Abstract:
An audio signal coding device, an audio signal decoding device and a method to improve audio quality. The audio signal coding device and method include a quantizer that quantizes a given signal according to a number of assigned bits in order to generate a codeword. The coding device includes an extractor that extracts core bits from the generated codeword. The coding device also includes a determiner that determines an optimal value of the number of assigned bits based on an energy level corresponding to the extracted core bits. The audio signal decoding device and method include a dequantizer that dequantizes a given codeword according to the number of assigned bits to generate a decoded signal. The decoding device includes an extractor that extracts core bits from the given codeword. The decoding device also includes a determiner that determines an optimal value of the number of assigned bits used in the dequantizer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a speech coding apparatus, speech decoding apparatus and speech coding/decoding method in sub-band ADPCM (Adaptive Differential Pulse Code Modulation). 
   2. Description of the Related Art 
   Conventionally, as a speech coding apparatus and speech decoding apparatus used in sub-band ADPCM, there are known apparatuses conforming to ITU-T (International Telecommunication Union Telecommunication sector) Recommendation G.722. 
     FIG. 1  is a block diagram illustrating configurations of speech coding apparatus  300  and speech decoding apparatus  400  used in two-sub-band ADPCM described in Recommendation G.722. 
   Speech coding apparatus  300  is comprised of 24-tap splitting filter bank  310  that splits a frequency band of an input signal to two sub-bands and outputs sub-band signals, ADPCM quantizers  320   a  and  320   b  that quantize respective two-split-sub-band signals, and multiplexer  330  that multiplexes codewords quantized in ADPCM quantizers  320   a  and  320   b  to produce a bit stream. 
   Meanwhile, speech decoding apparatus  400  is comprised of demultiplexer  410  that outputs codewords for each sub-band obtained from transmitted data streams, ADPCM dequantizers  420   a  and  420   b  that dequnantize respective codewords for each sub-band output from demuletiplexer  410  to output sub-band signals, and 24-tap synthesis filter bank  430  that performs synthesis filtering on the sub-band signals. 
   Operations of speech coding apparatus  300  and speech decoding apparatus  400  each configured as mentioned above will be described below. 
   A frequency band of an input signal is split to two sub-bands in splitting filter bank  310  and two sub-band signals are generated. Each of the sub-band signals is assigned a predetermined number of quantizing bits and quantized in respective one of ADPCM quantizers  320   a  and  320   b.  The codewords obtained by quantization are multiplexed in multiplexer  330  to be bit streams. 
   Meanwhile, in speech decoding apparatus  400 , the bit streams with a plurality of multiplexed codewords are demulitiplexed in demultiplexer  410  to be codewords for each sub-band. The codewords for each sub-band obtained by demultiplexing are dequantized in ADPCM dequantizers  420   a  and  420   b  to be sub-band signals. The sub-band signals are subjected to synthesis in synthesis filter bank  430  to be a decoded signal. 
   However, in the conventional speech coding apparatus and speech decoding apparatus as described above, since the number of quantizing bits is fixed which is assigned to each sub-band signal in an ADPCM quantizer in the speech coding apparatus, in particular, when a sampling frequency of an input signal becomes high, there is a risk that the bit assignment is not optimal and that audio quality of decoded signals may deteriorate in the speech decoding apparatus. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to improve the audio quality. 
   It is a subject matter of the present invention to in sub-band ADCPM coding in which residual signals between a plurality of sub-band signals for each frequency band split from an input signal and respective prediction values are each quantized, and each quantized output is dequantized to calculate a prediction value of a next frame of the sub-band signal, determine the number of quantizing bits assigned to a next frame of each residual signal in a process of calculating a prediction value of the next frame from a last frame, and thereby change the bit assignment adaptively. 
   According to an aspect of the invention, a speech coding apparatus that performs coding on speech signals in a sub-band ADPCM scheme has a generating section that quantizes a given sub-band signal according to the number of assigned bits to generate a codeword, and a determining section that determines an optimal value of the number of assigned bits used in the generating section. 
   According to another aspect of the invention, a speech decoding apparatus that performs decoding on speech signals in the sub-band ADPCM scheme has a generating section that dequantizes a given codeword according to the number of assigned bits to generate a decoded sub-band signal, and a determining section that determines an optimal value of the number of assigned bits used in the generating section. 
   According to still another aspect of the invention, a speech coding/decoding method for performing coding and decoding on speech signals in the sub-band ADPCM scheme has a determining step of determining an optimal value of the number of assigned bits to quantize a given sub-band signal, a quantizing step of quantizing the sub-band signal according to the determined optimal value of the number of assigned bits to generate a codeword, an acquiring step of acquiring the optimal value of the number of assigned bits based on the codeword, and a dequantizing step of dequantizing the codeword according to the acquired optimal value of the number of assigned bits to generate a decoded sub-band signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the invention will appear more fully hereinafter from a consideration of the following description taken in connection with the accompanying drawing wherein one example is illustrated by way of example, in which; 
       FIG. 1  is a block diagram illustrating configurations of a conventional speech coding apparatus and speech decoding apparatus used in two-sub-band ADPCM; 
       FIG. 2  is a block diagram illustrating a configuration of a speech coding apparatus according to first and second embodiments of the present invention; 
       FIG. 3  is a block diagram illustrating a primary configuration of the speech coding apparatus according to the first embodiment of the present invention; 
       FIG. 4  is a view showing an example of quantizing bit number assignment according to the first embodiment of the present invention; 
       FIG. 5  is a block diagram illustrating a configuration of a speech decoding apparatus according to the first and second embodiments of the present invention; 
       FIG. 6  is a block diagram illustrating a primary configuration of the speech decoding apparatus according to the first embodiment of the present invention; 
       FIG. 7  is a block diagram illustrating a primary configuration of the speech coding apparatus according to the second embodiment of the present invention; and 
       FIG. 8  is a block diagram illustrating a primary configuration of the speech decoding apparatus according to the second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention will be described below specifically with reference to accompanying drawings. 
   (First Embodiment) 
     FIG. 2  is a block diagram illustrating a configuration of a speech coding apparatus according to the first embodiment of the present invention. In  FIG. 2 , splitting filter bank  100  splits a frequency band of an input signal into four sub-bands with the same bandwidth, and performs thinning processing using “4” that is the number of splits, as a thinning number. Band splitting FIR filters  110   a  to  110   d  in splitting filter bank  100  perform splitting filtering on an input signal for predetermined frequency bands. Splitting filter bank  100  is a cosine modulation filter bank, and impulse responses of band splitting FIR filters  110   a  to  110   d  that are basic filters are asymmetric. 
   Further, downsamplers  120   a  to  120   d  in splitting filter bank  100  perform the thinning processing on respective outputs of band splitting FIR filters  110   a  to  110   d  for coding efficiency, using, as the number of thinning, “4” equal to the number of splits in splitting filter bank  100 , and output respective sub-band signals. 
   Each of ADPCM quantizers  130   a  to  130   d  quantizes a residual signal between the respective sub-band signal and a prediction value calculated from the last frame of the sub-band signal to output a scalable codeword. Further, each of ADPCM quantizers  130   a  to  130   d  calculates a dequantized value and scale factor from the residual signal. 
   Adaptive bit assigner  140  determines the number of quantizing bits to assign to each of residual signals based on an energy value of the dequantized value calculated in respective one of ADPCM quantizers  130   a  to  130   d.    
   Multiplexer  150  multiplexes codewords output from ADPCM quantizers  130   a  to  130   d  to produce a bit stream that is a multiplexed signal. 
     FIG. 3  is a block diagram illustrating a primary configuration of the speech coding apparatus according to the first embodiment of the present invention. While  FIG. 3  illustrates a configuration of ADPCM quantizer  130   a  and adaptive bit assigner  140 , the other ADPCM quantizers,  130   b  to  130   d,  have the same configuration as that of the quantizer  130   a  , and are connected to adaptive bit assigner  140 . 
   In  FIG. 3 , adder  131  calculates a difference between the sub-band signal input to respective one of ADPCM quantizers  130   a  to  130   d  and a prediction value to generate a residual signal. Quantizing section  132  quantizes the generated residual signal using the scale factor, and outputs a codeword with the number of quantizing bits determined in adaptive bit assigner  140 . Core bit extracting section  133  deletes least significant bits (hereinafter, referred to as “LSB”) from the codeword output from quantizing section  132  to extract core bits. Scale factor adapting section  134  calculates a scale factor from the extracted core bits. Dequantizing section  135  dequantizes the extracted core bits, and outputs a dequantized value to predicting section  136 , adder  137 , and adaptive bit assigner  140 . Predicting section  136  performs zero prediction and pole prediction using the dequantized value and an output of the predicting section  136 , and calculates a prediction value of a next frame of the sub-band signal. Adder  137  calculates the sum of the dequantized value and the prediction value calculated in predicting section  136 . 
   The operation of the speech coding apparatus configured as described above will be described next. 
   A speech signal input to the speech coding apparatus is split into four sub-band signals in splitting filter bank  100 . Since splitting filter bank  100  is a cosine modulation filter bank and impulse responses of band splitting FIR filters  110   a  to  110   d  that are basic filters are asymmetric, a group delay occurring in filtering is decreased, and it is thereby possible to reduce an amount of computation. The split sub-band signals are input to ACDCM quantizers  130   a  to  130   d  respectively. 
   Adder  131  calculates a residual signal between the sub-band signal input to respective one of ADPCM quantizers  130   a  to  130   d  and a prediction value calculated from the last frame in predicting section  136 , and inputs the calculated residual signal to quantizing section  132 . The residual signal is quantized in quantizing section  132  to be a codeword with the number of quantizing bits assigned by adaptive bit assigner  140 . Quantizing the residual signal uses the scale factor calculated in scale factor adapting section  134 . The codeword quantized in quantizing section  132  is output to multiplexer  150 , and also to core bit extracting section  133 . The section  133  deletes LSB to extract core bits. The extracted core bits are input to scale factor adapting section  134  to be used in calculating a scale factor, and also to dequantizing section  135 . Herein, the codeword quantized in quantizing section  132  becomes scalable to keep the consistency of the scale factor. 
   Dequantizing section  135  dequantizes the core bits using the scale factor calculated in scale factor adapting section  134 . The dequantized value obtained by dequantizing the core bits is input to predicting section  136 . This input value is called a zero prediction input value. The dequantized value is added in adder  137  to a prediction value of a last frame output from predicting section  136 , and is input again to predicting section  136 . This input value is called a pole prediction input value. Using the zero prediction input value and pole prediction input value, predicting section  136  calculates a prediction value of a next frame of the sub-band signal. 
   The dequantized value is input to adaptive bit assigner  140  per a predetermined number of frames such as a pitch period basis. Adaptive bit assigner  140  calculates an energy of the dequantized value, i.e., square sum of the dequantized value as a sample, output from each of ADPCM quantizers  130   a  to  130   d,  and based on the calculated energy of the dequantized value, determines the number of bits assigned to each residual signal to be quantized in respective one of ADPCM quantizers  130   a  to  130   d.    
   The determined numbers of quantizing bits are output to respective quantizing sections  132  in ADPCM quantizers  130   a  to  130   d.  As described above, each quantizing section  132  quantizes the residual signal of the next frame using the scale factor, and outputs a codeword with the number of assigned bits. Codewords quantized in ADPCM quantizers  130   a  to  130   d  are multiplexed in multiplexer  150  to be a bit stream that is a multiplexed signal. 
     FIG. 4  illustrates an example of quantizing bit number assignment. In  FIG. 4 , bits shown by oblique line indicate core bits in each band. The number of the core bits is five in the first band, four in the second band, three in the third band and two in the fourth band. The core bits are always constant in every band, and bits assigned adaptively by adaptive bit assigner  140  are two bits shown by white in  FIG. 4 . The two bits are assigned adaptively to each band corresponding to the energy of the dequantized value. 
   A speech decoding apparatus according to the first embodiment will be described below. 
     FIG. 5  is a block diagram illustrating a configuration of the speech decoding apparatus according to the first embodiment of the present invention. In  FIG. 5 , demultiplexer  200  decomposes an input bit stream every a number of bits assigned by adaptive bit assigner  220  described later and thus splits the bit stream into codewords for each sub-band. Each of ADPCM dequantizers  210   a  to  210   d  outputs a sum of a decoded residual signal obtained by dequantizing a respective codeword and a prediction value calculated from a codeword of a last frame as a decoded sub-band signal. Further, each of ADPCM dequantizers  210   a  to  210   d  calculates a dequantized value of only core bits obtained by deleting LSB from the codeword, and the scale factor. Based on the energy of the dequantized value of the core bits calculated in each of ADPCM dequantizers  210   a  to  210   d,  adaptive bit assigner  220  calculates the number of quantizing bits assigned to the respective residual signal in the speech coding apparatus. 
   Synthesis filter bank  230  combines decoded sub-band signals output from ADPCM dequantizers  210   a  to  210   d  to obtain a decoded signal. Upsamplers  240   a  to  240   d  in synthesis filter bank  230  perform interpolation of thinned respective decoded sub-band signals. Band synthesis FIR filters  250   a  to  250   d  in synthesis filter bank  230  perform synthesis filtering on respective interpolated decoded sub-band signals. Synthesis filter bank  230  is a cosine modulation filter bank, and impulse responses of band synthesis FIR filters  250   a  to  250   d  that are basic filters are asymmetric. 
     FIG. 6  is a block diagram illustrating a primary configuration of the speech decoding apparatus according to the first embodiment of the present invention. While  FIG. 6  illustrates a configuration of ADPCM dequantizer  210   a  and adaptive bit assigner  220 , the other ADPCM dequantizers,  210   b  to  210   d,  have the same configuration as that of the dequantizer  210   a  , and are connected to adaptive bit assigner  220 . 
   In  FIG. 6 , core bit extracting section  211  deletes LSB from the codeword input to respective one of ADPCM dequantizers  210   a  to  210   d  to extract core bits. Dequantizing section  212  dequantizes the extracted core bits, and outputs a dequantized value to adder  214 , predicting section  215 , and adaptive bit assigner  220 . Scale factor adapting section  213  calculates a scale factor from the extracted core bits. Adder  214  calculates the sum of the dequantized value and the prediction value calculated in predicting section  215 . Predicting section  215  performs zero prediction and pole prediction using the dequantized value and an output of the prediction section  215 , and calculates a prediction value of a next frame of the decoded sub-band signal. Dequantizing section  216  dequantizes the input codeword every a number of quantizing bits calculated in adaptive bit assigner  220  using the scale factor, and outputs a decoded residual signal. Adder  217  calculates the sum of the decoded residual signal output from dequantizing section  216  and the prediction value to generate a decoded sub-band signal. 
   The operation of the speech decoding apparatus configured as described above will be described next. 
   A bit stream input to the speech decoding apparatus is decomposed per a number of quantizing bits assigned by bit assigner  220 , and thus split into codewords every four sub-bands. The split codewords are input to respective ADPCM dequantizers  210   a  to  210   d.    
   The codeword input to each of the ADPCM dequantizers  210   a  to  210   d  is dequantized in dequantizing section  216  corresponding to the number of quantizing bits assigned by adaptive bit assigner  220  and output as a decoded residual signal. From the codeword input to respective one of ADPCM dequantizers  210   a  to  210   d,  LSB is deleted and core bits are extracted in core bit extracting section  211 . The extracted core bits are input to scale factor adapting section  213  to be used in calculating a scale factor, and also to dequantizing section  212 . In dequantizing section  212 , the core bits are dequantized using the scale factor calculated in scale factor adapting section  213 . The dequantized value obtained by dequantizing the core bits is input to predicting section  215 . This input value is called a zero prediction input value. The dequantized value is added in adder  214  to a prediction value of a last frame output from predicting section  215 , and is input again to predicting section  215 . This input value is called a pole prediction input value. Using the zero prediction input value and pole prediction input value, predicting section  215  calculates a prediction value of a next frame of the decoded sub-band signal. 
   The dequantized value is input to adaptive bit assigner  220  per a predetermined number of frames such as a pitch period basis. Adaptive bit assigner  220  calculates an energy of the dequantized value, i.e., square sum of the dequantized value as a sample, output from the each of ADPCM dequantizers  210   a  to  210   d,  and based on the calculated energy of the dequantized value, calculates the number of quantizing bits assigned to each residual signal quantized in respective one of ADPCM quantizers  130   a  to  130   d  in the speech coding apparatus. 
   The calculated numbers of quantizing bits are output to dequantizing section  216  in respective one of ADPCM dequantizers  210   a  to  210   d,  and as described above, dequantizing section  216  dequantizes a codeword of a next frame using the scale factor corresponding to the number of bits assigned in adaptive bit assigner  220  and outputs a decoded residual signal. The output decoded residual signal is added in adder  217  to the prediction value output from predicting section  215  to be a decoded sub-band signal, and the decoded sub-band signal is output from each of ADPCM dequantizers  210   a  to  210   d.    
   The decoded sub-band signals dequantized in ADPCM dequantizers  210   a  to  210   d  are subjected to interpolation in upsamplers  240   a  to  240   d  in synthesis filter bank  230 , and to synthesis filtering in band synthesis FIR filters  250   a  to  250   d.  The respective outputs from band synthesis FIR filters  250   a  to  250   d  are added in adders  260   a  to  260   c  to be a decoded signal. Herein, since synthesis filter bank  230  is a cosine modulation filter bank and impulse responses of band synthesis FIR filters  250   a  to  250   d  that are basic filters are asymmetric, a group delay occurring in filtering is decreased, and it is thereby possible to reduce an amount of computation. 
   Thus, according to the speech coding apparatus and speech decoding apparatus of this embodiment, in the speech coding apparatus, a residual signal between a sub-band signal for each frequency band and a prediction value is quantized to output to a codeword, the output codeword is dequantized to calculate an energy of the dequantized value, and the number of quantizing bits assigned in quantizing a next frame of each residual signal is determined based on the calculated energy. In the speech decoding apparatus, the same codeword as that dequantized in the speech coding apparatus is dequantized to calculate the energy of the dequantized value, and based on the calculated energy, the number of quantizing bits is calculated which is determined in the speech coding apparatus to assign to a next frame of each residual signal. As a result, the speech coding apparatus is capable of assigning the number of quantizing bits adaptively to each residual signal, and even when the speech coding apparatus changes the number of assigned quantizing bits, the speech decoding apparatus is capable of performing dequantization in sync with changes in the bit assignment in the speech coding apparatus without obtaining information of the changed bit assignment. Accordingly, since the speech coding apparatus does not need to notify the speech decoding apparatus of the information of the changed bit assignment to synchronize, it is possible to improve the audio quality without degrading the transmission efficiency of speech information. 
   (Second Embodiment) 
   It is a feature of the speech coding apparatus and speech decoding apparatus according to the second embodiment of the present invention to use a scale factor in determining an optimal value of the number of quantizing bits. In addition, configurations of the speech coding apparatus and speech decoding apparatus according to the second embodiment are the same as those of the speech coding apparatus and speech decoding apparatus illustrated in  FIGS. 2 and 5  of the first embodiment, respectively, and descriptions thereof are omitted. 
     FIG. 7  is a block diagram illustrating a primary configuration of the speech coding apparatus according to the second embodiment of the present invention. While  FIG. 7  illustrates a configuration of ADPCM quantizer  130   a  and adaptive bit assigner  140   a,  the other ADPCM quantizers,  130   b  to  130   d,  have the same configuration as that of the quantizer  130   a,  and are connected to adaptive bit assigner  140   a.  Further, the same sections as in  FIG. 3  are assigned the same reference numerals to omit descriptions thereof. 
   In  FIG. 7 , scale factor adapting section  134   a  calculates a scale factor from the core bits extracted in core bit extracting section  133  to output to adaptive bit assigner  140   a.  Dequantizing section  135   a  dequantizes the core bits extracted in core bit extracting section  133 , and outputs a dequantized value to predicting section  136  and adder  137 . Adaptive bit assigner  140   a  determines the number of quantizing bits to assign to each of residual signals based on a scale factor calculated in respective one of ADPCM quantizers  130   a  to  130   d.    
   The operation of the speech coding apparatus configured as described above will be described next. 
   Sub-band signals split in splitting filter bank  100  are input to ADPCM quantizers  130   a  to  130   d  respectively. Adder  131  calculates a residual signal between the sub-band signal input to respective one of the ADPCM quantizers  130   a  to  130   d  and a prediction value of a last frame calculated in predicting section  136 , and inputs the calculated residual signal to quantizing section  132 . The residual signal is quantized in quantizing section  132  to be a codeword with the number of quantizing bits assigned by adaptive bit assigner  140   a.  Quantizing the residual signal uses the scale factor calculated in scale factor adapting section  134   a.  The codeword quantized in quantizing section  132  is output to multiplexer  150 , and also to core bit extracting section  133 . The section  133  deletes LSB to extract core bits. The extracted core bits are input to scale factor adapting section  134   a  to be used in calculating a scale factor, and also to dequantizing section  135   a.  Herein, the codeword quantized in quantizing section  132  becomes scalable to keep the consistency of the scale factor. 
   Dequantizing section  135   a  dequantizes the core bits using the scale factor calculated in scale factor adapting section  134   a.  From the dequantized value obtained by dequantizing the core bits, predicting section  136  calculates a prediction value of a next frame of the sub-band signal. 
   The scale factor is input to adaptive bit assigner  140   a  per a predetermined number of frames such as a pitch period basis. Adaptive bit assigner  140   a  considers as an energy an average value of scale factors output from of ADPCM quantizers  130   a  to  130   d,  and as in the first embodiment, determines the number of quantizing bits assigned to each residual signal to be quantized in respective one of ADPCM quantizers  130   a  to  130   d.    
   The determined numbers of quantizing bits are output to respective quantizing sections  132  in ADPCM quantizers  130   a  to  130   d.  As described above, each quantizing section  132  quantizes the residual signal of the next frame using the scale factor, and outputs a codeword with the number of assigned bits. Codewords quantized in ADPCM quantizers  130   a  to  130   d  are multiplexed in multiplexer  150  to be a bit stream that is a multiplexed signal. 
   The speech decoding apparatus according to the second embodiment of the present invention will be described below. A configuration of the speech decoding apparatus according to the second embodiment is the same as that of the speech decoding apparatus illustrated in  FIG. 5  of the first embodiment, and descriptions thereof are omitted. 
     FIG. 8  is a block diagram illustrating a primary configuration of the speech decoding apparatus according to the second embodiment of the present invention. While  FIG. 8  illustrates a configuration of ADPCM dequantizer  210   a  and adaptive bit assigner  220   a,  the other ADPCM dequantizers,  210   b  to  210   d,  have the same configuration as that of the dequantizer  210   a,  and are connected to adaptive bit assigner  220   a.    
   In  FIG. 8 , core bit extracting section  211  deletes LSB from the codeword input to respective one of ADPCM dequantizers  210   a  to  210   d  to extract core bits. Dequantizing section  212   a  dequantizes the extracted core bits, and outputs a dequantized value to adder  214  and predicting section  215 . Scale factor adapting section  213   a  calculates a scale factor from the extracted core bits to output to adaptive bit assigner  220   a.  Adder  214  calculates the sum of the dequantized value and the prediction value calculated in predicting section  215 . Predicting section  215  performs zero prediction and pole prediction using the dequantized value and an output of the prediction section  215 , and calculates a prediction value of a next frame of the decoded sub-band signal. Dequantizing section  216  dequantizes the input codeword every a number of quantizing bits calculated in adaptive bit assigner  220   a  using the scale factor, and outputs a decoded residual signal. Adder  217  calculates the sum of the decoded residual signal output from dequantizing section  216  and the prediction value to generate a decoded sub-band signal. Adaptive bit assigner  220   a  determines the number of quantizing bits to assign to each of residual signals based on a scale factor calculated in respective one of ADPCM dequantizers  210   a  to  210   d.    
   The operation of the speech decoding apparatus configured as described above will be described next. 
   Codewords split in demultiplexer  200  are input to respective ADPCM dequantizers  210   a  to  210   d.  The codeword input to each of ADPCM dequantizers  210   a  to  210   d  is dequantized in dequantizing section  216  corresponding to the number of quantizing bits assigned by adaptive bit assigner  220   a,  and a decoded residual signal is output. From the codeword input to respective one of ADPCM dequantizers  210   a  to  210   d,  LSB is deleted and core bits are extracted in core bit extracting section  211 . The extracted core bits are input to scale factor adapting section  213   a  to be used in calculating a scale factor, and also to dequantizing section  212   a.  In dequantizing section  212   a,  the core bits are dequantized using the scale factor calculated in scale factor adapting section  213   a.  The dequantized value obtained by dequantizing the core bits is input to predicting section  215 . Predicting section  215  calculates a prediction value of a next frame of the decoded sub-band signal using the input dequantized value. 
   The scale factor is input to adaptive bit assigner  220   a  per a predetermined number of frames such as a pitch period basis. Adaptive bit assigner  220   a  considers as an energy an average value of scale factors output from of ADPCM dequantizers  210   a  to  210   d,  and as in the first embodiment, calculates the number of quantizing bits assigned to each residual signal quantized in respective one of ADPCM quantizers  130   a  to  130   d.    
   The calculated numbers of quantizing bits are output to dequantizing section  216  in respective one of ADPCM dequantizers  210   a  to  210   d,  and as described above, dequantizing section  216  dequantizes a codeword of a next frame using the scale factor corresponding to the number of bits assigned in adaptive bit assigner  220   a  and outputs a decoded residual signal. The output decoded residual signal is added in adder  217  to the prediction value output from predicting section  215  to be a decoded sub-band signal, and the decoded sub-band signal is output from each of ADPCM dequantizers  210   a  to  210   d.  The decoded sub-band signals dequantized in respective ADPCM dequantizers  210   a  to  210   d  are subjected to synthesis in synthesis filter bank  230  to be a decoded signal. 
   Thus, according to the speech coding apparatus and speech decoding apparatus of this embodiment, in the speech coding apparatus, a residual signal between a sub-band signal for each frequency band and a prediction value is quantized to output a codeword, a scale factor is calculated from core bits of the output codeword, and based on the calculated scale factor, the number of quantizing bits assigned in quantizing a next frame of each residual signal is determined. In the speech decoding apparatus, the scale factor is calculated using the same codeword as that dequantized in the speech coding apparatus, and based on the calculated scale factor, the number of quantizing bits is calculated which is determined in the speech coding apparatus to assign to a next frame of each residual signal. As a result, the speech coding apparatus is capable of assigning the number of quantizing bits adaptively to each residual signal, and even when the speech coding apparatus changes the number of assigned quantizing bits, the speech decoding apparatus is capable of performing dequantization in sync with changes in the bit assignment in the speech coding apparatus without obtaining information of the changed bit assignment. Accordingly, it is possible to improve the audio quality without degrading the transmission efficiency of speech information. 
   In addition, while each of the above-mentioned embodiments describes the case where an input signal is split into four sub-band signals in a splitting filter bank, the present invention is not limited to such a case, and it is only required to split an input signal into more than two signals corresponding to frequency band. In addition, increasing the number of splits provides smoothing on signals to be quantized, and improves the following characteristic of scale factor. Further, when a splitting filter bank is a cosine modulation filter, increasing the number of splits increases the number of taps of basic filter and suppress increases in delay amount. 
   As described above, according to the present invention, it is possible to provide a speech coding apparatus, speech decoding apparatus and speech coding/decoding method enabling improved audio quality. 
   The present invention is not limited to the above described embodiments, and various variations and modifications may be possible without departing from the scope of the present invention. 
   This application is based on the Japanese Patent Application No. 2001-347408 filed on Nov. 13, 2001, entire content of which is expressly incorporated by reference herein.