Abstract:
A decoding apparatus is provided. The decoding apparatus has a first decoding part for decoding a code word obtained by encoding an input signal using a Code-Excited Linear Prediction encoding method. A second decoding part decodes a code word obtained by encoding a signal with an encoding method other than the Code-Excited Linear Prediction encoding method. A rising-transition detection and notification part has a detection part that detects the existence of a rising-transition of amplitude of the input signal based on time variation of a gain of excitation vectors obtained by the first decoding part, and a notification part that notifies the second decoding part that the rising-transition of the amplitude exists.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a decoding apparatus, an encoding apparatus, a decoding method and an encoding method. More particularly, the present invention relates to a decoding apparatus, and an encoding apparatus in which an input signal is compressed highly-efficiently and encoded or decoded, and a decoding method and an encoding method in which the input signal is compressed highly-efficiently and encoded or decoded.  
           [0003]    2. Description of the Related Art  
           [0004]    Presently, there are various kinds of encoding and decoding apparatuses and methods that highly-efficiently compress speech and acoustic signals. One of such encoding and decoding methods is a scalable encoding method in which a part of an encoded sequence can be decoded according to a required quality or status of a network because it has scalable encoding characteristics. The scalable encoding process has an architecture to successively encode an input signal in such a way that an error signal between the input signal and a decoded signal of a lower layer encoder is further encoded by a higher layer encoder. The lowest layer is called a core layer and higher layers than the lowest layer are called enhancement layers. An example of a representative scalable encoding method is described in ISO/IEC14496-3, which is called MPEG-4 Audio, standardized by ISO/IEC. FIG. 1 shows a block diagram of the scalable encoding process. In FIG. 1, the Code-Excited Linear Prediction (CELP) encoding method, a parametric encoding method, such as for example, the Harmonic Vector Excitation Coding (HVXC) method and the Harmonic Individual Line with Noise (HILN) method or, a transform coding method, such as, for example, the Advanced Audio Coding (AAC) method and the Transform Domain Weighted Interleave Vector Quantization (TwinVQ) method is used in a core layer encoder  101 . The encoders that perform the transform coding method are used in enhancement layer encoders  104 .  
           [0005]    [0005]FIG. 2 shows a block diagram of a CELP encoder. The CELP encoder as shown in FIG. 2 mainly has a linear prediction analyzer  201 , a linear prediction coefficient quantization part  202 , a linear prediction synthesis filter  203 , an adaptive code book  204 , a fixed code book  206 , a perceptual weighting filter  208 , a controller  209 , an adder  212  and a subtracter  213 . An input signal  200  is supplied to the CELP encoder every 5 to 40 ms and linear prediction analysis is performed on the input signal by the linear prediction analyzer  201 . Then, the linear prediction coefficients  210  obtained by the linear prediction analysis are quantized by the linear prediction coefficient quantization part  202 . The linear prediction synthesis filter  203  is constructed using the quantized linear prediction coefficients obtained as described above. Excitation vectors  211  to drive the linear prediction synthesis filter  203  are stored in the adaptive code book  204 . The adaptive code book excitation vector is output from the adaptive code book  204  and the fixed code book excitation vector is output from the fixed code book  206  according to an output signal from the controller  209 . Each of the vectors is multiplied by an adaptive code book gain  205  or a fixed code book gain  207 , respectively. Then, the excitation vector  211  is generated at an output of an adder  212  by means of adding the results multiplied by each of the gains. The excitation vector  211  generated as described above is supplied to the linear prediction synthesis filter  203 . An output signal of the linear prediction synthesis filter  203  is a synthesis signal, and an error signal between the input signal and the synthesis signal is calculated by the subtracter  213  and then, the error signal is supplied to the perceptual weighting filter  208 . The perceptual weighting filter  208  supplies the perceptually weighted error signal to the controller  209 . The controller  209  searches the excitation vector  211  so that the power level of the perceptually weighted error signal has minimum value and then, determines the adaptive code book gain  205  and the fixed code book gain  207  using the selected adaptive code book excitation vector and the selected fixed code book excitation vector, respectively, by the searches so that the power level of the perceptually weighted error signal has minimum value.  
           [0006]    [0006]FIG. 3 shows a block diagram of a CELP decoder  300 . In the decoder  300  as shown in FIG. 3, the coefficients for a linear prediction synthesis filter  305 , an adaptive code book  301 , an adaptive code book gain  302 , a fixed code book  303 , and a fixed code book gain  304  are extracted from a code word sequence  311 . The adaptive code book excitation vector and the fixed code book excitation vector are respectively multiplied by each of the gains and then, they are added by the adder  307  and then, the signal is an excited vector  306 . The linear prediction synthesis filter  305  is driven by the excitation vector  306  and a decoded signal  312  is supplied as an output signal.  
           [0007]    On the other hand, FIG. 4 shows an encoder  400  for transform coding. The encoder  400  mainly has an orthogonal transformation part  401 , a transform coefficient quantization part  402  and a quantized transform coefficient encoding part  403 . The transform coefficients  405  are calculated by performing the orthogonal transform for the input signal at the orthogonal transformation part  401 . The transform coefficients  405  are quantized by the transform coefficient quantization part  402  and then, the quantized transform coefficients  406  are encoded to an encoded code sequence  407  by the quantized transform coefficient encoding part  403 .  
           [0008]    [0008]FIG. 5 shows a block diagram of a decoder  500  for decoding a transform-encoded code sequence  504 . In the decoder as shown in FIG. 5, the encoded code sequence  504  is decoded to the quantized transform coefficients by the quantized transform coefficient decoding part  501  and then, the quantized transform coefficients are de-quantized to the transform coefficients by the transform coefficient de-quantization part  502 . The transform coefficients obtained as described above are inverse-orthogonally-transformed to a decoded signal by the inverse orthogonal transformation part  503 .  
           [0009]    As described above, in the transform coding, the input signal in the time domain is orthogonally transformed into the coefficients in the frequency domain and then, the quantization and the encoding are performed. Therefore, when the encoded code sequence is inversely-transformed into the signal in the time domain, quantization noise that is generated by the quantization in the frequency domain spreads over a whole transform block (that is an unit of the transform coding) at approximately the same level. Therefore, if there is steep rising-transition of amplitude, which is so called ‘attack’, in a part of an input signal within the transform block, a pre-echo that is a jarring noise will occur at a part prior to the steep rising-transition of the amplitude. For example, if a transform block length is long, the interval in which the pre-echo occurs is also long. Therefore, the subjective quality is further degraded. When the transform coding is used in the scalable encoding as described above, the same problem as the problem generated by the transform coding arises.  
           [0010]    To solve this problem, a technology of an adaptive block length conversion is used in the MPEG-4 Audio (ISO/IEC14496-3) as described above. In the technology, if there is a steep rising-transition of the amplitude in the input signal, a short transform block is used and, if there is not a steep rising-transition of the amplitude in the input signal, a long transform block is used. However, it is necessary to detect whether a steep rising-transition of the amplitude in the input signal exists or not in order to perform switching of the length. There is an example of such a detection method below. At first, the input signal is divided into the transform blocks and a Fourier transformation is performed on the transform blocks. Next, the obtained Fourier transform coefficients are divided to some frequency bands. Then, a parameter called perceptual entropy is calculated based on a signal to masking ratio (SMR) that is a ratio between the minimum audible noise calculated using a psychoacoustic model and the input signal power for each of the frequency bands. The steep rising-transition of the amplitude is detected by comparing the perceptual entropy with a predetermined threshold value. This method is used in the scalable encoding in the MPEG-4 Audio (ISO/IEC14496-3).  
           [0011]    However, in the prior art method as described above, the length of the transform block is only adjusted to become short in order to shorten the interval in which the pre-echo exists. Further, because the transform block length varies, supplementary information that indicates the transform block length is required in order to decode the encoded code sequence at the decoding side. Therefore, the structure of the system becomes complex.  
         SUMMARY OF THE INVENTION  
         [0012]    It is a general object of the present invention to provide a decoding apparatus, an encoding apparatus, a decoding method and an encoding method in which the above disadvantages are eliminated.  
           [0013]    A more specific object of the present invention is to provide an apparatus and a method that detect the rising-transition of the amplitude of the input signal and notify encoding or decoding parts using another encoding method, in which, in an encoding and decoding apparatus or a method using the CELP encoding method and another encoding method, such as, for example, the scalable encoding method that uses the CELP encoding method as the core layer encoding method, it is possible to perform a process to cope with the pre-echo, which process is performed at a shorter time interval than the transform block used in the transform coding method, using the local decoded signal of the CELP encoded code sequence or the power of the decoded signal or the fixed code book gain that is a CELP encoding parameter.  
           [0014]    The present invention uses the fact that the time variation of the power of the input signal, the time variation of the local decoded signal of the CELP encoded code sequence, and the time variation of the fixed code book gain of the CELP encoding are strongly correlated.  
           [0015]    In the encoding and decoding apparatus or the method having the CELP encoding method and other encoding methods, such as, for example, the scalable encoding method that uses the CELP encoding method as the core layer encoding method, using the fact that the time variation of the power of the input signal, the time variation of the local decoded signal of the CELP encoded code sequence or the power of the decoded signal and the time variation of the fixed code book gain that is the CELP encoding parameter are strongly correlated, the present invention allows other encoding and decoding parts to perform a process that detects the rising-transition of the amplitude of the input signal, and provides a detected result to encoding or decoding parts of other encoding methods, and performs a process to cope with the pre-echo at a shorter time interval than the transform block used in the transform coding method, by means of observing the time variation of the local decoded signal or the power of the decoded signal or the fixed code book gain. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which:  
         [0017]    [0017]FIG. 1 shows a block diagram of a scalable encoding process;  
         [0018]    [0018]FIG. 2 shows a block diagram of a CELP encoder;  
         [0019]    [0019]FIG. 3 shows a block diagram of a CELP decoder of the CELP encoding method;  
         [0020]    [0020]FIG. 4 shows an encoder for transform coding;  
         [0021]    [0021]FIG. 5 shows a block diagram of a decoder of transform coding;  
         [0022]    [0022]FIG. 6 shows a relationship between the time variation of the power of the input signal and the time variation of the fixed code book gain of the CELP encoding;  
         [0023]    [0023]FIG. 7 shows a block diagram of a decoder according to the first embodiment of the present invention;  
         [0024]    [0024]FIG. 8 shows a relationship between a frame and a sub-frame used for the CELP encoding and a transform block used for the transform coding;  
         [0025]    [0025]FIG. 9 shows a block diagram of an encoder according to the second embodiment of the present invention;  
         [0026]    [0026]FIG. 10 shows a block diagram of an encoder according to the third embodiment of the present invention;  
         [0027]    [0027]FIG. 11 shows a block diagram of an encoder according to the fourth embodiment of the present invention;  
         [0028]    [0028]FIG. 12 shows a block diagram of an encoder according to the fifth embodiment of the present invention;  
         [0029]    [0029]FIG. 13 shows a block diagram of a rising-transition detection part according to the sixth embodiment of the present invention;  
         [0030]    [0030]FIG. 14 shows a block diagram of a rising-transition detection part according to the seventh embodiment of the present invention;  
         [0031]    [0031]FIG. 15 shows a block diagram of a rising-transition detection part according to the eighth embodiment of the present invention; and  
         [0032]    [0032]FIG. 16 shows a block diagram of a rising-transition detection part according to the ninth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    In the following, embodiments of the present invention will be described with reference to figures. In the following description of the embodiments, a signal means a digital signal converted by an analog/digital converter.  
         [0034]    First, a principle of rising-transition detection of the amplitude of the input signal will be explained.  
         [0035]    [0035]FIG. 6 shows a relationship between the time variation of the power of the input signal and the time variation of the fixed code book gain of the CELP encoding. The time variation of the power of the input signal and the time variation of the fixed code book gain of the CELP encoding are strongly correlated. Therefore, in the present invention, the fixed code book gain of the CELP encoding is observed and used to detect the rising-transition of the amplitude of the input signal.  
         [0036]    Next, the first embodiment of the present invention will be explained. FIG. 7 shows a block diagram of a decoder according to the first embodiment of the present invention, which decoder decodes an encoded code sequence encoded by means of the scalable encoding method in that the CELP encoding method is used as the core layer encoding method.  
         [0037]    The decoder  700  has a CELP decoding part  701 , a rising transition detection part  702 , an enhancement layer decoding part  703  and an adder  711 .  
         [0038]    [0038]FIG. 8 shows an example of a relationship between a frame and a sub-frame used in the CELP-encoding method that is used as the core layer and a transform block used for the transform coding method that is used as the enhancement layer. One transform block has four CELP frames and one CELP frame has four CELP sub-frames. One CELP sub-frame has 64 samples and one CELP frame has 256 samples, and one transform block has 1024 samples.  
         [0039]    As shown in FIG. 7, the CELP decoding part  701  receives the CELP code words  704  encoded by means of the CELP encoding method and decodes the CELP code words  704  and supplies the CELP decoded signal  708  to the adder  711 . At the same time, the CELP decoding part  701  supplies the fixed code book gain  706  to the rising transition detection part  702 . The rising transition detection part  702  observes the time variation of the fixed code book gain  706  corresponding to a length of one transform block used for transform coding for the enhancement layer and detects rising-transition of the fixed code book gain  706  and outputs the rising transition detection information  707 . The rising transition detection information  707  detected as described above is supplied to the enhancement layer decoding part  703 .  
         [0040]    On the other hand, the enhancement layer decoding part  703  receives the enhancement layer code words  705 , and decodes the enhancement layer code words  705  according to the rising transition detection information  707  and then, supplies the enhancement layer decoded signal  709  to the adder  711 . The adder  711  adds the CELP decoded signal  708  and the enhancement layer decoded signal  709  and outputs the decoded output signal  710 .  
         [0041]    For example, assuming that there is the relationship among the transform block, the CELP frame and the CELP sub-frame as shown in FIG. 8 The fixed code book gain is calculated for every CELP sub-frame during the CELP encoding process, and the fixed code book gains are encoded for every CELP frame. Therefore, in the enhancement layer decoding block  703 , it is possible to observe the time variation of 16 fixed code book gains  706  for 16 CELP sub-frames in the transform block and to detect the rising-transition of the fixed code book gain. Therefore, because it is possible to detect the rising-transition of the fixed code book gain with a time precision of  {fraction (1/16)} of the transform block, it is possible to detect the rising-transition of the amplitude of the original signal with a time precision of    {fraction (1/16)} of the transform block.    
         [0042]    Next, the second embodiment of the present invention will be explained. FIG. 9 shows a block diagram of an encoder  900  according to the second embodiment of the present invention, which encodes an input signal by means of the scalable encoding method in that the CELP encoding method is used as the core layer encoding method. The encoder  900  has a CELP encoding part  901 , an enhancement layer encoding part  902 , a rising transition detection part  903  and a subtracter  918 .  
         [0043]    The input signal  910  is supplied to the CELP encoding part  901  and is encoded. The CELP code words  913  are output from the CELP encoding part  901 , and at the same time, the fixed code book gain  911  is supplied to the rising transition detection part  903 . Further, during the encoding process, the CELP decoded signal  912  that is a local decoded signal of the CELP encoded signal is also output from the CELP encoding part  901 . In the subtracter  918 , the CELP residual signal  914  that is the difference between the input signal  910  and the locally decoded CELP signal  912  is calculated, and the CELP residual signal  914  is supplied to the enhancement layer encoding part  902 .  
         [0044]    On the other hand, the same as described in the first embodiment, the rising transition detection part  903  observes the time variation of the fixed code book gain  911  and detects rising-transition of the fixed code book gain  911  and outputs the rising transition detection information  915 . The rising transition detection information  915  is supplied to the enhancement layer encoding part  902  and the enhancement layer encoding part  902  refers to the rising transition detection information  915  to perform encoding of the enhancement layer.  
         [0045]    Next, the third embodiment of the present invention will be explained. FIG. 10 shows a block diagram of an encoder  920  according to the third embodiment of the present invention, in which the input signal is encoded using the CELP encoding method and another encoding method, such as, for example, the transform coding method, and either a code sequence encoded using the CELP encoding method or a code sequence encoded using the other encoding method is supplied as an output of the encoder.  
         [0046]    The encoder  920  has the CELP encoding part  901 , the rising transition detection part  903 , a transform coding part  950  and a selection part  951 .  
         [0047]    In FIG. 10, the input signal  910  is encoded by the CELP encoding part  901  and the CELP code words  913  are output and at the same time, the fixed code book gain  911  is supplied to the rising transition detection part  903 . On the other hand, the input signal  910  is also encoded by the transform coding part  950  and the transform coded code words  952  are output. At the same time, the same as described in the first embodiment, the rising transition detection part  903  observes the time variation of the fixed code book gain  911  and detects the rising-transition of the fixed code book gain  911  and outputs the rising transition detection information  915  to the transform coding part  950 . The rising transition detection information  915  is supplied to the transform coding part  950  and the transform coding part  950  refers to the rising transition detection information  915  to perform encoding of the input signal  910 .  
         [0048]    Next, the fourth embodiment of the present invention will be explained. FIG. 11 shows a block diagram of an encoder  930  according to the fourth embodiment of the present invention, in which the input signal is encoded using the CELP encoding method and another encoding method, such as, for example, the transform coding method, and either a code sequence encoded using the CELP encoding method or a code sequence encoded using the other encoding method is supplied as an output of the encoder.  
         [0049]    The encoder  930  has the CELP encoding part  901 , the rising transition detection part  903 , a transform coding part  950 , a selection part  951  and a rising-transition detection information encoding part  953 .  
         [0050]    In FIG. 11, the input signal  910  is encoded by the CELP encoding part  901  and the CELP code words  913  are output and at the same time, the fixed code book gain  911  is supplied to the rising transition detection part  903 . On the other hand, the input signal  910  is also encoded by the transform coding part  950  and the transform coded code words  952  are output. At the same time, the same as described in the first embodiment, the rising transition detection part  903  observes the time variation of the fixed code book gain  911  and detects the rising-transition of the fixed code book gain  911  and outputs the rising transition detection information  915 . The rising transition detection information  915  is provided to the rising-transition detection information encoding part  953 . The rising-transition detection information encoding part  953  encodes the rising transition detection information  915  and outputs the encoded rising transition detection information  954  when the transform coded code words  952  are selected by the selector  951  as the output of the encoder  930 . Then, the encoder  930  outputs both the encoded code sequence  955  selected by the selector  951  and the encoded rising transition detection information  954  as the output of the encoder  930 . Therefore, the encoder  930  supplies the encoded rising transition detection information  954 .  
         [0051]    Next, the fifth embodiment of the present invention will be explained. FIG. 12 shows a block diagram of an encoder  940  according to the fifth embodiment of the present invention, in which the input signal is encoded using the CELP encoding method and another encoding method, such as, for example, the transform coding method, and either a code sequence encoded using the CELP encoding method or a code sequence encoded using the other encoding method is supplied as an output of the encoder.  
         [0052]    The encoder  940  has the CELP encoding part  901 , the rising transition detection part  903 , a transform coding part  950 , a selection part  951  and a rising-transition detection information encoding part  953 .  
         [0053]    In FIG. 12, the input signal  910  is encoded by the CELP encoding part  901  and the CELP code words  913  are output and at the same time, the fixed code book gain  911  is supplied to the rising transition detection part  903 . On the other hand, the input signal  910  is also encoded by the transform coding part  950  and the transform coded code words  952  are output. At the same time, the same as described in the first embodiment, the rising transition detection part  903  observes the time variation of the fixed code book gain  911  and detects the rising-transition of the fixed code book gain  911  and outputs the rising transition detection information  915 . Then, the rising transition detection information  915  is provided to both the transform coding part  950  and the rising-transition detection information encoding part  953 . The transform coding part  950  encodes the input signal  910  with reference to the rising transition detection information  915 . On the other hand, the rising-transition detection information encoding part  953  encodes the rising transition detection information  915  and outputs the encoded rising transition detection information  954  when the transformation encoded code words  952  are selected by the selector  951  as the output of the encoder  940 . Then, the encoder  940  outputs both the encoded code sequence  955  selected by the selector  951  and the encoded rising transition detection information  954  as the output of the encoder  940 . Therefore, the encoder  940  supplies the encoded rising transition detection information  954 .  
         [0054]    Next, the other embodiments will be explained below. The following embodiments are embodiments of the rising transition detection part as described in the first embodiment through the fifth embodiment. The relationship among the transform block, the CELP frame and the CELP sub-frame is the same relationship as shown in FIG. 8.  
         [0055]    First, the sixth embodiment of the present invention will be explained. FIG. 13 shows a block diagram of a rising-transition detection part according to the sixth embodiment of the present invention. The rising-transition detection part as shown in FIG. 13 has an average fixed code book gain calculation part  1301 , a fixed code book gain variance calculation part  1302  and a rising-transition decision part  1303 .  
         [0056]    The average value of the fixed code book gains for one transform block is calculated by the average fixed code book gain calculation part  1301 . For example, assuming that the fixed code book gain is calculated for each CELP sub-frame. Therefore, in the case that the input signal is encoded for every CELP frame that consists of N CELP sub-frames (N=4 for the case shown in FIG. 8), because one transform block consists of M CELP frames (M=4 for the case shown in FIG. 8), the average fixed code book gain for k transform blocks is expressed as follow,  
                 g   _     k   c     =       1     M   ·   N              ∑     m   =   0       M   -   1              ∑     n   =   0       N   -   1            g     k   ,   m   ,   n     c                   (   1   )                               
 
         [0057]    ,where  
         g k,m,n   c   
         [0058]    is a fixed code book gain of the n-th CELP sub-frame in the m-th CELP frame of the collection of the CELP frames in the k-th transform block. The variance of the fixed code book gain is calculated by the fixed code book gain variance calculation part  1302  using both the average fixed code book gain and each of the fixed code book gains. The variance of the fixed code book gains in the k-th transform block is expressed as follows.  
               ν   k     =       1     MN   -   1              ∑     m   =   0       M   -   1              ∑     n   =   0       N   -   1              (       g     k   ,   m   ,   n     c     -       g   _     k   c       )     2                   (   2   )                               
 
         [0059]    Then, the rising-transition decision part  1303  determines whether the rising-transition of the fixed code book gain exists or not in the k-th transform block by means of comparing the variance of the fixed code book gain calculated using expression (2) with a predetermined threshold value. Further, it is possible to change the threshold value for every transform block according to the input signal. Then, the rising-transition detection information  1311  is output from the rising-transition decision part  1303 .  
         [0060]    Next, the seventh embodiment of the present invention will be explained. FIG. 14 shows a block diagram of a rising-transition detection part according to the seventh embodiment of the present invention. The rising-transition detection part as shown in FIG. 14 has an average fixed code book gain calculation part  1301 , a frame mean square distance calculation part  1401  and a rising-transition decision part  1303 . In this embodiment, the average fixed code book gain calculation part  1301  performs the same operation as described in the sixth embodiment as shown in FIG. 13. Next, the frame mean square distance calculation part  1401  calculates the frame mean square distance between the average fixed code book gain and the fixed code book gain for each CELP sub-frame, for each CELP frame. The frame mean square distance of m-th CELP frame within the k-th transform block is expressed as follows.  
               s     k   ,   m     2     =       1   N            ∑     n   =   0       N   -   1              (       g     k   ,   m   ,   n     c     -       g   _     k   c       )     2                 (   3   )                               
 
         [0061]    Then, the rising-transition decision part  1303  determines whether the rising-transition of the fixed code book gain exists or not in the k-th transform block by means of comparing the frame mean square distance calculated using expression (3) with a predetermined threshold value. Further, it is possible to change the threshold value for every transform block according to the input signal. Then, the rising-transition detection information  1311  as detected above is output from the rising-transition decision part  1303 .  
         [0062]    Next, the eighth embodiment of the present invention will be explained. FIG. 15 shows a block diagram of a rising-transition detection part according to the eighth embodiment of the present invention. The rising-transition detection part as shown in FIG. 15 has an average fixed code book gain calculation part  1301  and a rising-transition decision part  1501 . In this embodiment, the average fixed code book gain calculation part  1301  performs the same operation as described in the sixth embodiment as shown in FIG. 13. Then, the rising-transition decision part  1501  determines whether the rising-transition of the fixed code book gain exists or not by means of comparing the average fixed code book gain or a modified value that is, for example, the average fixed code book gain multiplied by a constant calculated by the average fixed code book gain calculation part  1301 , with the fixed code book gain for each CELP sub-frame in the transform block, and outputs the rising-transition detection information  1311 .  
         [0063]    Next, the ninth embodiment of the present invention will be explained. FIG. 16 shows a block diagram of a rising-transition detection part according to the ninth embodiment of the present invention. The rising-transition detection part as shown in FIG. 16 has a fixed code book gain prediction part  1601 , a fixed code book gain prediction residual detection part  1602  and a rising-transition decision part  1603 . The fixed code book gain prediction part  1601  predicts the fixed code book gain of the CELP sub-frame from the fixed code book gain of the past CELP sub-frames and calculates a predicted fixed code book gain  1604 . For example, the predicted fixed code book gain  1604  is calculated from an expressions (4) and (5) as follows.  
                 g   ^       k   ,   m   ,   n     c     =       ∑     p   =   1     P            a   p          g     k   ,   m   ,     n   -   1       c                 (   4   )                   g     k   ,   m   ,     -   1       c     =         g     k   ,     m   -   1     ,     N   -   1       c          (     m   ≠   0     )            
                =       g       k   -   1     ,     M   -   1     ,     N   -   1       c          (     m   =   0     )                              (   5   )                               
 
         [0064]    The fixed code book gain  1310  of the CELP sub-frame is kept in the fixed code book gain prediction part  1601  in order to calculate the predicted fixed code book gain  1604  of the next CELP sub-frame. At the same time, the fixed code book gain  1310  is supplied to the fixed code book gain prediction residual detection part  1602  and then, the fixed code book gain prediction residual detection part  1602  calculates a difference between the fixed code book gain  1310  and the predicted fixed code book gain  1604  to obtain the fixed code book gain prediction residual  1605 . Next, the rising-transition decision part  1603  compares the fixed code book gain prediction residual  1605  with a predetermined threshold value and determines whether the rising-transition of the fixed code book gain exists or not and then, outputs the rising-transition detection information  1311 .  
         [0065]    In the description above, the fixed code book gain is used to describe the embodiments of the present invention. However, it is understood by those who are skilled in the art that it is possible to use the power of the decoded signal instead of the fixed code book gain. In the case that the power of the decoded signal is used instead of the fixed code book gain, examples of methods to determine whether the rising-transition of the power of the input signal exists or not are as follows. For example, it is possible to use a method in which an average power of the decoded signals for every CELP sub-frame is calculated and then, it is decided whether the rising-transition of the power of the input signal exists or not by comparing the time variation of the average power with a predetermined threshold value. Furthermore, it is possible to use a method in which a moving average is calculated using a predetermined number of samples and the time variation of the moving average is observed and then, determining whether the rising-transition of the amplitude of the input signal exists or not. Furthermore, in the case that the encoder performs the process, it is possible to send the rising-transition detection information, which is supplied to the second encoding part, to a decoding side as a part of the encoded sequence.  
         [0066]    In the description above, embodiments that process speech or audio signals are described. However, it is understood that the present invention is applied to other apparatuses or methods that process other digital signals having characteristics similar to speech or audio signals.  
         [0067]    It is possible to provide the encoding or the decoding apparatuses and methods, which use the CELP encoding method and another encoding method, such as, for example, the scalable encoding method that uses the CELP encoding method as the core layer encoding method and other encoding methods as the enhancement layer encoding methods, that observe the time variation of the fixed code book gain and detect the rising-transition of the amplitude of the input signal and notify the enhancement layers.  
         [0068]    In the decoding apparatus, the time variation of the decoded signal may be time variation of power level of the decoded signal.  
         [0069]    In the decoding apparatus, the input signal may be one of a speech signal and an audio signal.  
         [0070]    In the encoding apparatus, the time variation of the local decoded signal may be time variation of power level of the decoded signal.  
         [0071]    In the encoding apparatus, the input signal is one of a speech signal and an audio signal.  
         [0072]    In the decoding method, the gain of excitation vectors may be one of a gain of a fixed code book and a parameter of the gain of a fixed code book.  
         [0073]    In the decoding method, the time variation of the decoded signal may be time variation of power level of the decoded signal.  
         [0074]    In the decoding method, the input signal is one of a speech signal and an audio signal.  
         [0075]    In the encoding method, the gain of excitation vectors is one of a gain of a fixed code book and a parameter of the gain of a fixed code book.  
         [0076]    In the encoding method, the time variation of the local decoded signal may be time variation of power level of the decoded signal.  
         [0077]    In the encoding method, the input signal is one of a speech signal and an audio signal.  
         [0078]    The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.  
         [0079]    The present application is based on Japanese priority application No.2002-033154 filed on Feb. 08, 2002, the entire contents of which are hereby incorporated by reference.