# Patent

## Title
Voice audio encoding device, voice audio decoding device, voice audio encoding method, and voice audio decoding method

## Abstract

Provided are a voice audio encoding device, voice audio decoding device, voice audio encoding method, and voice audio decoding method that efficiently perform bit distribution and improve sound quality. Dominant frequency band identification unit identifies a dominant frequency band having a norm factor value that is the maximum value within the spectrum of an input voice audio signal. Dominant group determination units and non-dominant group determination unit group all sub-bands into a dominant group that contains the dominant frequency band and a non-dominant group that contains no dominant frequency band. Group bit distribution unit distributes bits to each group on the basis of the energy and norm variance of each group. Sub-band bit distribution unit redistributes the bits that have been distributed to each group to each sub-band in accordance with the ratio of the norm to the energy of the groups.

## Background
TECHNICAL FIELD
The present invention relates to a speech/audio coding apparatus, a speech/audio decoding apparatus, a speech/audio coding method and a speech/audio decoding method using a transform coding scheme.
BACKGROUND ART
As a scheme capable of efficiently encoding a speech signal or music signal in a full band (FB) of 0.02 to 20 kHz, there is a technique standardized in ITU-T (International Telecommunication Union Telecommunication Standardization Sector). This technique transforms an input signal into a frequency-domain signal and encodes a band of up to 20 kHz (transform coding).
Here, transform coding is a coding scheme that transforms an input signal from a time domain into a frequency domain using time/frequency transformation such as discrete cosine transform (DCT) or modified discrete cosine transform (MDCT) to enable a signal to be mapped in precise correspondence with auditory characteristics.
In transform coding, a spectral coefficient is split into a plurality of frequency subbands. In coding of each subband, allocating more quantization bits to a band which is perceptually important to human ears makes it possible to improve overall sound quality.
In order to attain this object, studies are being carried out on efficient bit allocation schemes, and for example, a technique disclosed in Non-Patent Literature (hereinafter, referred to as “NPL”) 1 is known. Hereinafter, the bit allocation scheme disclosed in Patent Literature (hereinafter, referred to as “PTL”) 1 will be described using FIG. 1 and FIG. 2.
FIG. 1 is a block diagram illustrating a configuration of a speech/audio coding apparatus disclosed in PTL 1. An input signal sampled at 48 kHz is inputted to transient detector 11 and transformation section 12 of the speech/audio coding apparatus.
Transient detector 11 detects, from the input signal, either a transient frame corresponding to a leading edge or an end edge of speech or a stationary frame corresponding to a speech section other than that, and transformation section 12 applies, to the frame of the input signal, high-frequency resolution transformation or low-frequency resolution transformation depending on whether the frame detected by transient detector 11 is a transient frame or stationary frame, and acquires a spectral coefficient (or transform coefficient).
Norm estimation section 13 splits the spectral coefficient obtained in transformation section 12 into bands of different bandwidths. Norm estimation section 13 estimates a norm (or energy) of each split band.
Norm quantization section 14 determines a spectral envelope made up of the norms of all bands based on the norm of each band estimated by norm estimation section 13 and quantizes the determined spectral envelope.
Spectrum normalization section 15 normalizes the spectral coefficient obtained by transformation section 12 according to the norm quantized by norm quantization section 14.
Norm adjustment section 16 adjusts the norm quantized by norm quantization section 14 based on adaptive spectral weighting.
Bit allocation section 17 allocates available bits for each band in a frame using the quantization norm adjusted by norm adjustment section 16.
Lattice-vector coding section 18 performs lattice-vector coding on the spectral coefficient normalized by spectrum normalization section 15 using bits allocated for each band by bit allocation section 17.
Noise level adjustment section 19 estimates the level of the spectral coefficient before coding in lattice-vector coding section 18 and encodes the estimated level. A noise level adjustment index is obtained in this way.
Multiplexer 20 multiplexes a frame configuration of the input signal acquired by transformation section 12, that is, a transient signal flag indicating whether the frame is a stationary frame or transient frame, the norm quantized by norm quantization section 14, the lattice coding vector obtained by lattice-vector coding section 18 and the noise level adjustment index obtained by noise level adjustment section 19, and forms a bit stream and transmits the bit stream to a speech/audio decoding apparatus.
FIG. 2 is a block diagram illustrating a configuration of the speech/audio decoding apparatus disclosed in PTL 1. The speech/audio decoding apparatus receives the bit stream transmitted from the speech/audio coding apparatus and demultiplexer 21 demultiplexes the bit stream.
Norm de-quantization section 22 de-quantizes the quantized norm, acquires a spectral envelope made up of norms of all bands, and norm adjustment section 23 adjusts the norm de-quantized by norm de-quantization section 22 based on adaptive spectral weighting.
Bit allocation section 24 allocates available bits for each band in a frame using the norms adjusted by norm adjustment section 23. That is, bit allocation section 24 recalculates bit allocation indispensable to decode the lattice-vector code of the normalized spectral coefficient.
Lattice decoding section 25 decodes a transient signal flag, decodes the lattice coding vector based on a frame configuration indicated by the decoded transient signal flag and the bits allocated by bit allocation section 24 and acquires a spectral coefficient.
Spectral-fill generator 26 regenerates a low-frequency spectral coefficient to which no bit has been allocated using a codebook created based on the spectral coefficient decoded by lattice decoding section 25. Spectral-fill generator 26 adjusts the level of the spectral coefficient regenerated using a noise level adjustment index. Furthermore, spectral-fill generator 26 regenerates a high-frequency uncoded spectral coefficient using a low-frequency coded spectral coefficient.
Adder 27 adds up the decoded spectral coefficient and the regenerated spectral coefficient, and generates a normalized spectral coefficient.
Envelope shaping section 28 applies the spectral envelope de-quantized by norm de-quantization section 22 to the normalized spectral coefficient generated by adder 27 and generates a full-band spectral coefficient.
Inverse transformation section 29 applies inverse transform such as inverse modified discrete cosine transform (IMDCT) to the full-band spectral coefficient generated by envelope shaping section 28 to transform it into a time-domain signal. Here, inverse transform with high-frequency resolution is applied to a case with a stationary frame and inverse transform with low-frequency resolution is applied to a case with a transient frame.
In G.719, the spectral coefficients are split into spectrum groups. Each spectrum group is split into bands of equal length sub-vectors as shown in FIG. 3. Sub-vectors are different in length from one group to another and this length increases as the frequency increases. Regarding transform resolution, higher frequency resolution is used for low frequencies, while lower frequency resolution is used for high frequencies. As described in G.719, the grouping allows an efficient use of the available bit-budget during encoding.
In G.719, the bit allocation scheme is identical in a coding apparatus and a decoding apparatus. Here, the bit allocation scheme will be described using FIG. 4.
As shown in FIG. 4, in step (hereinafter abbreviated as “ST”) 31, quantized norms are adjusted prior to bit allocation to adjust psycho-acoustical weighting and masking effects.
In ST32, subbands having a maximum norm are identified from among all subbands and in ST33, one bit is allocated to each spectral coefficient for the subbands having the maximum norm. That is, as many bits as spectral coefficients are allocated.
In ST34, the norms are reduced according to the bits allocated, and in ST35, it is determined whether the remaining number of allocatable bits is 8 or more. When the remaining number of allocatable bits is 8 or more, the flow returns to ST32 and when the remaining number of allocatable bits is less than 8, the bit allocation procedure is terminated.
Thus, in the bit allocation scheme, available bits within a frame are allocated among subbands using the adjusted quantization norms. Normalized spectral coefficients are encoded by lattice-vector coding using the bits allocated to each subband.
CITATION LIST
Patent Literature


    NPL 1
    ITU-T Recommendation G.719, “Low-complexity full-band audio coding for high-quality conversational applications,” ITU-T, 2009.


SUMMARY OF INVENTION
Technical Problem
However, the above bit allocation scheme does not take into consideration input signal characteristics when grouping spectral bands, and therefore has a problem in that efficient bit allocation is not possible and further improvement of sound quality cannot be expected.
An object of the present invention is to provide a speech/audio coding apparatus, a speech/audio decoding apparatus, a speech/audio coding method and a speech/audio decoding method capable of realizing efficient bit allocation and improving sound quality.
Solution to Problem
A speech/audio coding apparatus of the present invention includes: a transformation section that transforms an input signal from a time domain to a frequency domain; an estimation section that estimates an energy envelope which represents an energy level for each of a plurality of subbands obtained by splitting a frequency spectrum of the input signal; a quantization section that quantizes the energy envelopes; a group determining section that groups the quantized energy envelopes into a plurality of groups; a first bit allocation section that allocates bits to the plurality of groups; a second bit allocation section that allocates the bits allocated to the plurality of groups to subbands on a group-by-group basis; and a coding section that encodes the frequency spectrum using bits allocated to the subbands.
A speech/audio decoding apparatus according to the present invention includes: a de-quantization section that de-quantizes a quantized spectral envelope; a group determining section that groups the quantized spectral envelopes into a plurality of groups; a first bit allocation section that allocates bits to the plurality of groups; a second bit allocation section that allocates the bits allocated to the plurality of groups to subbands on a group-by-group basis; a decoding section that decodes a frequency spectrum of a speech/audio signal using the bits allocated to the subbands; an envelope shaping section that applies the de-quantized spectral envelope to the decoded frequency spectrum and reproduces a decoded spectrum; and an inverse transformation section that inversely transforms the decoded spectrum from a frequency domain to a time domain.
A speech/audio coding method according to the present invention includes: transforming an input signal from a time domain to a frequency domain; estimating an energy envelope that represents an energy level for each of a plurality of subbands obtained by splitting a frequency spectrum of the input signal; quantizing the energy envelopes; grouping the quantized energy envelopes into a plurality of groups; allocating bits to the plurality of groups; allocating the bits allocated to the plurality of groups to subbands on a group-by-group basis; and encoding the frequency spectrum using bits allocated to the subbands.
A speech/audio decoding method according to the present invention includes: de-quantizing a quantized spectral envelope; grouping the quantized spectral envelope into a plurality of groups; allocating bits to the plurality of groups; allocating the bits allocated to the plurality of groups to subbands on a group-by-group basis; decoding a frequency spectrum of a speech/audio signal using the bits allocated to the subbands; applying the de-quantized spectral envelope to the decoded frequency spectrum and reproducing a decoded spectrum; and inversely transforming the decoded spectrum from a frequency domain to a time domain.
Advantageous Effects of Invention
According to the present invention, it is possible to realize efficient bit allocation and improve sound quality.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a block diagram illustrating a configuration of a speech/audio coding apparatus disclosed in PTL 1;
FIG. 2 is a block diagram illustrating a configuration of a speech/audio decoding apparatus disclosed in PTL 1;
FIG. 3 is a diagram illustrating grouping of spectral coefficients in a stationary mode disclosed in PTL 1;
FIG. 4 is a flowchart illustrating a bit allocation scheme disclosed in PTL 1;
FIG. 5 is a block diagram illustrating a configuration of a speech/audio coding apparatus according to an embodiment of the present invention;
FIG. 6 is a block diagram illustrating a configuration of a speech/audio decoding apparatus according to an embodiment of the present invention;
FIG. 7 is a block diagram illustrating an internal configuration of the bit allocation section shown in FIG. 5;
FIGS. 8A to 8C are diagrams provided for describing a grouping method according to an embodiment of the present invention; and
FIG. 9 is a diagram illustrating a norm variance.



DESCRIPTION OF EMBODIMENTS
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
(Embodiment)
FIG. 5 is a block diagram illustrating a configuration of speech/audio coding apparatus 100 according to an embodiment of the present invention. An input signal sampled at 48 kHz is inputted to transient detector 101 and transformation section 102 of speech/audio coding apparatus 100.
Transient detector 101 detects, from an input signal, either a transient frame corresponding to a leading edge or an end edge of speech or a stationary frame corresponding to a speech section other than that, and outputs the detection result to transformation section 102. Transformation section 102 applies, to the frame of the input signal, high-frequency resolution transformation or low-frequency resolution transformation depending on whether the detection result outputted from transient detector 101 is a transient frame or stationary frame, and acquires a spectral coefficient (or transform coefficient) and outputs the spectral coefficient to norm estimation section 103 and spectrum normalization section 105. Transformation section 102 outputs a frame configuration which is the detection result outputted from transient detector 101, that is, a transient signal flag indicating whether the frame is a stationary frame or a transient frame to multiplexer 110.
Norm estimation section 103 splits the spectral coefficient outputted from transformation section 102 into bands of different bandwidths and estimates a norm (or energy) of each split band. Norm estimation section 103 outputs the estimated norm of each band to norm quantization section 104.
Norm quantization section 104 determines a spectral envelope made up of norms of all bands based on norms of respective bands outputted from norm estimation section 103, quantizes the determined spectral envelope and outputs the quantized spectral envelope to spectrum normalization section 105 and norm adjustment section 106.
Spectrum normalization section 105 normalizes the spectral coefficient outputted from transformation section 102 according to the quantized spectral envelope outputted from norm quantization section 104 and outputs the normalized spectral coefficient to lattice-vector coding section 108.
Norm adjustment section 106 adjusts the quantized spectral envelope outputted from norm quantization section 104 based on adaptive spectral weighting and outputs the adjusted quantized spectral envelope to bit allocation section 107.
Bit allocation section 107 allocates available bits for each band in a frame using the adjusted quantized spectral envelope outputted from norm adjustment section 106 and outputs the allocated bits to lattice-vector coding section 108. Details of bit allocation section 107 will be described later.
Lattice-vector coding section 108 performs lattice-vector coding on the spectral coefficient normalized by spectrum normalization section 105 using the bits allocated for each band in bit allocation section 107 and outputs the lattice coding vector to noise level adjustment section 109 and multiplexer 110.
Noise level adjustment section 109 estimates the level of the spectral coefficient prior to coding in lattice-vector coding section 108 and encodes the estimated level. A noise level adjustment index is determined in this way. The noise level adjustment index is outputted to multiplexer 110.
Multiplexer 110 multiplexes the transient signal flag outputted from transformation section 102, quantized spectral envelope outputted from norm quantization section 104, lattice coding vector outputted from lattice-vector coding section 108 and noise level adjustment index outputted from noise level adjustment section 109, and forms a bit stream and transmits the bit stream to a speech/audio decoding apparatus.
FIG. 6 is a block diagram illustrating a configuration of speech/audio decoding apparatus 200 according to an embodiment of the present invention. A bit stream transmitted from speech/audio coding apparatus 100 is received by speech/audio decoding apparatus 200 and demultiplexed by demultiplexer 201.
Norm de-quantization section 202 de-quantizes the quantized spectral envelope (that is, norm) outputted from the multiplexer, obtains a spectral envelope made up of norms of all bands and outputs the spectral envelope obtained to norm adjustment section 203.
Norm adjustment section 203 adjusts the spectral envelope outputted from norm de-quantization section 202 based on adaptive spectral weighting and outputs the adjusted spectral envelope to bit allocation section 204.
Bit allocation section 204 allocates available bits for each band in a frame using the spectral envelope outputted from norm adjustment section 203. That is, bit allocation section 204 recalculates bit allocation indispensable to decode the lattice-vector code of the normalized spectral coefficient. The allocated bits are outputted to lattice decoding section 205.
Lattice decoding section 205 decodes the lattice coding vector outputted from demultiplexer 201 based on a frame configuration indicated by the transient signal flag outputted from demultiplexer 201 and the bits outputted from bit allocation section 204 and acquires a spectral coefficient. The spectral coefficient is outputted to spectral-fill generator 206 and adder 207.
Spectral-fill generator 206 regenerates a low-frequency spectral coefficient to which no bit has been allocated using a codebook created based on the spectral coefficient outputted from lattice decoding section 205. Spectral-fill generator 206 adjusts the level of the regenerated spectral coefficient using the noise level adjustment index outputted from demultiplexer 201. Furthermore, spectral-fill generator 206 regenerates the spectral coefficient not subjected to high-frequency coding using a low-frequency coded spectral coefficient. The level-adjusted low-frequency spectral coefficient and regenerated high-frequency spectral coefficient are outputted to adder 207.
Adder 207 adds up the spectral coefficient outputted from lattice decoding section 205 and the spectral coefficient outputted from spectral-fill generator 206, generates a normalized spectral coefficient and outputs the normalized spectral coefficient to envelope shaping section 208.
Envelope shaping section 208 applies the spectral envelope outputted from norm de-quantization section 202 to the normalized spectral coefficient generated by adder 207 and generates a full-band spectral coefficient (corresponding to the decoded spectrum). The full-band spectral coefficient generated is outputted to inverse transformation section 209.
Inverse transformation section 209 applies inverse transform such as inverse modified discrete cosine transform (IMDCT) to the full-band spectral coefficient outputted from envelope shaping section 208, transforms it to a time-domain signal and outputs an output signal. Here, inverse transform with high-frequency resolution is applied to a case of a stationary frame and inverse transform with low-frequency resolution is applied to a case of a transient frame.
Next, the details of bit allocation section 107 will be described using FIG. 7. Note that bit allocation section 107 of speech/audio coding apparatus 100 is identical in configuration to bit allocation section 204 of speech/audio decoding apparatus 200, and therefore only bit allocation section 107 will be described and description of bit allocation section 204 will be omitted here.
FIG. 7 is a block diagram illustrating an internal configuration of bit allocation section 107 shown in FIG. 5. Dominant frequency band identification section 301 identifies, based on the quantized spectral envelope outputted from norm adjustment section 106, a dominant frequency band which is a subband in which a norm coefficient value in the spectrum has a local maximum value, and outputs each identified dominant frequency band to dominant group determining sections 302-1 to 302N. In addition to designating a frequency band for which a norm coefficient value has a local maximum value, examples of the method of determining a dominant frequency band may include designating, a band among all subbands in which a norm coefficient value has a maximum value as a dominant frequency band or designating as a dominant frequency band, a band having a norm coefficient value exceeding a predetermined threshold or a threshold calculated from norms of all subbands.
Dominant group determining sections 302-1 to 302N adaptively determine group widths according to input signal characteristics centered on the dominant frequency band outputted from dominant frequency band identification section 301. More specifically, the group width is defined as the width of a group of subbands centered on and on both sides of the dominant frequency band up to subbands where a descending slope of the norm coefficient value stops. Dominant group determining sections 302-1 to 302N determine frequency bands included in group widths as dominant groups and output the determined dominant groups to non-dominant group determining section 303. Note that when a dominant frequency band is located at an edge (end of an available frequency), only one side of the descending slope is included in the group.
Non-dominant group determining section 303 determines continuous subbands outputted from dominant group determining sections 302-1 to 302N other than the dominant groups as non-dominant groups without dominant frequency bands. Non-dominant group determining section 303 outputs the dominant groups and the non-dominant groups to group energy calculation section 304 and norm variance calculation section 306.
Group energy calculation section 304 calculates group-specific energy of the dominant groups and the non-dominant groups outputted from non-dominant group determining section 303 and outputs the calculated energy to total energy calculation section 305 and group bit distribution section 308. The group-specific energy is calculated by following equation 1.

Energy(G(k))=Σi=1MNorm(i)  (Equation 1)

Here, k denotes an index of each group, Energy(G(k)) denotes energy of group k, i denotes a subband index of group k, M denotes the total number of subbands of group k and Norm(i) denotes a norm coefficient value of subband i of group k.
Total energy calculation section 305 adds up all group-specific energy outputted from group energy calculation section 304 and calculates total energy of all groups. The total energy calculated is outputted to group bit distribution section 308. The total energy is calculated by following equation 2.

Energytotal=Σk=1NEnergy(G(k))  (Equation 2)

Here, Energytotal denotes total energy of all groups, N denotes the total number of groups in a spectrum, k denotes an index of each group, and Energy(G(k)) denotes energy of group k.
Norm variance calculation section 306 calculates group-specific norm variance for the dominant groups and the non-dominant groups outputted from non-dominant group determining section 303, and outputs the calculated norm variance to total norm variance calculation section 307 and group bit distribution section 308. The group-specific norm variance is calculated by following equation 3.

Normvar(G(k))=Normmax(G(k))−Normmin(G(k))   (Equation 3)

Here, k denotes an index of each group, Normvar(G(k)) denotes a norm variance of group k, Normmax(G(k)) denotes a maximum norm coefficient value of group k, and Normmin(G(k)) denotes a minimum norm coefficient value of group k.
Total norm variance calculation section 307 calculates a total norm variance of all groups based on the group-specific norm variance outputted from norm variance calculation section 306. The calculated total norm variance is outputted to group bit distribution section 308. The total norm variance is calculated by following equation 4.

Normvartotal=Σk=1NNormvar(G(k))  (Equation 4)

Here, Normvartotal denotes a total norm variance of all groups, N denotes the total number of groups in a spectrum, k denotes an index of each group, and Normvar(G(k)) denotes a norm variance of group k.
Group bit distribution section 308 (corresponding to a first bit allocation section) distributes bits on a group-by-group basis based on group-specific energy outputted from group energy calculation section 304, total energy of all groups outputted from total energy calculation section 305, group-specific norm variance outputted from norm variance calculation section 306 and total norm variance of all groups outputted from total norm variance calculation section 307, and outputs bits distributed on a group-by-group basis to subband bit distribution section 309. Bits distributed on a group-by-group basis are calculated by following equation 5.




  
    
      
        
          
        
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Here, k denotes an index of each group, Bits(G(k)) denotes the number of bits distributed to group k, Bitstotal denotes the total number of available bits, scale1 denotes the ratio of bits allocated by energy, Energy(G(k)) denotes energy of group k, Energytotal denotes total energy of all groups, and Normvar(G(k)) denotes a norm variance of group k.
Furthermore, scale1 in equation 5 above takes on a value within a range of [0, 1] and adjusts the ratio of bits allocated by energy or norm variance. The greater the value of scale1, the more bits are allocated by energy, and in an extreme case, if the value is 1, all bits are allocated by energy. The smaller the value of scale1, the more bits are allocated by norm variance, and in an extreme case, if the value is 0, all bits are allocated by norm variance.
By distributing bits on a group-by-group basis as described above, group bit distribution section 308 can distribute more bits to dominant groups and distribute fewer bits to non-dominant groups.
Thus, group bit distribution section 308 can determine the perceptual importance of each group by energy and norm variance and enhance dominant groups more. The norm variance matches a masking theory and can determine the perceptual importance more accurately.
Subband bit distribution section 309 (corresponding to a second bit allocation section) distributes bits to subbands in each group based on group-specific bits outputted from group bit distribution section 308 and outputs the bits allocated to group-specific subbands to lattice-vector coding section 108 as the bit allocation result. Here, more bits are distributed to perceptually important subbands and fewer bits are distributed to perceptually less important subbands. Bits distributed to each subband in a group are calculated by following equation 6.




  
    
      
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Here, BitsG(k)sb(i) denotes a bit allocated to subband i of group k, i denotes a subband index of group k, Bits(G(k)) denotes a bit allocated to group k, Energy(G(k)) denotes energy of group k, and Norm(i) denotes a norm coefficient value of subband i of group k.
Next, a grouping method will be described using FIGS. 8A to 8C. Suppose that a quantized spectral envelope shown in FIG. 8A is inputted to peak frequency band identification section 301. Peak frequency band identification section 301 identifies dominant frequency bands 9 and 20 based on the inputted quantized spectral envelope (see FIG. 8B).
Dominant group generation sections 302-1 to 302-N determine subbands centered on and on both sides of dominant frequency bands 9 and 20 up to subbands where a descending slope of the norm coefficient value stops as an identical dominant group. In examples in FIGS. 8A to 8C, as for dominant frequency band 9, subbands 6 to 12 are determined as dominant group (group 2), while as for dominant frequency band 20, subband 17 to 22 are determined as dominant group (group 4) (see FIG. 8C).
Non-dominant group determining section 303 determines continuous frequency bands other than the dominant groups as non-dominant groups without the dominant frequency bands. In the example in FIGS. 8A to 8C, subbands 1 to 5 (group 1), subbands 13 to 16 (group 3) and subbands 23 to 25 (group 5) are determined as non-dominant groups respectively (see FIG. 8C).
As a result, the quantized spectral envelopes are split into five groups, that is, two dominant groups (groups 2 and 4) and three non-dominant groups (groups 1, 3 and 5).
Using such a grouping method, it is possible to adaptively determine group widths according to input signal characteristics. According to this method, the speech/audio decoding apparatus also uses available quantized norm coefficients, and therefore additional information need not be transmitted to the speech/audio decoding apparatus.
Note that norm variance calculation section 306 calculates a group-specific norm variance. In the examples in FIGS. 8A to 8C, norm variance Energyvar(G(2)) in group 2 is shown in FIG. 9 as a reference.
Next, the perceptual importance will be described. A spectrum of a speech/audio signal generally includes a plurality of peaks (mountains) and valleys. A peak is made up of a spectrum component located at a dominant frequency of the speech/audio signal (dominant sound component). The peak is perceptually very important. The perceptual importance of the peak can be determined by a difference between energy of the peak and energy of the valley, that is, by a norm variance. Theoretically, when a peak has sufficiently large energy compared to neighboring frequency bands, the peak should be encoded with a sufficient number of bits, but if the peak is encoded with an insufficient number of bits, coding noise that mixes in becomes outstanding, causing sound quality to deteriorate. On the other hand, a valley is not made up of any dominant sound component of a speech/audio signal and is perceptually not important.
According to the frequency band grouping method of the present embodiment, a dominant frequency band corresponds to a peak of a spectrum and grouping frequency bands means separating peaks (dominant groups including dominant frequency bands) from valleys (non-dominant groups without dominant frequency bands).
Group bit distribution section 308 determines perceptual importance of a peak. In contrast to the G.719 technique in which perceptual importance is determined only by energy, the present embodiment determines perceptual importance based on both energy and norm (energy) distributions and determines bits to be distributed to each group based on the determined perceptual importance.
In subband bit distribution section 309, when a norm variance in a group is large, this means that this group is one of peaks, the peak is perceptually more important and a norm coefficient having a maximum value should be accurately encoded. For this reason, more bits are distributed to each subband of this peak. On the other hand, when a norm variance in a group is very small, this means that this group is one of valleys, and the valley is perceptually not important and need not be accurately encoded. For this reason, fewer bits are distributed to each subband of this group.
Thus, the present embodiment identifies a dominant frequency band in which a norm coefficient value in a spectrum of an input speech/audio signal has a local maximum value, groups all subbands into dominant groups including a dominant frequency band and non-dominant groups not including any dominant frequency band, distributes bits to each group based on group-specific energy and norm variances, and further distributes the bits distributed on a group-by-group basis to each subband according to a ratio of a norm to energy of each group. In this way, it is possible to allocate more bits to perceptually important groups and subbands and perform an efficient bit distribution. As a result, sound quality can be improved.
Note that the norm coefficient in the present embodiment represents subband energy and is also referred to as “energy envelope.”
The disclosure of Japanese Patent Application No. 2012-272571, filed on Dec. 13, 2012, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
INDUSTRIAL APPLICABILITY
The speech/audio coding apparatus, speech/audio decoding apparatus, speech/audio coding method and speech/audio decoding method according to the present invention are applicable to a radio communication terminal apparatus, radio communication base station apparatus, telephone conference terminal apparatus, video conference terminal apparatus and voice over Internet protocol (VoIP) terminal apparatus or the like.
REFERENCE SIGNS LIST


    101 Transient detector
    102 Transformation section
    103 Norm estimation section
    104 Norm quantization section
    105 Spectrum normalization section
    106, 203 Norm adjustment section
    107, 204 Bit allocation section
    108 Lattice-vector coding section
    109 Noise level adjustment section
    110 Multiplexer
    201 Demultiplexer
    202 Norm de-quantization section
    205 Lattice decoding section
    206 Spectral-fill generator
    207 Adder
    208 Envelope shaping section
    209 Inverse transformation section
    301 Dominant frequency band identification section
    302-1 to 302-N Dominant group determining section
    303 Non-dominant group determining section
    304 Group energy calculation section
    305 Total energy calculation section
    306 Norm variance calculation section
    307 Total norm variance calculation section
    308 Group bit distribution section
    309 Subband bit distribution section

## Claims

1. A speech/audio coding apparatus comprising:
a receiver that receives a time-domain speech/audio input signal;
a memory; and
a processor that
transforms the speech/audio input signal into a frequency domain;
splits a frequency spectrum of the speech/audio signal to obtain a plurality of subbands;
estimates an energy envelope which represents an energy level for each of the plurality of subbands;
quantizes the energy envelope;
determines a plurality of groups from the quantized energy envelope, each of the plurality of groups being composed of a plurality of subbands;
allocates bits to the determined plurality of groups on a group-by-group basis;
allocates the bits allocated to each of the plurality of groups to the plurality of subbands included in each of the groups on a subband-by-subband basis; and
encodes the frequency spectrum using the bits allocated to the subbands,

wherein, when determining the plurality of groups, the processor
identifies one or more dominant groups which are composed of a dominant frequency subband in which an energy envelope of the frequency spectrum has a local maximum value and mutually adjacent subbands on both sides of the dominant frequency subband, the mutually adjacent subbands each forming a descending slope of an energy envelope, and
identifies one or more non-dominant groups which are composed of mutually adjacent subbands other than those included in the one or more dominant groups.

2. The speech/audio coding apparatus according to claim 1,
wherein the processor further calculates group-specific energy, and
wherein the processor allocates, based on the calculated group-specific energy, more bits to a group when the energy is greater and allocates fewer bits to a group when the energy is smaller.

3. The speech/audio coding apparatus according to claim 1, wherein the processor allocates more bits to a subband having a greater energy envelope and allocates fewer bits to a subband having a smaller energy envelope.

4. The speech/audio coding apparatus according to claim 1, wherein a group width of the dominant group is defined as a width of a group of subbands centered on both sides of the dominant frequency band up to subbands where a descending slope of a norm coefficient value ends.

5. The speech/audio coding apparatus according to claim 1, wherein when the dominant frequency band is the highest frequency band or the lowest frequency band among available frequency bands, only one side of the descending slope is included in the dominant group.

6. A speech/audio decoding apparatus comprising:
a receiver that receives encoded speech/audio data;
a memory; and
a processor that
de-quantizes a quantized spectral envelope;
determines a plurality of groups from the quantized spectral envelope, each of the plurality of groups being composed of a plurality of subbands;
allocates bits to the determined plurality of groups on a group-by-group basis;
allocates the bits allocated to each of the plurality of groups to the plurality of subbands included in each of the groups on a subband-by-subband basis;
decodes a frequency spectrum of a speech/audio signal using the bits allocated to the subbands;
applies the de-quantized spectral envelope to the decoded frequency spectrum and reproduces a decoded spectrum; and
inversely transforms the decoded spectrum from a frequency domain to a time domain,
wherein, when determining the plurality of groups, the processor
identifies one or more dominant groups which are composed of a dominant frequency subband in which an energy envelope of the frequency spectrum has a local maximum value and mutually adjacent subbands on both sides of the dominant frequency subband, the mutually adjacent subbands each forming a descending slope of an energy envelope, and
identifies one or more non-dominant groups which are composed of mutually adjacent subbands other than those included in the one or more dominant groups.

7. The speech/audio decoding apparatus according to claim 6, wherein the processor further calculates group-specific energy, and
wherein the processor allocates, based on the calculated group-specific energy, more bits to the groups when the energy is greater and allocates fewer bits to the groups when the energy is smaller.

8. The speech/audio decoding apparatus according to claim 6, wherein the processor allocates more bits to subbands having a greater energy envelope and allocates fewer bits to subbands having a smaller energy envelope.

9. A speech/audio coding method comprising:
receiving a time-domain speech/audio input signal;
transforming the speech/audio input signal into a frequency domain;
splitting a frequency spectrum of the speech/audio signal to obtain a plurality of subbands;
estimating an energy envelope that represents an energy level for each of the plurality of subbands;
quantizing the energy envelope;
determining, from the quantized energy envelope, a plurality of groups, each of the plurality of groups being composed of a plurality of subbands;
allocating bits to the determined plurality of groups on a group-by-group basis;
allocating the bits allocated to each of the plurality of groups to the plurality of subbands included in each of the groups on a subband-by-subband basis; and
encoding the frequency spectrum using the bits allocated to the subbands,
wherein, when determining the plurality of groups,
identifying one or more dominant groups which are composed of a dominant frequency subband in which an energy envelope of the frequency spectrum has a local maximum value and mutually adjacent subbands on both sides of the dominant frequency subband, the mutually adjacent subbands each forming a descending slope of an energy envelope, and
identifying one or more non-dominant groups which are composed of mutually adjacent subbands other than those included in the one or more dominant groups.

10. A speech/audio decoding method comprising:
receiving encoded speech/audio data;
de-quantizing a quantized spectral envelope;
determining a plurality of groups from the quantized spectral envelope, each of the plurality of groups being composed of a plurality of subbands;
allocating bits to the determined plurality of groups on a group-by-group basis;
allocating the bits allocated to each of the plurality of groups to the plurality of subbands included in each of the groups on a subband-by-subband basis;
decoding a frequency spectrum of a speech/audio signal using the bits allocated to the subbands;
applying the de-quantized spectral envelope to the decoded frequency spectrum and reproducing a decoded spectrum; and
inversely transforming the decoded spectrum from a frequency domain to a time domain,
wherein, when determining the plurality of groups,
identifying one or more dominant groups which are composed of a dominant frequency subband in which an energy envelope of the frequency spectrum has a local maximum value and mutually adjacent subbands on both sides of the dominant frequency subband, the mutually adjacent subbands each forming a descending slope of an energy envelope, and
identifying one or more non-dominant groups which are composed of mutually adjacent subbands other than those included in the one or more dominant groups.