 5, the PS encoder 41 in Fig. 6 does not only output a downmix signal DMX but also outputs a resi 30 dual signal RES. The downmix signal DIX is used by an SBR encoder 32 to de termine SBR parameters 33 of the dowumnix signal DMX. A fixed DMX/RES to ~ 27 pseudo L/R transform (i.e. an M/S to L/R transform) is applied to the downmix DMX and the residual RES signals in a transform stage 2. The transform stage 2 in Fig. 6 corresponds to the transform stage 2 in Fig. 1. The transform stage 2 creates a "pseudo" left and right channel signal L,, Rp for the core encoder 48 to 5 operate on. In this embodiment, the inverse L/R to M/S transform is applied in the QMF domain, prior to the subband synthesis by filter banks 35. Preferably, the number N (e.g. N= 32) of subbands for the synthesis corresponds to half the number M (e.g. M = 64) of subbands used for the analysis and the core coder 48 operates at half the sampling rate. It should be noted that there is no restriction to 10 use 64 subband channels for the QMF analysis in the encoder, and 32 subbands for the synthesis, other values are possible as well, depending on which sampling rate is desired for the signal received by the core coder 48. The core stereo encod er 48 performs perceptual encoding of the signal of the filter banks 35 to generate a bitstream signal 46. The PS parameters 5 are embedded in the bitstream signal 15 46 by a multiplexer (not shown). Optionally, the PS parameters and/or the original L/R input signal may be used by the core encoder 48, Such information indicates to the core encoder 48 how the PS encoder 41 rotated the stereo space. The infor mation may guide the core encoder 48 how to control quantization in a percep tually optimal way. This is indicated in Fig. 6 by the dashed lines, 20 Fig. 7 illustrates a further embodiment of an encoder system which is similar to the embodiment in Fig. 6, In comparison to the embodiment of Fig. 6, in Fig. 7 the SBR encoder 42 is connected upstream of the PS encoder 41. In Fig. 7 the SBR encoder 42 has been moved prior to the PS encoder 41, thus operating on the 25 left and right channels (here: in the QMF domain), instead of operating on the dowarnix signal DMX as in Fig. 6. Due to the re-arrangement of the SBR encoder 42, the PS encoder 41 may be con figured to operate not on the full bandwidth of the input signal but eg. only on the 30 frequency range below the SBR crossover frequency. In Fig. 7, the SBR parame ters 43 are in stereo for the SBR range, and the output from the corresponding PS -28 decoder as will be discussed later on in connection with Fig. 15 produces a stereo source frequency range for the SBR decoder to operate on. This modification, i.e. connecting the SBR encoder module 42 upstream of the PS encoder module 41 in the encoder system and correspondingly placing the SBR decoder module after 5 the PS decoder module in the decoder system (see Fig 15), has the benefit that the use of a decorrelated signal for generating the stereo output can be reduced. Please note that in case no residual signal exists at all or for a particular frequency band, a decorrelated version of the downmix signal DMX is used instead in the PS de coder, However, a reconstruction based on a decorrelated signal reduces the audio 10 quality. Thus, reducing the use of the decorrelated signal increases the audio qual ity. This advantage of the embodiment in Fig, 7 in comparison to the embodiment in Fig. 6 will be now explained more in detail with reference to Figs. 8a to 8d. 15 In Fig, 8a, a time frequency representation of one of the two output channels L, R (at the decoder side) is visualized, In ease of Fig. 8a, an encoder is used where the PS encoding module is placed in front of the SBR encoding module such as the encoder in Fig. 5 or Fig. 6 (in the decoder the PS decoder is placed after the SBR. 20 decoder, see Fig. 14). Moreover, the residual is coded only in a low bandwidth frequency range 50, which is smaller than the frequency range 51 of the core cod er. As evident from the spectrogram visualization in Fig. 8a, the frequency range 52 where a decorrelated signal is to be used by the PS decoder covers all of the frequency range apart from the lower frequency range 50 covered by the use of 25 the residual signal. Moreover, the SBR covers a frequency range 53 starting sig nificantly higher than that of the decorrelated signal. Thus, the entire frequency range separates in the following frequency ranges: in the lower frequency range (see range 50 in Fig. 8a), waveform coding is used; in the middle frequency range (see intersection of frequency ranges 51 and 52), waveform coding in combination 30 with a decorrelated signal is used; and in the higher frequency range (see frequen cy range 53), a SBR regenerated signal which is regenerated from the lower fre- -29 quencies is used in combination with the decorrelated signal produced by the PS decoder. In Fig. 8b, a time frequency representation of one of the two output channels L, R. 5 (at the decoder side) is visualized for the case when the SBR encoder is connected upstream of the PS encoder in the encoder system (and the SBR decoder is located after the PS decoder in the decoder systern), In Fig 8b a low bitrate scenario is illustrated, with the residual signal bandwidth 60 (where residual coding is per formed) being lower than the bandwidth of the core coder 61. Since the SBR de 10 coding process operates on the decoder side after the PS decoder (see Fig, 15), the residual signal used for the low -frequencies is also used for the reconstruction of at least a part (see frequency range 64) of the higher frequencies in the SBR range 63, 15 The advantage becomes even more apparent when operating on intermediate bi trates where the residual signal bandwidth approaches or is equal to the core coder bandwidth. In this case, the time frequency representation of Fig. 8a (where the order of PS encoding and SBR encoding as shown in Fig, 6 is used) results in the time frequency representation shown in Fig, 8c, In Fig. Sc, the residual signal es 20 sentially covers the entire lowband range 51 of the core coder; in the SBR fre quency range 53 the decorrelated signal is used by the PS decoder, In Fig. 8d, the time frequency representation in case of the preferred order of the encod ing/decoding modules (i.e. SBR encoding operating on a stereo signal before PS encoding, as shown in Fig. 7) is visualized. Here, the PS decoding module oper 25 ates prior to the SBR decoding module in the decoder, as shown in Fig. 15. Thus, the residual signal is part of the low band used for high frequency reconstruction. When the residual signal bandwidth equals that of the mono downmix signal bandwidth, no decorrelated signal information will be needed to decoder the out put signal (see the fRill frequency range being hatched in Fig. 8d). 30 -30 In Fig. 9a, an embodiment of the stereo core encoder 48 with adaptively selectable L/R or M/S stereo encoding in the MDCT transform domain is illustrated. Such stereo encoder 48 may be used in Figs, 6 and 7. A mono core encoder 34 as shown in Fig. 5 can be considered as a special case of the stereo core encoder 48 5 in Fig. 9a, where only a single mono input channel is processed (ie. where the second input channel, shown as dashed line in Fig. 9a, is not present) In Fig, 9b, an embodiment of a more generalized encoder is illustrated. For mono signals, encoding can be switched between coding in a linear predictive domain to (see block 71) and coding in a transform domain (see block 48). Such type of core coder introduces several coding methods which can adaptively be used dependent upon the characteristics of the input signal Here, the coder can choose to code the signal using either an AAC style transform coder 48 (available for mono and ste reo signals, with adaptively selectable L/R. or M/S coding in case of stereo sig 15 nals) or an AMR-WB+ (Adaptive Multi Rate - WideBand Plus) style core coder 71 (only available for mono signals). The AMR-WB+ core coder 71 evaluates the residual of a linear predictor 72, and in turn also chooses between a transform coding approach of the linear prediction residual or a classic speech coder ACELP (Algebraic Code Excited Linear Prediction) approach for coding the linear predic 20 tion residual, For deciding between AAC style transform coder 48 and the AMR WB+ style core coder 71, a mode decision stage 73 is used which decides based on the input signal between both coders 48 and 71. The encoder 48 is a stereo AAC style MDCT based coder. When the mode deci 25 sion 73 steers the input signal to use MDCT based coding, the mono input signal or the stereo input signals are coded by the AAC based MDCT coder 48. The MDCT coder 48 does an MDCT analysis of the one or two signals in MDCT stages 74. In case of a stereo signal, further, an M/S or L/R decision on a frequen cy band basis is performed in a stage 75 prior to quantization and coding. L/R 30 stereo encoding or M/S stereo encoding is selectable in a frequency-variant man ner The stage 75 also performs a L/R to M/S transform. If M/S encoding is de- -.31 cided for a particular frequency band the stage 75'butputs an M/S signal for this frequency band. Otherwise, the stage 75 outputs a L/R signal for this frequency band. 5 Hence, when the transform coding mode is used, the fill efficiency of the stereo coding functionality of the underlying core coder can be used for stereo, When the mode decision 73 steers the mono signal to the linear predictive domain coder 71, the mono signal is subsequently analyzed by means of linear predictive 10 analysis in block 72, Subsequently, a decision is made on whether to code the LP residual by means of a time-domain ACELP style coder 76 or a TCX style coder 77 (Transform Coded eXcitation) operating in the MDCT domain. The linear pre dictive domain coder 71 does not have any inherent stereo coding capability. Hence, to allow coding of stereo signal with the linear predictive domain coder 15 71, an encoder configuration similar to that shown in Fig. 5 can be used, In this configuration, a PS encoder generates PS parameters 5 and a mono downinix sig nal DMX, which is then encoded by the linear predictive domain coder. Fig. 10 illustrates a further embodiment of an encoder system, wherein parts of 20 Fig. 7 and Fig, 9 are combined in a new fashion, The DMX/RES to pseudo L/R block 2, as outlined in Fig. 7, is arranged within the AAC style downmix coder 70 prior to the stereo MDCT analysis 74. This embodiment has the advantage that the DMX/RES to pseudo L/R transform 2 is applied only when the stereo MDCT core coder is used. Hence, when the transform coding mode is used, the full efficiency 25 of the stereo coding functionality of the underlying core coder can be used for stereo coding of the frequency range covered by the residual signal. While the mode decision 73 in Fig. 9b operates either on the mono input signal or on the input stereo signal, the mode decision 73' in Fig. 10 operates on the 30 downmix signal DMX and the residual signal RES. In case of a mono input sig- -32 nal, the mono signal can directly be used as the DMX signal, the RES signal is set to zero, and the PS parameters can default to IID=; 0 dB and ICC= 1. When the mode decision 73' steers the downmix signal DMX to the linear predic 5 tive domain coder 71, the downmix signal DMX is subsequently analyzed by means of linear predictive analysis in block 72. Subsequently, a decision is made on whether to code the LP residual by means of a time-domain ACELP style cod er 76 or a TCX style coder 77 (Transform Coded eXcitation) operating in the MDCT domain. The linear predictive domain coder 71 does not have any inherent 10 stereo coding capability that can be used for coding the residual signal in addition to the downmix signal DMX Hence, a dedicated residual coder 78 is employed for encoding the residual signal RES when the downmix signal DMX is encoded by the predictive domain coder 71. E.g, such coder 78 may be a mono AAC cod er. 15 It should be noted that the coder 71 and 78 in Fig. 10 may be omitted (in this case the mode decision stage 73' is not necessary anymore). Fig. 11 a illustrates a detail of an alternative further embodiment of an encoder 20 system which achieves the same advantage as the embodiment in Fig, 10, In con trast to the embodiment of Fig. 10, in Fig. I a the DMXIRES to pseudo L!R trans form 2 is placed after the MDCT analysis 74 of the core coder 70, i.e the trans form operates in the MDCT domain. The transform in block 2 is linear and time invariant and thus can be placed after the MDCT analysis 74. The remaining 25 blocks of Fig. 10 which are not shown in Fig. II can be optionally added in the same way in Fig. 11 a. The MDCT analysis blocks 74 may be also alternatively placed after the transform block 2.. Fig, 1 lb illustrates an implementation of the embodiment in Fig. 11 a, In Fig. 11 b, 30 an exemplary implementation of the stage 75 for selecting between M/S or I/R encoding is showti. The stage 75 comprises a sum and difference transform stage - 33 98 (more precisely a L/R to MIS transform stage) which receives the pseudo ste reo signal L,, R,. The transform stage 98 generates a pseudo mid/side signal M., SP by performing ail L/R. to M/S transform. Except for a possible gain factor, the following applies: My= DMX and Sp= RES. The stage 75 decides between LR or M/S encoding. Based on the decision, either the pseudo stereo signal L., Rp or the pseudo mid/side signal 1p, S, are selected (see selection switch) and encoded in AAC block 97. It should be noted that also two AAC blocks 97 may be used (not shown in Fig. 11b), with the first AAC 10 block 97 assigned to the pseudo stereo signal I, Ry and the second AAC block 97 assigned to the pseudo mid/side signal M,, S, In this case, the L/R or M/S selec tion is performed by selecting either the output of the first AAC block 97 or the output of the second AAC block 97, 15 Fig. 11 c shows an alternative to the embodiment in Fig. I 1a. Here, no explicit transform stage 2 is used. Rather; the transform stage 2 and the stage 75 is com bined in a single stage 75'. The dow-nmix signal DMX and the residual signal RES are fed to a sum and difference transform stage 99 (onre precisely a DMX/RES to pseudo L/R transform stage) as part of stage 75'. The transform 20 stage 99 generates a pseudo stereo signal L,, R,. The DMX/RES to pseudo L/R transform stage 99 in Fig, 11 c is similar to the L/R to M/S transfonn stage 98 in Fig 1 lb (expect for a possibly different gain factor). Nevertheless, in Fig, I1c the selection between MIS and L/R decoding needs to be inverted in comparison to Fig, 11 b. Note that in both Fig. I1 b and Fig. 11, the position of the switch for the 25 L/R or M/S selection is shown in LP/R, position, which is the tpper one in Fig. 11 b and the lower one in Fig. 11e. This visualizes the notion of the inverted mean ing of the L/R or MIS selection. It should be noted that the switch in Figs. 11 b and I Ic preferably exists indivi 30 dually -for each frequency band in the MDCT domain such that the selection be tween L/R and MI/S can be both time- and frequency-variant. In other words; the -34 position of the switch is preferably frequency-variant, The transform stages 98 and 99 may transform the whole used frequency range or may only transform a single frequency band. 5 Moreover, it should be noted that all blocks 2, 98 and 99 can be called "sum and difference transform blocks" since all blocks implement a transform matrix in the form of l0 Merely, the gain factor c may be different in the blocks 2, 98, 99. In Fig. 12, a further embodiment of an encoder system is outlined. It uses an ex tended set of PS parameters which, in addition to lID an ICC (described above), 15 includes two further parameters IPD (inter channel phase difference, see pipd be low) and OPD (overall phase difference, see pa below) that allow to characterize the phase relationship between the two channels L and R of a stereo signal. An example for these phase parameters is given in ISO/IEC 14496-3 subclause 8.6,4.6.3 which is hereby incorporated by reference. When phase parameters are 20 used, the resulting uprnix matrix HcoM,,A.- (and its inverse H ) becomes complex-valued, according to: Hco =HAU A 1 11, where exp(fjpo) 0 exp jps, 25 and where ?PPPd -Po -35 The stage 80 of the PS encoder which operates in the complex QMF domain only takes care of phase dependencies between the channels L, R, The dowomix rota tion (ie. the transformation from the L/R domain to the DMX/RES domain which was described by the matrix -H above) is taken care of in the MDCT domain as 5 part of the stereo core coder 81. Hence, the phase dependencies between the two channels are extracted in the complex QMF domain, while other, real-valued, waveform dependencies are extracted in the real-valued critically sampled MDCT domain as part of the stereo coding mechanism of the core coder used. This has the advantage that the extraction of linear dependencies between the channels can 10 be tightly integrated in the stereo coding of the core coder (though, to prevent aliasing in the critical sampled MDCT domain, only for the frequency range that is covered by residual coding, possibly minus a "guard band" on the frequency axis). 15 The phase adjustment stage 80 of the PS encoder in Fig. 12 extracts phase related PS parameters, e.g. the parameters IPD (inter channel phase difference) and OPD (overall phase difference). Hence, the phase adjustment matrix Hff that it pro duces may be according to the following: fexpexp( 20 As discussed before, the downmix rotation part of the PS module is dealt with in the stereo coding module 81 of the core coder in Fig. 12. The stereo coding mod ule 81 operates in the MDCT domain and is shown in Fig. 1 The stereo coding module 81 receives the phase adjusted stereo signal L 9 , Rp in the MDCT domain, This signal is dowmnixed in a downmix stage 82 by a downmix rotation matrix 25 LV' which is the real-valued part of a complex downmix matrix H P as discussed above, thereby generating the downmix signal DMX and residual signal RES. The downmix operation is followed by the inverse LR to M/S transform according to the present application (see transform stage 2), thereby generating a pseudo stereo signal L, R,. The pseudo stereo signal L., R, is processed by the -36 stereo coding algorithm (see adaptive MIS or LR stereo encoder 83), in this par ticular embodiment a stereo coding mechanism that depending on perceptual en tropy criteria decides to code either an L/R representation or an M/S representa tion of the signal. This decision is preferably time- and frequency-variant. 5 In Fig. 14 an embodiment of a decoder system is shown which is suitable to de code a bitstream 46 as generated by the encoder system shown in Fig. 6. This em bodiment is merely illustrative for the principles of the present application. It is understood that modifications and variations of the embodiment will be apparent 10 to others skilled in the art. A core decoder 90 decodes the bitstream 46 into pseu do left and right channels, which are transfonned in the QMF domain by filter banks 91. Subsequently, a fixed pseudo L/R to DMX/RES transform of the result ing pseudo stereo signal L, R, is performed in transform stage 12, thus creating a downrmix signal DMX and a residual signal RES When using SBR coding, these is signals are low band signals, e,g, the downmix signal DMX and residual signal RES may only contain audio information for the low frequency band up to ap proximately 8 kHz. The downmix signal DMX is used by an SBR decoder 93 to reconstruct the high frequency band based on received SBR parameters (not shown), Both the output signal (including the low and reconstructed high frequen 20 cy bands of the dowmnix signal DMX) from the SBR decoder 93 and the residual signal RES are input to a PS decoder 94 operating in the QMF domain (in particu lar in the hybrid QMF+Nyquist filter domain). The dowunix signal DMX at the input of the PS decoder 94 also contains audio information in the high frequency band (e~g. up to 20 kHz), whereas the residual signal RES at the input of the PS 25 decoder 94 is a low band signal (e.g. limited up to 8 kz), Thus, for the high fre quency band (e.g. for the band from 8 kflz to 20 kz), the PS decoder 94 uses a decorrelated version of the downmix signal DMX instead of using the band li mited residual signal RES. The decoded signals at the output of the PS decoder 94 are therefore based on a residual signal only up to 8 kHz. After PS decoding, the 30 two output channels of the PS decoder 94 are transformed in the time domain by filter banks 95, thereby generating the output stereo signal L, R, - 37 In Fig. 15 an embodiment of a decoder system is shownwhich is suitable to de code the bitstream 46 as generated by the encoder system shown in Fig. 7. This embodiment is merely illustrative for the principles of the present application. It is 5 understood that modifications and variations of the embodiment will be apparent to others skilled in the art. The principle operation of the embodiment in Fig, 15 is similar to that of the decoder system outlined in Fig. 14, In contrast to Fig. 14, the SPR decoder 96 in Fig. 15 is located at the output of the PS decoder 94. Moreo ver, the SBR decoder makes use of SBR parameters (not shown) forming stereo 10 envelope data in contrast to the mono SBR parameters in Fig, 14. The downmix and residual signal at the input of the PS decoder 94 are typically low band sig nals, eg. the downmix signal DMIX and residual signal RES may contain audio information only for the low frequency-band, e.g. up to approximately 8 kIz. Based on the low band downmix signal DMX and residual signal RES, the PS 15 encoder 94 determines a low band stereo signal, e.g. up to approximately 8 kz. Based on the low band stereo signal and stereo SBR parameters, the SBR decoder 96 reconstructs the high frequency part of the stereo signal. In comparison to the embodiment in Fig.14, the embodiment in Fig. 15 offers the advantage that no decorrelated signal is needed (see also Fig. 8d) and thus an enhanced audio quality 20 is achieved, whereas in Fig. 14 for the high frequency part a decolTelated signal is needed (see also Fig. 8c), thereby reducing the audio quality. Fig. 16a shows an embodiment of a decoding system which is inverse to the en coding system shown in Fig. I I a, The incoming bitstream signal is fed to a de 25 coder block 100, which generates a first decoded signal 102 and a second decoded signal 103. At the encoder either MS coding or L/R coding was selected. This is indicated in the received bitstream. Based on this infonnation, either M/S orLa is selected in the selection stage 101. In case MiS was selected in the encoder, the first 102 and second 103 signals are converted into a (pseudo) L/I signal. In case 30 L/R was selected in the encoder, the first 102 and second 103 signals may pass the stage 101 without transformation, The pseudo L/R signal L,, Rp at the output of -38 stage 101 is converted into an DMIXRES signal by the transform stage 12 (this stage quasi performs a L/R to MIS transfonn). Preferably, the stages 100, 101 and 12 in Fig. 16a operate in the MDCT domain. For transforming the downmix sig nal DMX and residual signals RES into the time domain, conversion blocks 104 5 may be used, Thereafter, the resulting signal is fed to a PS decoder (not shown) and optionally to an SBR decoder as shown in Figs. 14 and 15. The blocks 104 may be also alternatively placed before block 12. Fig. 16b illustrates an implementation of the embodiment in Fig. 16a. In Fig. 16b, 10 an exemplary implementation of the stage 101 for selecting between M/S or L/R. decoding is shown. The stage 101 comprises a sum and difference transform stage 105 (M/S to L/R transform) which receives the first 102 and second 103 signals. Based on the encoding information given in the bitstream, the stage 101 selects ts either L/P. or M/S decoding. When L/R decoding is selected, the output signal of the decoding block 100 is fed to the transform stage 12. Fig. 16c shows an alternative to the embodiment in Fig. 16a. Here, no explicit transform stage 12 is used. Rather, the transform stage 12 and the stage 101 are 20 merged in a single stage 101' The first 102 and second 103 signals are fed to a sum and difference transform stage 105' (more precisely a pseudo LIR to DMX/RES transform stage) as part of stage 101'. The transform stage 105' gene rates a DMX/RES signal. The transform stage 105' in Fig. 16e is similar or iden tical to the transform stage 105 in Fig. 16b (expect for a possibly different gain 25 factor). In Fig. 16 the selection between M/S and L/R. decoding needs to be in verted in comparison to Fig. 16b. In Fig. 16e the switch is in the lower position, whereas in Fig. 16b the switch is in the upper position, This visualizes the inver sion of the LIR or M/S selection (the selection signal may be simply inverted by an inverter). 30 - 39~ It should be noted that the switch in Figs. 16b and 16c preferably exists indivi dually for each frequency band in the MDCT domain such that the selection be tween L/R and M/S can be both time- and frequency-variant. The transform stages 105 and 105' may transform the whole used frequency range or may only trans 5 form a single frequency band. Fig. 17 shows a further embodiment of an encoding system for coding a stereo signal L, R into a bitstream signal. The encoding system comprises a downmix stage 8 for generating a downmix signal DMX and a residual signal RES based on 10 the stereo signal. Further, the encoding system comprises a parameter determining stage 9 for determining one or more parametric stereo parameters 5. Further, the encoding system comprises means 110 for perceptual encoding downstream of the downmix stage 8. The encoding is selectable: - encoding based on a sum signal of the downmix signal DMX and the resi 15 dual signal RES and based on a difference signal of the downnix signal DMX and the residual signal RES, or - encoding based on the downinix signal DMX and the residual signal RES. Preferably, the selection is time- and frequency-variant. 20 The encoding means 110 comprises a sum and difference tiansfon stage 111 which generates the sum and difference signals. Further, the encoding means 110 comprise a selection block 112 for selecting encoding based on the sum and dif ference signals or based on the .downmix signal DMX and the residual signal 25 RES. Furthermore, an encoding block 113 is provided. Alternatively, two encod ing blocks 113 may be used, with the first encoding block 113 encoding the DMX and RES signals and the second encoding block 113 encoding the sum and differ ence signals, In this case the selection 112 is downstream of the two encoding blocks 113. 30 The sum and difference transform in block 111 is of the form 0 1j The transform block 11 may correspond to transform block 99 in Fig. 11e. 5 The output of the perceptual encoder 110 is combined with the parametric stereo parameters 5 in the multiplexer 7 to form the resulting bitstream 6. In contrast to the structure in Fig. 17, encoding based on the downmix signal 10 DMX and residual signal RES may be realized when encoding a resulting signal which is generated by transforming the downmix signal DMX and residual signal RES by two serial sum and difference transforms as shown in Fig. 11 b (see the two transform blocks 2 and 98). The resulting signal after two sum and difference transforms corresponds to the downmix signal DMX and residual signal RES (ex 15 cept for a possible different gain factor). Fig. 18 shows an embodiment of a decoder system which is inverse to the encoder system in Fig, 17. The decoder system comprises means 120 for perceptual decod ing based on bitstream signal. Before decoding, the PS parameters are separated 20 from the bitstream signal 6 in demultiplexer 10, The decoding means 120 com prise a core decoder 121 which generates a first signal 122 and a second signal 123. (by decoding). The decoding means output a downmix signal DMX and a residual signal RES. 25 The downumix signal DMX and the residual signal RES are selectively - based on the sum of the first signal 122 and of the second signal 123 and based on the difference of the first signal 122 and of the second signal 123 or - based on the first signal 122 and based on the second signal 123. 30 -41 Preferably, the selection is time- and frequency-variant. The selection is per formed in the selection stage 125. The decoding means 120 comprise a sum and difference transform stage 124 5 which generates sum and difference signals. The sum and difference transform in block 124 is of the form 10 The transform block 124 may correspond to transform block 105 in Fig. 16c. After selection, the DMX and RES signals are fed to an upmix stage 126 for gene rating the stereo signal L, R based on the downmix signal DMX and the residual 15 signal RES. The upmix operation is dependent on the PS parameters 5. Preferably, in Figs. 47 and 18 the selection is frequency-variant. In Fig. 17, e.g. a time to frequency transform (e.g. by a MDCT or analysis filter bank) may be per formed as first step in the perceptual encoding means 110. In Fig. 18, e.g. a fte 20 quency to time transform (e.g. by an inverse MDCT or synthesis filter bank) may be performed as the last step in the perceptual decoding means 120. It should be noted that in the above-described embodiments, the signals, parame ters and matrices may be frequency-variant or frequency-invariant and/or time 25 variant or time-invariant. The described computing steps may be carried out fre quency-wise or for the complete audio band. Moreover, it should be noted that -the various sum and difference transforms, i.e. the DMX/RES to pseudo LR transform, the pseudo L/R to DMX/RES transform, 30 the L/R to M/S transform and the M/S to L/R transfonn, are all of the form -42 11 Merely, the gain factor c may be different. Therefore, in principle, each of these 5 transforms may be exchanged by a different transform of these transforms. If the gain is not correct during the encoding processing, this may be compensated in the decoding process. Moreover, when placing two same or two different of the sum and difference transforms is series, the resulting transform corresponds to the identity matrix' (possibly, multiplied by ? gain factor). In an encoder system comprising both a PS encoder and a SBR encoder, different PS/SBR configurations are possible. In a first configuration, shown in Fig. 6, the SBR encoder 32 is connected downstream of the PS encoder 41. In a second con figuration, shown in Fig. 7, the SBR encoder 42 is connected upstream of the PS 15 encoder 41, Depending upon e.g. the desired target bitrate, the properties of the core encoder, and/or one or more various other factors, one of the configurations can be preferred over the other in order to provide best performance. Typically, for lower bitrates; the first configuration can be preferred, while for higher bi trates, the second configuration can be preferred. Hence, it is desirable if an en 20 coder system supports both different configurations to be able to choose a pre ferred configuration depending upon e.g. desired target bitrate and/or one or more other criteria. Also in a decoder system comprising both a PS decoder and a SBR decoder, dif 25 ferent PS/SBR configurations are possible. In a first configuration, shown in Fig, 14, the SBR decoder 93 is connected upstream of the PS decoder 94. In a second configuration, shown in Fig. 15, the SBR decoder 96 is connected down stream of the PS decoder 94, In order to achieve correct operation, the configura tion of the decoder system has to match that of the encoder system. If the encoder 30 is configured according to Fig. 6, then the decoder is correspondingly configured - 43 according to Fig. 14, If the encoder is configured according to Fig, 7, then the decoder is correspondingly configured according to Fig. 15, In order to ensure correct operation, the encoder preferably signals to the decoder which PS/SBR configuration was chosen -for encoding (and thus which PS/SBR configuration is 5 to be chosen for decoding). Based on this information, the decoder selects the appropriate decoder configuration As discussed above, in order to ensure correct decoder operation, there is prefera bly a mechanism to signal from the encoder to the decoder which configuration is 10 to be used in the decoder. This can be done explicitly (e.g. by means of an dedi cated bit or field in the configuration header of the bitstrearn as discussed below) or implicitly (e.g. by checking whether the SBR data is mono or stereo in case of PS data being present), 15 As discussed above, to signal the chosen PS/SBR configuration, a dedicated ele inent in the bitstream header of the bitstream conveyed from the encoder to the decoder may be used. Such a bitstream header carries necessary configuration information that is needed to enable the decoder to correctly decode the data in the bitstream. The dedicated element in the bitstream header may be e,g. a one bit 20 flag, a field, or it may be an index pointing to a specific entry in a table that speci fies different decoder configurations. Instead of including in the bitstream header an additional dedicated element for signaling the PS/SBR configuration, information already present in the bitstream 25 may be evaluated at the decoding system for selecting the correct PS/SBR confi guration. E.g. the chosen PS/SBR configuration may be derived from bitstream header configuration information for the PS decoder and SBR decoder. This con figuration information typically indicates whether the SBR. decoder is to be confi gured for mono operation or stereo operation. If, for example, a PS decoder is 30 enabled and the SBR decoder is configured for mono operation (as indicated in the configuration information), the PS/SBR configuration according to Fig. 14 can -44 be selected, If a PS decoder is enabled and the SBR decoder is configured for stereo operation, the PS/S BR configuration according to Fig. 15 can be selected. The above-described embodiments are merely illustrative for the principles of the present application, It is understood that modifications and variations of the arrangements and the 5 details described herein will be apparent to others skilled in the art. It is the intent, therefore, that the scope of the application is not limited by the specific details presented by way of description and explanation of the embodiments herein. The systems and methods disclosed in the application may be implemented as software, firmware, hardware or a combination thereof. Certain components or all components may be 10 implemented as software running on a digital signal processor or microprocessor, or implemented as hardware and or as application specific integrated circuits. Typical devices making use of the disclosed systems and methods are portable audioplayers, mobile communication devices, set-top-boxes, TV-sets, AVRs (audio-video receiver), personal computers etc. 15 The above references to and descriptions of prior proposals or products are not intended to be, and are not to be construed as, statements or admissions of common general knowledge in the art, In particular, the above prior art discussion does not relate to what is conmonly or well known by the person skilled in the art, but assists in the understanding of the inventive step of the present invention of which the identification of pertinent prior art proposals is but 20 one pait Throughout the specification and claims the word "comprise" and its derivatives are intended to have an inclusive rather than exclusive meaning unless the contrary is expressly stated or the context requires otherwise, That is, the word "comprise" and its derivatives will be taken to indicate the inclusion of not only the listed components, steps or features that it directly 25 references, but also other components, steps or features not specifically listed, unless the contrary is expressly stated or the context requires otherwise.

Claims (20)

1. An encoder system configured for encoding a stereo signal to a bitstream sig 5 nal, the encoder system comprising: - a downmixing means configured for generating a downmix signal and a re sidual signal based on the stereo signal; - a parameter determining means configured for determining one or more parametric stereo parameters; wherein the encoder system is configured to 10 select in a frequency-variant or frequency-invariant manner between para metric stereo encoding the stereo signal to the bitstream signal or left/right encoding the stereo signal to the bitstream signal; and - perceptual encoding means downstream of the downmixing means, where in the perceptual encoding means are configured for selecting 15 - encoding based on a sum of the downmix signal and the residual signal and based on a difference of the downmix signal and the re sidual signal, or - encoding based on the downmix signal and based on the residual signal 20 in a frequency-variant or frequency-invariant manner.

2. The encoder system of claim 1, wherein the perceptual encoding means com prise: - a transform means configured for performing a transform based on the 25 downmix signal and the residual signal, thereby generating a pseudo stereo signal; and - a perceptual encoder configured for encoding the pseudo stereo signal, wherein the perceptual encoder is configured for selecting - left/right perceptual encoding or 30 - mid/side perceptual encoding in a frequency-variant or frequency-invariant manner. - 46

3. The encoder system of claim 2, wherein the perceptual encoder is configured to perform a left/right to mid/side transform based on the pseudo stereo signal.

4. The encoder system of any one of the preceding claims, wherein the parametric 5 stereo parameters comprise - a frequency-variant or a frequency-invariant parameter indicating an in ter-channel intensity difference and - a frequency-variant or a frequency-invariant parameter indicating an in ter-channel cross-correlation. 10

5. The encoder system of any one of claims 2 to 4, wherein the pseudo stereo signal is proportional to the stereo signal for a frequency band if, for the fre quency band, the left and right channels of the stereo signal are independent and have the same level. 15

6. The encoder system of any one of claims 2 to 5, wherein - a first channel of the pseudo stereo signal is proportional to the sum of the downmix signal and the residual signal; and - a second channel of the pseudo stereo signal is proportional to the dif 20 ference of the downmix signal and the residual signal.

7. The encoder system of any one of the preceding claims, wherein the perceptual encoding means comprise an Advanced Audio Coding based stereo encoder. 25

8. The encoder system of any one of the preceding claims, wherein the perceptual encoding means comprise a psycho-acoustic control mechanism, and the psy cho-acoustic control mechanism has access - to one or more of the parametric stereo parameters and/or - to the stereo signal. 30

9. The encoder system of any one of the preceding claims, - 47 wherein the encoder system further comprises deactivation means configured to effectively deactivate parametric stereo encoding in a frequency-variant or frequency-invariant manner. 5 10. The encoder system of claim 9, wherein the deactivation means is configured to receive parametric stereo parameter values from the parameter determining means, and the deactivating means is configured to send - for effectively deac tivating parametric stereo encoding - modified parametric stereo parameter values to the downmixing means.

10

11. The encoder system of claim 10, wherein the modified parametric stereo pa rameter values comprise - an inter-channel intensity difference value of roughly 0 dB and - an inter-channel cross-correlation value of roughly 0. 15

12. The encoder system of any one of the preceding claims, wherein the encoder system further comprises a Spectral Band Replication encoder.

13. The encoder system of claim 12, wherein the Spectral Band Replication encod 20 er is connected upstream of the downmixing means.

14. The encoder system of any one of the preceding claims, wherein the downmix ing means and the parameter determining means are configured to operate in an oversampled frequency domain. 25

15. The encoder system of any one of the preceding claims, wherein the perceptual encoding in the perceptual encoding means is carried out in a critically sam pled MDCT domain. - 48

16. A decoder system configured for decoding a bitstream signal including one or more parametric stereo parameters to a stereo signal, the decoder system com prising: - perceptual decoding means configured for decoding based on the bitstream 5 signal, wherein the decoding means are configured to generate by decod ing a first signal and a second signal and to output a downmix signal and a residual signal, wherein the decoding means are configured to select the downmix signal and the residual signal - based on a sum of the first signal and of the second signal and 10 based on a difference of the first signal and of the second signal or - based on the first signal and based on the second signal in a frequency-variant or frequency-invariant manner; and - upmixing means configured for performing an upmix operation for gener 15 ating the stereo signal based on the downmix signal and the residual sig nal, with the upmix operation of the upmixing means being dependent on the one or more parametric stereo parameters; wherein the decoder system is configured to switch in a frequency-variant or frequency-invariant manner between 20 - parametric stereo decoding the bitstream signal to the stereo signal or - left/right decoding the bitstream signal to the stereo signal.

17. The decoder system of claim 16, wherein the perceptual decoding means com prise: 25 - a perceptual stereo decoder configured for decoding based on the bitstream signal, the decoder generating a pseudo stereo signal, wherein the decoder is configured to selectively perform - left/right perceptual decoding or - mid/side perceptual decoding 30 in a frequency-variant or frequency-invariant manner; and - 49 - a transform means configured for performing a transform based on the pseudo stereo signal, thereby generating the downmix signal and the resid ual signal. 5

18. The decoder system of any one of claims 16 to 17, wherein the parametric ste reo parameters comprise - a frequency-variant or a frequency-invariant parameter indicating an in ter-channel intensity difference, and - a frequency-variant or a frequency-invariant parameter indicating an 10 inter-channel cross-correlation.

19. A method for encoding a stereo signal to a bitstream signal, the method com prising: - generating a downmix signal and a residual signal based on the stereo sig 15 nal; - determining one or more parametric stereo parameters; - perceptual encoding downstream of generating the downmix signal and the residual signal, wherein - encoding based on a sum of the downmix signal and the residual 20 signal and based on a difference of the downmix signal and the re sidual signal or - encoding based on the downmix signal and based on the residual signal is selectable in a frequency-variant or frequency-invariant manner; wherein 25 the method allows to select in a frequency-variant or frequency-invariant manner between parametric stereo encoding the stereo signal to the bit stream signal or left/right encoding the stereo signal to the bitstream signal.

20. A method for decoding a bitstream signal including parametric stereo parame 30 ters to a stereo signal, the method comprising: - 50 perceptual decoding based on the bitstream signal, wherein a first signal and a second signal is generated by decoding and a downmix signal and a residual signal is output after perceptual decoding, the downmix signal and the residual signal being selectively 5 - based on the sum of the first signal and of the second signal and based on the difference of the first signal and of the second sig nal or - based on the first signal and based on the second signal in a frequency-variant or frequency-invariant manner; and 10 - generating the stereo signal based on the downmix signal and the residual signal by an upmix operation, with the upmix operation being dependent on the parametric stereo parameters; wherein the method allows to switch in a frequency-variant or frequency-invariant manner between parametric stereo decoding the bitstream signal to the stereo signal or left/right decoding the 15 bitstream signal to the stereo signal.