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.

Description:
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 ST 32 , subbands having a maximum norm are identified from among all subbands and in ST 33 , 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 ST 34 , the norms are reduced according to the bits allocated, and in ST 35 , 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 ST 32  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  302 N. 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  302 N 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  302 N 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  302 N 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 ))=Σ i=1   M Norm( 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.
 
Energy total =Σ k=1   N Energy( G ( k ))  (Equation 2)
 
     Here, Energy total  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.
 
Norm var ( G ( k ))=Norm max ( G ( k ))−Norm min ( G ( k ))   (Equation 3)
 
     Here, k denotes an index of each group, Norm var (G(k)) denotes a norm variance of group k, Norm max (G(k)) denotes a maximum norm coefficient value of group k, and Norm min (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.
 
Norm vartotal =Σ k=1   N Norm var ( G ( k ))  (Equation 4)
 
     Here, Norm vartotal  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 Norm var (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, Bits total  denotes the total number of available bits, scale1 denotes the ratio of bits allocated by energy, Energy(G(k)) denotes energy of group k, Energy total  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, Bits G(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 Energy var (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