JP2013050663A - Multi-channel sound coding device and program thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multi-channel sound coding device and a program thereof for performing bit allocation based on the relevance between channels of a transmission signal when encoding the transmission signal into which an input signal of a multi-channel sound method is converted by matrix conversion.SOLUTION: A multi-channel sound coding device 1 according to the present invention comprises: a matrix converting section 20 that converts, by matrix conversion, an input signal of a multi-channel sound method into a transmission signal having a plurality of channels; a calculating section 32 that calculates a scale factor for each of the channels on the basis of an auditory model for each channel of the transmission signal; a correlation calculating section 34 that calculates a correlation coefficient between channels of the transmission signal; a selecting section 35 that selects, for a channel group of the transmission signal having the correlation coefficient equal to or greater than a threshold, an applied scale factor based on the scale factors of the channel group; and a bit allocating section 36 that performs bit allocation according to the applied scale factor for the transmission signals of the channel group.

Description

  The present invention relates to a multi-channel acoustic encoding apparatus and a program thereof.

  The Japan Radio Industry Association (ARIB) standardizes a method using existing AAC (Advanced Audio Coding) coding to transmit and code 22.2 channel sound. The AAC method is a method capable of encoding at a bit rate of about 1/10 while maintaining the quality of a CD (compact disc) (for example, see Non-Patent Document 1).

  When performing transmission / coding of 22.2 channel sound, it is necessary to perform signal conversion such as matrix conversion in order to ensure compatibility with the conventional 2-channel stereo without increasing the number of transmission channels. . Conventional AAC encoding using matrix transformation will be described with reference to FIG. FIG. 3 is a diagram showing an outline of processing for converting an input signal, which is a 22.2 channel acoustic signal, into a 2-channel transmission signal by matrix conversion and performing AAC transmission. First, in the transmission block, the 22.2 channel input signal is subjected to matrix conversion, for example, converted into a transmission signal including two channels of a basic signal and an auxiliary signal. Here, the basic signal is a signal of 8 to 10 channels representing main spatial information of the 22.2 channel acoustic signal, and the auxiliary signal is the original 22.2 channel acoustic signal by complementing the basic signal. This is a signal for restoration.

  Next, in the transmission block, AAC encoding of the basic signal and the auxiliary signal is performed independently. Here, in AAC coding, after frequency analysis of each transmission signal, significant frequency components are detected, and a masking curve representing the upper limit of frequency components that cannot be heard (masked) by this component is calculated. Bit allocation for the following frequency components is reduced, and bit allocation that allows quantization noise that falls below the masking curve is performed.

  When an AAC-encoded transmission signal is transmitted from the transmission block to the reception block, the basic signal and the auxiliary signal are independently AAC decoded in the reception block, and the 22.2 channel acoustic signal is restored by inverse matrix transformation. Is done.

Marina Bosi, Richard E. Goldberg, "Introduction to Digital Audio Coding and Standards" Springer, 2002-12-31

  Here, in the conventional AAC coding, processing for signal coding accompanied by matrix transformation is examined, as in the case of transmitting a 22.2 channel acoustic input signal by downmixing to a transmission signal having a smaller number of channels. It has not been.

  For example, in the conventional AAC encoding shown in FIG. 3, each channel (basic signal and auxiliary signal) of the transmission signal after matrix conversion is processed independently in the transmission block. That is, a bit allocation process based on a masking curve is individually performed for each channel after matrix transformation on a transmission signal in which a plurality of multi-channel acoustic signals are mixed by matrix transformation. For this reason, depending on the channel after matrix transformation, a phenomenon may occur in which a specific component is left or deleted, and as a result, after the inverse matrix transformation, the original multi-channel acoustic signal component cannot be restored, or There was a problem that noise was generated because there was no component to be canceled.

  Accordingly, an object of the present invention made in view of such a point is to perform bit allocation based on the relationship between channels of a transmission signal when encoding a transmission signal obtained by performing matrix transformation on an input signal of a multi-channel acoustic system. It is to provide a multi-channel acoustic encoding device and a program thereof.

  In order to solve the above-described problems, a multi-channel acoustic encoding apparatus according to the present invention includes a matrix conversion unit that converts a multi-channel acoustic input signal into a transmission signal of a plurality of channels by matrix conversion, A multi-channel acoustic encoding device comprising: an encoding unit that performs bit allocation for the transmission signal for each channel, wherein the encoding unit is configured to perform the channel-based audio model for each channel based on an auditory model for each channel of the transmission signal. A calculation unit that calculates a scale factor; a correlation calculation unit that calculates a correlation coefficient between the channels of the transmission signal; and a channel group of the transmission signal in which the correlation coefficient is equal to or greater than a threshold value. A selection unit that selects an applicable scale factor based on a scale factor; and bit allocation based on the applicable scale factor for the transmission signals of the channel group. A bit allocation unit which performs hand, in that it comprises the features.

  In addition, the correlation calculation unit calculates a correlation coefficient for each subband of each channel of the transmission signal, and the selection unit selects the applicable scale factor according to the correlation coefficient for each subband. It is preferable to change the method.

  The selection unit may perform smoothing of at least one of smoothing of an applied scale factor based on a past applied scale factor and smoothing of an applied scale factor between channels in adjacent frequency bands. Is preferred.

  As described above, the solution of the present invention has been described as an apparatus. However, the present invention can be realized as a method, a program, and a storage medium storing the program, which are substantially equivalent thereto, and the scope of the present invention. It should be understood that these are also included.

  For example, the invention that implements the present invention as a program is a computer that converts a multi-channel acoustic input signal into a transmission signal of a plurality of channels by matrix transformation and a hearing model for each channel of the transmission signal. A procedure for calculating a scale factor for each channel; a procedure for calculating a correlation coefficient between the channels of the transmission signal; and a channel group of the transmission signal for which the correlation coefficient is equal to or greater than a threshold. A procedure for selecting an applicable scale factor based on a scale factor and a procedure for assigning bits according to the applicable scale factor to the transmission signals of the channel group are executed.

  According to the multi-channel acoustic encoding apparatus and the program thereof according to the present invention, when encoding a transmission signal obtained by performing matrix conversion on an input signal of a multi-channel acoustic scheme, bit allocation based on the relationship between each channel of the transmission signal is performed. Can be done.

It is a figure which shows the functional block of the multi-channel acoustic coding apparatus which concerns on one Embodiment of this invention. It is a figure which shows the detailed functional block of an encoding part. It is a figure which shows the outline | summary of the process which converts a multi-channel input signal into a 2-channel transmission signal by matrix transformation, and carries out AAC transmission.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Here, in the following description, the 22.2 channel sound which is the sound method for Super Hi-Vision will be described as an example of the multi-channel sound method, but the present invention is limited to only 22.2 channel sound. Note that this is not the case.

  FIG. 1 is a functional block diagram of a multi-channel acoustic encoding apparatus 1 according to an embodiment of the present invention. The multi-channel acoustic encoding apparatus 1 includes an acoustic signal input unit 10, a matrix conversion unit 20, an encoding unit 30, and a transmission unit 40.

  Here, in order to function as the multi-channel acoustic encoding device 1, a computer can be suitably used, and such a computer is a program describing processing contents for realizing each function of the multi-channel acoustic encoding device 1. Is stored in a storage unit (not shown) of the computer, and this program is read and executed by a central processing unit (CPU) of the computer.

  The acoustic signal input unit 10 performs A / D conversion on the input 22.2 channel acoustic signal and outputs a digital acoustic signal to the matrix conversion unit 20 as an input signal.

  The matrix converter 20 downmixes the input signal, which is a 22.2 channel acoustic signal, into a transmission signal with a smaller number of channels by matrix conversion. The matrix conversion unit 20 outputs the transmission signal after the matrix conversion to the encoding unit 30.

  The encoding unit 30 assigns bits to the transmission signal for each channel of the transmission signal. FIG. 2 is a diagram illustrating detailed functional blocks of the encoding unit 30. The encoding unit 30 includes a masking curve calculation unit 31, a subband scale factor calculation unit 32, a subband division unit 33, an inter-channel correlation calculation unit 34, a scale factor selection unit 35, and a bit allocation unit 36. Prepare. Here, the masking curve calculation unit 31, the subband scale factor calculation unit 32, the subband division unit 33, and the bit allocation unit 36 can be configured independently for each channel of the transmission signal. For convenience of explanation, channel numbers (for example, a to c) are attached to the functional blocks of each channel in FIG. In FIG. 2, functional blocks corresponding to the b channel and the c channel are schematically described, but the masking curve calculation unit 31 and the sub channel are also applied to the b channel and the c channel as in the a channel. It should be noted that the band scale factor calculation unit 32, the subband division unit 33a, and the bit allocation unit 36 can be configured.

  The masking curve calculation unit 31 calculates an auditory model of each channel of the transmission signal. Here, the auditory model of each channel is a masking curve based on an auditory characteristic that “a sound having relatively low energy cannot be heard in a frequency band near a high-energy signal” It is calculated based on the minimum audible curve based on the auditory characteristic that there is a sound of energy that cannot be performed. Note that the calculation of the masking curve and the minimum audible curve is known to those skilled in the art and will not be described in detail in this article. Hereinafter, for convenience of explanation, a case where a masking curve is used as an auditory model will be described as an example, but the present invention is not limited to this. The masking curve calculation unit 31 outputs the calculated masking curve of each channel to the subband scale factor calculation unit 32.

  The subband scale factor calculation unit 32 calculates a scale factor for each subband of each channel from the masking curve of each channel. Specifically, the subband scale factor calculation unit 32 searches the subband signal of each channel for a sample having the maximum absolute value, and obtains a scale factor obtained by converting the value into a logarithm. . Note that the calculation of the scale factor for each subband is known to those skilled in the art, and will not be described in detail in this article. The subband scale factor calculation unit 32 outputs the calculated scale factor for each subband of each channel to the scale factor selection unit 35.

  The subband division unit 33 divides the transmission signal into subbands for each channel of the transmission signal and outputs the subband to the interchannel correlation calculation unit 34.

  The inter-channel correlation calculation unit 34 calculates a correlation coefficient between each channel of the transmission signal. In particular, the inter-channel correlation calculation unit 34 calculates the correlation coefficient between the respective channels from the change in the transmission signal in a certain time interval of each channel for each subband. That is, the correlation coefficient between channels whose transmission signal changes in a certain time interval is close, and the correlation coefficient between channels whose change is different is calculated low. The inter-channel correlation calculation unit 34 supplies the calculated correlation coefficient to the scale factor selection unit 35.

  The scale factor selection unit 35 selects an applicable scale factor based on a scale factor for each subband of the transmission signal for a transmission signal of a channel group having a correlation coefficient equal to or greater than a certain value. Here, the applied scale factor means a scale factor used for bit allocation of a transmission signal of a channel group in which a correlation coefficient is a certain value or more.

For example, the scale factor selection unit 35 can select the maximum value of the scale factor of each channel belonging to the channel group for each subband as shown in Equation 1, and set it as the applicable scale factor. Here, G (k) represents the applicable scale factor for the kth subband, and _Fj (k) represents the scale factor of the kth subband of the jth channel belonging to the channel group. In this case, the applicable scale factor is common to the transmission signals of the channels belonging to the channel group. When the maximum scale factor of each channel is combined as the applied scale factor, the quantization granularity becomes the finest, so that the quantization accuracy is improved, but the quantization efficiency is lowered.

  Also, the scale factor selection unit 35 can select the minimum value of the scale factor of each channel belonging to the channel group for each subband, as Equation 2, and set it as the applicable scale factor. Also in this case, the applicable scale factor is common to the transmission signals of the respective channels belonging to the channel group. When the minimum value of the scale factor of each channel is synthesized as the applied scale factor, the quantization granularity becomes the roughest, and thus the quantization accuracy is lowered, but the quantization efficiency is improved.

Furthermore, the scale factor selection unit 35 can select the applicable scale factor of each channel based on the correlation coefficient between channels as shown in Equation 3. In Equation 3, G _i (k) represents an applied scale factor for the transmission signal of the i th channel in the k th subband, and r _ij (k) represents the i th channel and the j th channel in the k th subband. Represents the correlation coefficient with the channel. In this case, the applicable scale factor has a different value for each channel of the transmission signal. By combining each scale factor based on the correlation coefficient between each channel as an applicable scale factor, it is possible to perform quantization along each channel while suppressing the difference in quantization accuracy between channels, and to improve the quantization efficiency. Can be improved.

Furthermore, the scale factor selection unit 35 can change the selection method of the applied scale factor for each subband according to the correlation coefficient. Specifically, threshold values Th ₁ and Th ₂ (Th ₁ > Th ₂ ) are set in advance, and when the correlation coefficient in a certain subband k is higher than the threshold value Th ₁ as shown in Equation 4, between each channel If the application scale factor is selected in the same manner as in the equation (1) and the correlation coefficient is higher than the threshold Th _{2 and} equal to or lower than the threshold Th ₁ , the degree of correlation between the channels is used. As in the equation (3), the applicable scale factor is selected as an application scale factor that considers Incidentally, when the correlation coefficient is less than the threshold Th ₂ is a correlation as low between channels, and those using the scale factor for each channel as it is a bit allocation for each channel.

  The selection of the applicable scale factor is not limited to the formulas (1) to (4). For example, the scale factor selection unit 35 can select the average value of the scale factors of each channel as the applied scale factor.

  Furthermore, the scale factor selection unit 35 can perform various smoothing related to the scale factor, such as smoothing of the applied scale factor based on the past applied scale factor, and smoothing of the applied scale factor between channels in adjacent frequency bands. it can.

  The scale factor selection unit 35 outputs the applied scale factor obtained by the equations (1) to (4) to the bit allocation unit 36 for the transmission signals of the channel group in which the correlation coefficient is a certain value or more, For channels with a correlation coefficient less than a certain value, the scale factor of each channel is output to the bit allocation unit 36 as it is.

  The bit allocation unit 36 performs bit allocation to the transmission signal for each channel of the transmission signal based on the applied scale factor or the scale factor from the scale factor selection unit 35. The bit allocation unit 36 outputs the encoded transmission signal to the transmission unit 60.

  The transmission unit 60 transmits the transmission signal encoded by the encoding unit 50 to the reception side.

  Thus, according to the present embodiment, the inter-channel correlation calculation unit 34 calculates the correlation coefficient between the channels of the transmission signal, and the scale factor selection unit 35 transmits the transmission signal whose correlation coefficient is equal to or greater than the threshold value. For this channel group, an applied scale factor based on the scale factor of the channel group is selected, and the bit allocation unit 36 performs bit allocation based on the applied scale factor for the transmission signal of the channel group. Accordingly, when encoding a transmission signal obtained by performing matrix conversion on an input signal of a multi-channel acoustic system, it is possible to perform bit allocation based on the relationship between the channels of the transmission signal. For example, for a transmission signal of a channel group in which the correlation coefficient is equal to or greater than a certain value, bit allocation is performed from the minimum value or maximum value of the scale factor of each channel using an applicable scale factor common to each channel. As a result, it is possible to adjust the quantization accuracy and the quantization efficiency as required. For this reason, when the quantization accuracy is increased, it is possible to reduce signal loss and noise generation due to inverse matrix transformation, which has been a problem in the past. In particular, for example, a 22.2 channel acoustic signal is transmitted by being separated into an 8-10 channel basic signal representing main spatial information and an auxiliary signal for restoring the original signal. Therefore, even when a high bit rate is assigned and the bit rate is suppressed for the auxiliary signal, encoding / transmission with less quantization noise is possible. In addition, for example, even when transmitting 22.2 channel audio signals for Super Hi-Vision as 2 channel stereo signals or 5.1 channel signals, encoding and transmission with high quality and a bit rate of about 1/10. Is possible.

  Further, according to the present embodiment, the inter-channel correlation calculation unit 34 calculates the correlation coefficient for each subband of each channel of the transmission signal, and the scale factor selection unit 35 calculates the correlation coefficient for each subband. Change the application scale factor selection method accordingly. As a result, a common scale factor can be used for subbands with a high correlation coefficient, and a scale factor based on the correlation coefficient can be set for subbands with a medium correlation coefficient. It becomes possible to control the quantization efficiency.

  In addition, according to the present embodiment, the scale factor selection unit 35 performs various types of scale factors such as smoothing of an applied scale factor based on past applied scale factors and smoothing of an applied scale factor between channels in adjacent frequency bands. Can be smoothed. As a result, the difference in scale factor between adjacent time zones and frequency bands is reduced, and the loss of signals and the occurrence of noise due to a sudden change in scale factor can be reduced.

  Although the present invention has been described based on the drawings and examples, it should be noted that those skilled in the art can easily make various modifications and corrections based on the present disclosure. Therefore, it should be noted that these variations and modifications are included in the scope of the present invention. For example, functions included in each member, each means, each step, etc. can be rearranged so as not to be logically contradictory, and a plurality of means, steps, etc. can be combined or divided into one. Is possible.

  For example, in the above embodiment, the input signal of the multi-channel acoustic system is described as 22.2 channel sound, and the transmission signal after matrix conversion is described as 2 channel stereo signal. However, the present invention is not limited to other 5.1 channel sound. It is needless to say that any multi-channel sound system such as 7.1-channel sound can be applied to all processes involving signal matrix transformation with respect to coded transmission. Moreover, it is applicable also to acoustic signals, such as an ambisonics, as a signal accompanied by linear processes, such as matrix transformation.

  In the above embodiment, AAC has been described as an example of the encoding method, but AAC encoding in the present invention includes all versions of AAC such as MPEG2-AAC, MPEG4-AAC, HE-AAC. It is. In addition, the encoding that can be supported by the present invention is not limited to AAC, and any encoding scheme can be used as long as it encodes with high quality based on human auditory characteristics.

  According to the present invention, when encoding a transmission signal obtained by performing matrix conversion on an input signal of a multi-channel acoustic system, it is possible to perform bit allocation based on the relationship between each channel of the transmission signal. is there.

DESCRIPTION OF SYMBOLS 1 Multichannel acoustic coding apparatus 10 Acoustic signal input part 20 Matrix transformation part 30 Coding part 31 Masking curve calculation part 32 Subband scale factor calculation part (calculation part)
33 Subband division unit 34 Interchannel correlation calculation unit (correlation calculation unit)
35 Scale factor selection part (selection part)
36 bit allocation unit 40 transmission unit

Claims (4)

A matrix converter that converts a multi-channel acoustic input signal into a transmission signal of a plurality of channels by matrix conversion;
A multi-channel acoustic encoding device comprising: an encoding unit that performs bit allocation to the transmission signal for each channel of the transmission signal;
The encoding unit includes:
A calculation unit for calculating a scale factor for each channel based on an auditory model for each channel of the transmission signal;
A correlation calculator for calculating a correlation coefficient between each channel of the transmission signal;
A selection unit that selects an applied scale factor based on a scale factor of the channel group for a channel group of the transmission signal in which the correlation coefficient is equal to or greater than a threshold;
A multi-channel acoustic coding apparatus comprising: a bit allocation unit that performs bit allocation based on the applied scale factor for the transmission signals of the channel group. The correlation calculation unit calculates a correlation coefficient for each subband of each channel of the transmission signal,
The multi-channel acoustic encoding apparatus according to claim 1, wherein the selection unit changes a selection method of the applied scale factor according to the correlation coefficient for each subband.   The selection unit performs smoothing of at least one of smoothing of an applied scale factor based on a past applied scale factor and smoothing of an applied scale factor between channels in adjacent frequency bands. Or the multi-channel acoustic encoding apparatus according to 2; On the computer,
A procedure for converting a multi-channel acoustic input signal into a transmission signal of a plurality of channels by matrix conversion,
Calculating a scale factor for each channel based on an auditory model for each channel of the transmitted signal;
Calculating a correlation coefficient between each channel of the transmission signal;
Selecting an applicable scale factor based on a scale factor of the channel group for the channel group of the transmission signal in which the correlation coefficient is equal to or greater than a threshold;
A program for executing a procedure for performing bit allocation according to the applied scale factor for the transmission signals of the channel group.

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