Abstract:
Disclosed is an encoding device whereby it is possible to improve the quality of an encoded signal, even when encoding music signals. In the encoding device, a Code-Excited Linear Prediction (CELP) encoder ( 101 ) generates first encoded data by encoding an input signal, a CELP decoder ( 102 ) generates a decoded signal by decoding the first encoded data input from the CELP encoder ( 101 ), and a characteristic parameter encoder ( 106 ) calculates a parameter that expresses the degree of fluctuation in the ratio of the peak components and the floor components between the spectra of the decoded signal and the input signal.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an encoding apparatus, a decoding apparatus, a spectrum fluctuation calculation method and a spectrum amplitude adjustment method. 
       BACKGROUND ART 
       [0002]    For effective utilization of radio wave resources or the like, mobile communication systems require a technique of compressing a speech signal to a low bit rate and transmitting the signal. On the other hand, speech codec capable of encoding signals at a low bit rate and with high quality is required for not only speech signals but also signals other than speech signals such as music signals. This is a technique indispensable for realizing high quality in a service of streaming music (melody call or the like) as a ringing back tone, for example. 
         [0003]    CELP (Code Excited Linear Prediction) encoding is an effective scheme that encodes a speech signal at a low bit rate with high efficiency (e.g., see Non-Patent Literature 1). CELP encoding is a scheme that causes an excitation signal recorded in a codebook to pass through a pitch filter corresponding to the strength of periodicity and a synthesis filter corresponding to a vocal tract characteristic and determines encoding parameters so that a square error between output and input signals thereof is minimized under a weight of perceptual characteristics based on an engineering simulation model of a human speech generation model. In CELP encoding, using this model allows a speech signal to be encoded at a low bit rate and with high sound quality. Many of latest standard speech encoding schemes are based on CELP encoding and typical examples thereof include G729, G718 of ITU (International Telecommunication Union or AMR, AMR-WB of 3GPP (The 3rd Generation Partnership Project). 
       CITATION LIST 
     Non-Patent Literature 
     NPL 1 
       [0000]    
       
         M. R. Schoder and B. S. Atal, “Code-excited linear prediction (CELP); high-quality speech at very low bit rates”, Proc. ICASSP 85, pp. 937-940, 1985. 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    However, CELP encoding is a speech codec capable of encoding a speech signal at a low bit rate and with high sound quality, but since CELP encoding is based on a model not suitable for a music signal, applying CELP encoding to a music signal causes sound quality to considerably degrade. 
         [0006]    To be more specific, as described above, CELP encoding causes an excitation signal recorded in a codebook to pass through a pitch filter corresponding to the strength of periodicity and a synthesis filter corresponding to a vocal tract characteristic and generates a synthesis signal. This model is suitable for expressing a high energy component (spectrum envelope) at a resonance frequency corresponding to a formant of a speech signal and a component with relatively strong peak performance appearing at an integer multiple of a fundamental frequency (harmonic structure or harmonics). However, a formant or harmonic structure in the speech signal does not always exist in a general music signal. Moreover, components having much stronger peak performance than the harmonic structure of the speech signal appear in the music signal, whereas CELP encoding cannot express such components with accuracy. 
         [0007]    For example,  FIG. 1A  and  FIG. 1B  show a spectrum resulting from frequency-analyzing a signal which is a vowel part of a speech signal recorded at a sampling rate of 16 kHz (original signal spectrum (speech) shown in  FIG. 1A ) and a spectrum of decoded sound resulting from processing the signal in an 8 kbit/s mode of ITU-T G718 (decoded signal spectrum (speech) shown in  FIG. 1B ). The 8 kbit/s mode of G718 is an encoding scheme based on CELP encoding. It is clear from a comparison between the original signal spectrum shown in  FIG. 1A  and the decoded signal spectrum shown in  FIG. 1B  that the two spectra are generally very similar to each other although there is a minor difference in a high frequency region. 
         [0008]    On the other hand,  FIG. 1C  and  FIG. 1D  show a spectrum resulting from frequency-analyzing a piano sound (music signal) recorded at a sampling rate of 16 kHz (original signal spectrum (piano) shown in  FIG. 1C ) and a spectrum of a decoded sound after processing the signal in an 8 kbit/s mode of ITU-T G718 (decoded signal spectrum (piano) shown in  FIG. 1D ). A comparison between the original signal spectrum shown in  FIG. 1C  and the decoded signal spectrum shown in  FIG. 1D  shows that peak (tone) shapes of the spectrum clearly appear in the entire original signal spectrum. On the other hand, in the decoded signal spectrum, peak shapes of the spectrum start to collapse at approximately 1.5 kHz and the spectrum shape greatly differs from the original signal spectrum at 3.5 kHz or above. Thus, the peak shapes of the decoded signal spectrum collapse and the sizes of crests and troughs of peaks of the spectrum are suppressed, and when a user listens to the decoded signal, the user feels as if he/she were hearing noise and the sound quality is considerably degraded. 
         [0009]    Thus, as a technique of improving quality of a decoded signal in CELP encoding, a technique is proposed which frequency-analyzes a decoded signal of CELP encoding, suppresses inter-tone components in subband units and thereby improves sound quality of a music signal (e.g., see Tommy Vaillancourt, et. al., “Inter-tone noise reduction in a low bit rate CELP decoder”, Proc. ICASSP2009, pp. 4113-4116, 2009). 
         [0010]    However, since this technique determines the amount of suppression of inter-tone components in subband units, there is a problem that the frequency resolution is lowered. Moreover, since this technique frequency-analyzes the decoded signal (that is, the signal of degraded quality) and thereby calculates the amount of suppression of inter-tone components, there is a problem that it is difficult to calculate the accurate amount of suppression to improve sound quality. For these reasons, it is not possible to obtain sufficient sound quality improvement effects. 
         [0011]    It is an object of the present invention to provide an encoding apparatus, a decoding apparatus, a spectrum fluctuation calculation method and a spectrum amplitude adjustment method capable of improving quality of a decoded signal even when encoding a music signal. 
       Solution to Problem 
       [0012]    An encoding apparatus according to the present invention adopts a configuration including a first encoding section that encodes an input signal to generate first encoded data, a decoding section that decodes the first encoded data to generate a decoded signal and a calculation section that calculates a parameter indicating the amount of fluctuation in a ratio of peak components and floor components between spectra of the decoded signal and the input signal. 
         [0013]    A decoding apparatus according to the present invention adopts a configuration including a first decoding section that decodes first encoded data obtained by encoding an input signal in an encoding apparatus, to generate a decoded signal, and an adjustment section that adjusts amplitude of peak components of a spectrum of the decoded signal using a parameter indicating the amount of fluctuation in a ratio of peak components and floor components between spectra of the decoded signal and the input signal. 
         [0014]    A spectrum fluctuation calculation method according to the present invention adopts a configuration including an encoding step of encoding an input signal to generate first encoded data, a decoding step of decoding the first encoded data to generate a decoded signal, and a calculating step of calculating a parameter indicating the amount of fluctuation in a ratio of peak components and floor components between spectra of the decoded signal and the input signal. 
         [0015]    A spectrum amplitude adjustment method according to the present invention includes a decoding step of decoding first encoded data obtained by encoding an input signal in an encoding apparatus, to generate a decoded signal, and an adjusting step of adjusting amplitude of peak components of a spectrum of the decoded signal using a parameter indicating the amount of fluctuation in a ratio of peak components and floor components between spectra of the decoded signal and the input signal. 
       Advantageous Effects of Invention 
       [0016]    According to the present invention, it is possible to improve quality of a decoded signal even when encoding a music signal. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]      FIG. 1  are diagrams illustrating shapes of an original signal spectrum and a decoded signal spectrum of a speech signal and a music signal; 
           [0018]      FIG. 2  is a block diagram showing a configuration of an encoding apparatus according to Embodiment 1 of the present invention; 
           [0019]      FIG. 3  is a block diagram showing an internal configuration of a characteristic parameter encoding section according to Embodiment 1 of the present invention; 
           [0020]      FIG. 4  is a block diagram showing a configuration of a decoding apparatus according to Embodiment 1 of the present invention; 
           [0021]      FIG. 5  is a block diagram showing an internal configuration of a transform coefficient emphasizing section according to Embodiment 1 of the present invention; 
           [0022]      FIG. 6  are diagrams illustrating a processing flow in the transform coefficient emphasizing section according to Embodiment 1 of the present invention; 
           [0023]      FIG. 7  is a block diagram showing a configuration of an encoding apparatus according to Embodiment 2 of the present invention; 
           [0024]      FIG. 8  is a block diagram showing an internal configuration of a characteristic parameter encoding section according to Embodiment 2 of the present invention; 
           [0025]      FIG. 9  is a block diagram showing a configuration of a decoding apparatus according to Embodiment 2 of the present invention; 
           [0026]      FIG. 10  is a block diagram showing an internal configuration of a transform coefficient emphasizing section according to Embodiment 2 of the present invention; 
           [0027]      FIG. 11  is a block diagram showing an internal configuration of a characteristic parameter encoding section according to Embodiment 3 of the present invention; 
           [0028]      FIG. 12  is a block diagram showing an internal configuration of a transform coefficient emphasizing section according to Embodiment 3 of the present invention; 
           [0029]      FIG. 13  is a block diagram showing a configuration of an encoding apparatus according to Embodiment 4 of the present invention; 
           [0030]      FIG. 14  is a block diagram showing a configuration of a decoding apparatus according to Embodiment 4 of the present invention; 
           [0031]      FIG. 15  is a block diagram showing an internal configuration of a transform coefficient emphasizing section according to Embodiment 4 of the present invention; and 
           [0032]      FIG. 16  are diagrams illustrating a processing flow of the transform coefficient emphasizing section according to Embodiment 4 of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0033]    Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, a variable using n (e.g., s(n)) represents a time domain signal and a variable using k (e.g., S(k)) represents a frequency domain signal. Furthermore, a speech signal or music signal is inputted to an encoding apparatus according to the present invention as an input signal. 
       Embodiment 1 
       [0034]      FIG. 2  is a block diagram showing a configuration of main parts of an encoding apparatus according to the present embodiment. Encoding apparatus  100  in  FIG. 2  performs encoding processing on an input signal in predetermined time interval (frame) units to generate a bit stream and transmits the bit stream generated to a decoding apparatus which will be described later. 
         [0035]    In encoding apparatus  100  shown in  FIG. 2 , CELP encoding section  101  performs encoding processing on an input signal using CELP encoding to generate CELP encoded data (first encoded data). CELP encoding section  101  outputs the CELP encoded data to CELP decoding section  102  and multiplexing section  107 . 
         [0036]    CELP decoding section  102  performs CELP decoding processing on the CELP encoded data inputted from CELP encoding section  101  to generate a CELP decoded signal. CELP decoding section  102  outputs the CELP decoded signal to T/F transform section  103 . 
         [0037]    T/F transform section  103  transforms the CELP decoded signal inputted from CELP decoding section  102  to a frequency domain signal to calculate a CELP decoded transform coefficient and outputs the CELP decoded transform coefficient to characteristic parameter encoding section  106 . Here, MDCT (Modified Discrete Cosine Transform) is used for transforming to the frequency domain. 
         [0038]    Delay section  104  causes the input signal to delay by a time corresponding to a delay produced in CELP encoding section  101  and CELP decoding section  102  and outputs the delay-adjusted input signal to T/F transform section  105 . 
         [0039]    T/F transform section  105  transforms the input signal delay-adjusted in delay section  104  to a frequency domain signal to calculate an input transform coefficient and outputs the input transform coefficient to characteristic parameter encoding section  106 . MDCT is used for transforming to the frequency domain as in the case of T/F transform section  103 . 
         [0040]    Characteristic parameter encoding section  106  calculates and encodes a characteristic parameter using the CELP decoded transform coefficient inputted from T/F transform section  103  and the input transform coefficient inputted from T/F transform section  105  and generates characteristic parameter encoded data (second encoded data). Here, the characteristic parameter indicates the amount of fluctuation in the ratio of peak components and floor components between the spectra of the CELP decoded signal and the input signal. Characteristic parameter encoding section  106  outputs the characteristic parameter encoded data to multiplexing section  107 . Details of the processing of characteristic parameter encoding section  106  will be described later. 
         [0041]    Multiplexing section  107  multiplexes the CELP encoded data (first encoded data) inputted from CELP encoding section  101  and the characteristic parameter encoded data (second encoded data) inputted from characteristic parameter encoding section  106  to generate a bit stream and outputs the bit stream to a transmission channel (not shown). 
         [0042]    Next, details of the processing of characteristic parameter encoding section  106  in encoding apparatus  100  shown in  FIG. 2  will be described.  FIG. 3  is a block diagram showing an internal configuration of characteristic parameter encoding section  106 . 
         [0043]    Envelope component removing section  111  in characteristic parameter encoding section  106  shown in  FIG. 3  removes an envelope component (outline component of the spectrum) of the input transform coefficient. For example, envelope component removing section  111  transforms the input transform coefficient from a linear region to a logarithmic region and then performs smoothing processing such as moving average or the like on the transformed input transform coefficient. Envelope component removing section  111  then transforms the input transform coefficient after the smoothing processing from the logarithmic region to the linear region again. Thus, envelope component removing section  111  can obtain an envelope component of the input transform coefficient by performing smoothing processing in the logarithmic region. Envelope component removing section  111  then removes the envelope component obtained from the input transform coefficient and outputs the input transform coefficient after the removal of the envelope component to threshold calculation section  112  and transform coefficient classification section  113 . 
         [0044]    Threshold calculation section  112  calculates a threshold to classify the input transform coefficient into peak components and floor components using the input transform coefficient after the removal of the envelope component inputted from envelope component removing section  111  and outputs the calculated threshold to transform coefficient classification section  113 . To be more specific, threshold calculation section  112  calculates the threshold by performing statistic processing on the input transform coefficient after the removal of the envelope component. Here, a case will be described as an example where as shown in equation 1 below, threshold Th is calculated using standard deviation σ of the absolute value of the input transform coefficient after the removal of the envelope component. 
         [0000]      [1] 
         [0000]        Th=c·σ   (Equation 1)
 
         [0045]    Here, c represents a coefficient to determine threshold Th. Furthermore, standard deviation σ of the absolute value of the input transform coefficient is calculated according to following equation 2. 
         [0000]      [2] 
         [0000]    
       
         
           
             
               
                 
                   σ 
                   = 
                   
                     
                       
                         
                           1 
                           N 
                         
                          
                         
                           
                             ∑ 
                             k 
                           
                            
                           
                             
                                
                               
                                 
                                   S 
                                   R 
                                 
                                  
                                 
                                   ( 
                                   k 
                                   ) 
                                 
                               
                                
                             
                             2 
                           
                         
                       
                       - 
                       
                         
                           ( 
                           
                             M 
                             s 
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
         [0046]    Here, S R (k) represents an input transform coefficient after the removal of the envelope component, N represents the number of input transform coefficients and M S  represents a mean value of the absolute value of the input transform coefficient after the removal of the envelope component. Threshold calculation section  112  calculates threshold Th using equations 1 and 2 and outputs calculated threshold Th to transform coefficient classification section  113 . 
         [0047]    Transform coefficient classification section  113  classifies the input transform coefficient after the removal of the envelope component inputted from envelope component removing section  111  into peak components and floor components using threshold Th inputted from threshold calculation section  112 . Transform coefficient classification section  113  outputs an input transform coefficient classified as a peak component and an input transform coefficient classified as a floor component to characteristic parameter calculation section  117  as a first transform coefficient and a second transform coefficient respectively. To be more specific, when the absolute value of input transform coefficient S R (k) after the removal of the envelope component is equal to or above threshold Th (|S R (k)|≧Th), transform coefficient classification section  113  classifies input transform coefficient S R (k) as a peak component. On the other hand, when the absolute value of input transform coefficient S R (k) after the removal of the envelope component is less than threshold Th (other than |S R (k)|≧Th, that is, |S R (k)|&lt;Th), transform coefficient classification section  113  classifies input transform coefficient S R (k) as a floor component. 
         [0048]    The magnitude of coefficient c shown in equation 1 has an influences on the classification of peak components and floor components. This coefficient c may be a predetermined fixed value or a variable. When coefficient c is a variable, it may be such a variable that varies according to the pitch gain of CELP encoding, for example (which will be described later). 
         [0049]    On the other hand, envelope component removing section  114 , threshold calculation section  115  and transform coefficient classification section  116  perform processing similar to processing of envelope component removing section  111 , threshold calculation section  112  and transform coefficient classification section  113  on the CELP decoded transform coefficient. That is, envelope component removing section  114  removes the envelope component of the CELP decoded transform coefficient, threshold calculation section  115  calculates a threshold to classify the CELP decoded transform coefficient after the removal of the envelope component into peak components and floor components, transform coefficient classification section  116  classifies the CELP decoded transform coefficient after the removal of the envelope component into peak components and floor components. Transform coefficient classification section  116  outputs a CELP decoded transform coefficient classified as a peak component and a CELP decoded transform coefficient classified as a floor component to characteristic parameter calculation section  117  as a third transform coefficient and a fourth transform coefficient respectively. 
         [0050]    Characteristic parameter calculation section  117  calculates a characteristic parameter using the first transform coefficient and the second transform coefficient inputted from transform coefficient classification section  113 , and the third transform coefficient and the fourth transform coefficient inputted from transform coefficient classification section  116 . To be more specific, characteristic parameter calculation section  117  calculates a ratio of a peak component (first transform coefficient) and a floor component (second transform coefficient) of the input transform coefficient after the removal of the envelope component and a ratio of a peak component (third transform coefficient) and a floor component (fourth transform coefficient) of the CELP decoded transform coefficient after the removal of the envelope component. Characteristic parameter calculation section  117  then calculates the amount of fluctuation in both ratios as a characteristic parameter. 
         [0051]    To be more specific, characteristic parameter calculation section  117  calculates a ratio of average energy of the peak components to average energy of the floor components regarding the input transform coefficient after the removal of the envelope component. For example, suppose the first transform coefficient (peak component of the input transform coefficient) is S 1 (k) and the second transform coefficient (floor component of the input transform coefficient) is S 2 (k). In this case, characteristic parameter calculation section  117  calculates ratio R 12  of first transform coefficient S 1 (k) and second transform coefficient S 2 (k) (that is, ratio of the peak components and the floor components in the spectrum of the input signal) according to following equation 3. 
         [0000]      [3] 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     12 
                   
                   = 
                   
                     
                       
                         
                           1 
                           
                             N 
                             1 
                           
                         
                          
                         
                           
                             ∑ 
                             k 
                           
                            
                           
                             
                                
                               
                                 
                                   S 
                                   1 
                                 
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                                   ( 
                                   k 
                                   ) 
                                 
                               
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                             2 
                           
                         
                       
                       
                         
                           1 
                           
                             N 
                             2 
                           
                         
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                             k 
                           
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                                   2 
                                 
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                   ( 
                   
                     Equation 
                      
                     
                         
                     
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                     3 
                   
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         [0052]    Here, N 1  represents the number of first transform coefficients and N 2  represents the number of second transform coefficients. 
         [0053]    Similarly, characteristic parameter calculation section  117  calculates a ratio of average energy of the peak components to average energy of the floor components regarding the CELP decoded transform coefficient after the removal of the envelope component. For example, suppose third transform coefficient (peak component of the CELP decoded transform coefficient) is S 3 (k) and fourth transform coefficient (floor component of the CELP decoded transform coefficient) is S 4 (k). In this case, characteristic parameter calculation section  117  calculates ratio R 34  of third transform coefficient S 3 (k) and fourth transform coefficient S 4 (k) (that is, ratio of the peak components and the floor components in the spectrum of the CELP decoded signal) according to following equation 4. 
         [0000]      [4] 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     34 
                   
                   = 
                   
                     
                       
                         
                           1 
                           
                             N 
                             3 
                           
                         
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                                   3 
                                 
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                           1 
                           
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                                   4 
                                 
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         [0054]    Here, N 3  represents the number of third transform coefficients and N 4  represents the number of fourth transform coefficients. 
         [0055]    Characteristic parameter calculation section  117  then calculates characteristic parameter R indicating the amount of fluctuation in ratio R 12  of average energy of the peak components (first transform coefficient S 1 (k)) to average energy of the floor components (second transform coefficient S 2 (k)) of the input transform coefficient after the removal of the envelope component, and ratio R 34  of average energy of the peak components (third transform coefficient S 3 (k)) to average energy of the floor components (fourth transform coefficient S 4 (k)) of the CELP decoded transform coefficient after the removal of the envelope component according to next equation 5. 
         [0000]      [5] 
         [0000]    
       
         
           
             
               
                 
                   R 
                   = 
                   
                     
                       R 
                       12 
                     
                     
                       R 
                       34 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     5 
                   
                   ) 
                 
               
             
           
         
       
     
         [0056]    That is, characteristic parameter calculation section  117  calculates characteristic parameter R indicating the amount of fluctuation in the ratio of the peak components and the floor components between the spectra of the CELP decoded signal and the input signal. Characteristic parameter calculation section  117  then outputs calculated characteristic parameter R to characteristic parameter encoding section  118 . 
         [0057]    Characteristic parameter encoding section  118  encodes the characteristic parameter inputted from characteristic parameter calculation section  117  and generates characteristic parameter encoded data. Characteristic parameter encoding section  118  outputs the characteristic parameter encoded data to multiplexing section  107  shown in  FIG. 2 . For example, characteristic parameter encoding section  118  makes matching between a quantization table provided beforehand and the characteristic parameter. Characteristic parameter encoding section  118  outputs an index indicating a parameter candidate having the smallest error from the characteristic parameter among a plurality of parameter candidates included in the quantization table as the characteristic parameter encoded data. Alternatively, characteristic parameter encoding section  118  may also directly generate the characteristic parameter encoded data from the characteristic parameter through predetermined arithmetic processing. 
         [0058]      FIG. 4  is a block diagram showing a configuration of main parts of a decoding apparatus according to the present embodiment. Decoding apparatus  200  in  FIG. 4  receives and decodes a bit stream outputted from encoding apparatus  100  ( FIG. 2 ). 
         [0059]    In decoding apparatus  200  shown in  FIG. 4 , demultiplexing section  201  demultiplexes the bit stream inputted via a transmission channel (not shown) into CELP encoded data and characteristic parameter encoded data. Demultiplexing section  201  outputs the CELP encoded data to CELP decoding section  202  and outputs the characteristic parameter encoded data to characteristic parameter decoding section  204 . 
         [0060]    CELP decoding section  202  performs decoding processing on the CELP encoded data inputted from demultiplexing section  201  (encoded data obtained by encoding the input signal in encoding apparatus  100 ), generates a CELP decoded signal and outputs the generated CELP decoded signal to T/F transform section  203 . 
         [0061]    T/F transform section  203  transforms the CELP decoded signal inputted from CELP decoding section  202  to a frequency domain signal, calculates a CELP decoded transform coefficient and outputs the CELP decoded transform coefficient to transform coefficient emphasizing section  205 . Here, MDCT is used for transforming to the frequency domain. 
         [0062]    Characteristic parameter decoding section  204  performs decoding processing on the characteristic parameter encoded data inputted from demultiplexing section  201 , generates a decoded characteristic parameter and outputs the generated decoded characteristic parameter to transform coefficient emphasizing section  205 . 
         [0063]    Transform coefficient emphasizing section  205  emphasizes peak performance of the CELP decoded transform coefficient inputted from T/F transform section  203  using the decoded characteristic parameter inputted from characteristic parameter decoding section  204 . To be more specific, transform coefficient emphasizing section  205  adjusts the amplitude of peak components of the spectrum (CELP decoded transform coefficient) of the CELP decoded signal using a decoded characteristic parameter indicating the amount of fluctuation in the ratio of the peak components and the floor components between the spectra of the CELP decoded signal and the input signal. Transform coefficient emphasizing section  205  outputs the CELP decoded transform coefficient whose peak performance has been emphasized (hereinafter referred to as “emphasized transform coefficient”) to F/T transform section  206 . Details of the processing in transform coefficient emphasizing section  205  will be described later. 
         [0064]    F/T transform section  206  transforms the emphasized transform coefficient inputted from transform coefficient emphasizing section  205  to a time domain signal, calculates a decoded signal and outputs the calculated decoded signal. 
         [0065]    Next, details of the processing of transforms coefficient emphasizing section  205  of decoding apparatus  200  shown in  FIG. 4  will be described.  FIG. 5  is a block diagram showing an internal configuration of transform coefficient emphasizing section  205 . 
         [0066]    In transform coefficient emphasizing section  205  shown in  FIG. 5 , envelope component removing section  211  removes the envelope component of the CELP decoded transform coefficient inputted from T/F transform section  203  ( FIG. 4 ) in the same way as in envelope component removing section  114  ( FIG. 3 ). Envelope component removing section  211  then outputs the CELP decoded transform coefficient after the removal of the envelope component to threshold calculation section  212  and transform coefficient classification section  213 . Furthermore, envelope component removing section  211  outputs the envelope component of the CELP decoded transform coefficient and the CELP decoded transform coefficient after the removal of the envelope component to envelope component adding section  215 . Envelope component removing section  211  is different from envelope component removing section  114  ( FIG. 3 ) in that it outputs the envelope component of the CELP decoded transform coefficient and the CELP decoded transform coefficient after the removal of the envelope component to envelope component adding section  215 . 
         [0067]    Threshold calculation section  212  calculates a threshold to classify the CELP decoded transform coefficient into peak components and floor components using the CELP decoded transform coefficient after the removal of the envelope component inputted from envelope component removing section  211  in the same way as in threshold calculation section  115  ( FIG. 3 ). Threshold calculation section  212  outputs the calculated threshold to transform coefficient classification section  213 . 
         [0068]    Transform coefficient classification section  213  classifies the peak components from the CELP decoded transform coefficient after the removal of the envelope component inputted from envelope component removing section  211  using the threshold inputted from threshold calculation section  212  in the same way as in transform coefficient classification section  116  ( FIG. 3 ) and outputs the CELP decoded transform coefficient classified as the peak components to emphasizing section  214  as a third transform coefficient. Thus, transform coefficient classification section  213  is different from transform coefficient classification section  116  ( FIG. 3 ) in that it classifies and outputs only the peak components. 
         [0069]    Emphasizing section  214  emphasizes the third transform coefficient (peak components of the CELP decoded transform coefficient after the removal of the envelope component) inputted from transform coefficient classification section  213  using the decoded characteristic parameter inputted from characteristic parameter decoding section  204  ( FIG. 4 ). For example, emphasizing section  214  multiplies third transform coefficient S 3 (k) by decoded characteristic parameter R q  as shown in following equation 6. [6] 
         [0000]        S′   3 ( k )= S   3 ( k )· R   q   (Equation 6)
 
         [0070]    In this way, emphasizing section  214  adjusts the amplitude of the peak components of the spectrum of the CELP decoded signal using the characteristic parameter. Emphasizing section  214  then outputs emphasized third transform coefficient S 3 ′(k) to envelope component adding section  215 . 
         [0071]    Envelope component adding section  215  multiplies the emphasized third transform coefficient inputted from emphasizing section  214  by the envelope component of the CELP decoded transform coefficient inputted from envelope component removing section  211 , and thereby adds the envelope component to the emphasized third transform coefficient. Envelope component adding section  215  outputs the third transform coefficient with the envelope component added thereto to energy adjusting section  216 . 
         [0072]    For example, suppose the CELP decoded transform coefficient from which the envelope component has been removed is S R (k). In this case, envelope component adding section  215  substitutes the emphasized third transform coefficient S 3 ′(k) (that is, peak components whose amplitude has been adjusted) for the components at the positions corresponding to the peak components of the CELP decoded transform coefficient among components of CELP decoded transform coefficient S R (k) after the removal of the envelope component according to following equation 7 first and generates transform coefficient S R ′(k). 
         [0000]      [7] 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       S 
                       R 
                       ′ 
                     
                      
                     
                       ( 
                       k 
                       ) 
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             
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                      
                     
                         
                     
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                     7 
                   
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         [0073]    Where, k′ represents the position corresponding to a peak component. 
         [0074]    Next, envelope component adding section  215  multiplies transform coefficient S R ′(k) shown in equation 7 by the envelope component obtained in envelope component removing section  211 , and thereby adds the envelope component to transform coefficient S R ′(k) to generate transform coefficient S C ′(k). Envelope component adding section  215  outputs generated transform coefficient S C ′(k) to energy adjusting section  216 . 
         [0075]    Energy adjusting section  216  adjusts the energy of transform coefficient S C ′(k) so that the energy of transform coefficient S C ′(k) inputted from envelope component adding section  215  matches the energy of the original CELP decoded transform coefficient. Energy adjusting section  216  then outputs transform coefficient S C ′(k) after the energy adjustment to FIT transform section  206  ( FIG. 4 ) as the emphasized transform coefficient. 
         [0076]    For example, energy adjusting section  216  calculates energy adjusting coefficient g according to following equation 8 so that the energy of transform coefficient S C ′(k) matches the energy of original CELP decoded transform coefficient S C (k). 
         [0000]      [8] 
         [0000]    
       
         
           
             
               
                 
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                     8 
                   
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         [0077]    Energy adjusting section  216  multiplies transform coefficient S C ′(k) by energy adjusting coefficient g as shown in following equation 9 to generate emphasized transform coefficient S E (k). 
         [0000]      [9] 
         [0000]        S   E ( k )= g·S′   C ( k )  (Equation 9).
 
         [0078]    Next, a processing flow of transform coefficient emphasizing section  205  ( FIG. 5 ) will be described in detail using  FIG. 6A  to  FIG. 6D .  FIG. 6A  to  FIG. 6D  show a situation until an emphasized transform coefficient is generated from the CELP decoded transform coefficient inputted to transform coefficient emphasizing section  205 . 
         [0079]    To be more specific, as shown in  FIG. 6A , transform coefficient classification section  213  of transform coefficient emphasizing section  205  classifies the peak components of the CELP decoded transform coefficient whose envelope component has been removed in envelope component removing section  211  to generate a third transform coefficient. 
         [0080]    Next, as shown in  FIG. 6A , emphasizing section  214  emphasizes the peak components by adjusting the amplitude of the third transform coefficient, that is, the peak components of the CELP decoded transform coefficient after the removal of the envelope component. Envelope component adding section  215  then substitutes the emphasized third transform coefficient for the peak components of the CELP decoded transform coefficient after the removal of the envelope component according to equation 7. Thus, CELP decoded transform coefficient (S R ′(k) shown in equation 7) after the emphasis of the peak components is generated as shown in  FIG. 6B . 
         [0081]    Next, envelope component adding section  215  adds the envelope component to the CELP decoded transform coefficient after the emphasis of the peak components (CELP decoded transform coefficient whose envelope component has been removed) shown in  FIG. 6B  to generate transform coefficient S C ′(k) shown in  FIG. 6C . 
         [0082]    Energy adjusting section  216  adjusts the energy of transform coefficient S C ′(k) so that the energy of transform coefficient S C ′(k) shown in  FIG. 6C  matches the energy of the CELP decoded transform coefficient to generate emphasized transform coefficient S E (k) shown in  FIG. 6D . 
         [0083]    Thus, encoding apparatus  100  calculates the amount of fluctuation in the ratio of the peak components (third transform coefficient) and floor components (fourth transform coefficient) of the spectrum (CELP decoded transform coefficient) of the CELP decoded signal and the ratio of the peak components (first transform coefficient) and floor components (second transform coefficient) of the spectrum (input transform coefficient) of the input signal as a characteristic parameter. Encoding apparatus  100  transmits characteristic parameter encoded data obtained by encoding the characteristic parameter to decoding apparatus  200 . On the other hand, decoding apparatus  200  decodes the characteristic parameter encoded data transmitted from encoding apparatus  100  to obtain the characteristic parameter (decoded characteristic parameter) and emphasizes (adjusts the amplitude of) the peak components (third transform coefficient) of the CELP decoded signal (CELP decoded transform coefficient) using the characteristic parameter. 
         [0084]    That is, decoding apparatus  200  controls the ratio of the peak components and floor components of the CELP decoded signal using the characteristic parameter to thereby cause the ratio of the peak components and floor components of the CELP decoded signal to approximate to the ratio of the peak components and floor components of the input signal. This prevents a peak shape of the decoded signal spectrum from collapsing and reduces noiseness of the CELP decoded signal due to the suppression (increase of floor components) of the sizes of crests and troughs of peaks of the spectrum, and can thereby improve the quality of the decoded signal. 
         [0085]    In other words, encoding apparatus  100  frequency-analyzes the input signal, expresses the intensity of peak performance of the spectrum (input transform coefficient) of the input signal as a characteristic parameter, encodes the characteristic parameter and transmits the encoded characteristic parameter to decoding apparatus  200 . In this way, decoding apparatus  200  can generate a decoded signal having the intensity of peak performance similar to the intensity of peak performance of the spectrum (input transform coefficient) of the input signal using the characteristic parameter transmitted from encoding apparatus  100 , and can thereby improve the quality of the decoded signal. That is, a sound quality improvement effect can also be achieved for a music signal in which performing CELP encoding causes the peak shapes of the decoded signal spectrum to collapse, increasing the floor components and making the sound quality more likely to degrade a great deal. 
         [0086]    Thus, even when encoding a music signal using CELP encoding, the present embodiment can improve the quality of the decoded signal. 
         [0087]    Furthermore, encoding apparatus  100  obtains the intensity of peak performance as a characteristic parameter for each frequency component of an input signal and decoding apparatus  200  controls the intensity of peak performance of the CELP decoded signal for each frequency component to generate a decoded signal, and it is thereby possible to realize accurate control to improve sound quality. Thus, according to the present embodiment, decoding apparatus  200  can control the intensity of peak performance of the spectrum of the CELP decoded signal for each frequency component, and can thereby improve sound quality of a music signal. 
         [0088]    In the present embodiment, the encoding apparatus (characteristic parameter encoding section) may perform non-linear transform such as logarithmic transform on the characteristic parameter and perform encoding processing on the characteristic parameter after the non-linear transform. 
         [0089]    Furthermore, a case has been described in the present embodiment where a threshold is calculated to classify the transform coefficient into peak components and floor components using a standard deviation of the absolute value of the transform coefficient (input transform coefficient or CELP decoded transform coefficient) after the removal of the envelope component. However, when calculating a threshold, a mean value of the absolute value of the transform coefficient (input transform coefficient or CELP decoded transform coefficient) after the removal of the envelope component may also be used. 
         [0090]    The present embodiment has described a configuration using CELP encoding for the encoding apparatus. However, other time domain encoding schemes other than CELP encoding or encoding schemes having a low bit rate also have a problem that quality with respect to a music signal is low. The present invention is also applicable to such encoding schemes other than CELP encoding and applying the present invention allows the music quality to be improved. 
         [0091]    Furthermore, a feature of the present invention is to attenuate floor components which are increased through encoding processing, generate a decoded signal having the intensity of peak performance similar to the intensity of peak performance of the spectrum of the input signal and improve the quality. Therefore, the present embodiment has described the present invention on the premise of validity with respect to a music signal. However, the present invention can exert the quality improvement effect due to attenuation of floor components with respect to not only a music signal but also a speech signal. In a speech signal on which a signal such as background noise is superimposed in particular, floor components tend to increase by performing encoding processing and the present invention is further effective for such a case. 
       Embodiment 2 
       [0092]    The present embodiment will describe a case where a characteristic parameter is calculated further using a pitch gain in CELP encoding in addition to Embodiment 1. 
         [0093]    Hereinafter, the present embodiment will be described more specifically.  FIG. 7  is a block diagram showing a configuration of main parts of an encoding apparatus according to the present embodiment. In encoding apparatus  300  in  FIG. 7 , components common to those of encoding apparatus  100  shown in  FIG. 2  will be assigned the same reference numerals as those in  FIG. 2  and descriptions thereof will be omitted. 
         [0094]    In encoding apparatus  300  shown in  FIG. 7 , CELP decoding section  301  performs decoding processing on CELP encoded data inputted from CELP encoding section  101 , generates a CELP decoded signal, outputs the generated CELP decoded signal to T/F transform section  103 , decodes a pitch gain generated upon decoding processing and outputs the decoded pitch gain to characteristic parameter encoding section  302 . Here, the pitch gain is a gain value by which an adaptive vector used for CELP encoding (vector generated in an adaptive codebook that stores past excitation signals) is multiplied. Furthermore, the pitch gain corresponds to the strength of periodicity of an input signal. The pitch gain increases when, for example, the input signal has strong periodicity such as a vowel, whereas the pitch gain decreases when the input signal has weak periodicity such as a consonant. 
         [0095]    Characteristic parameter encoding section  302  calculates a characteristic parameter and performs encoding to generate characteristic parameter encoded data using the CELP decoded transform coefficient inputted from T/F transform section  103 , the input transform coefficient inputted from T/F transform section  105  and the pitch gain inputted from CELP decoding section  301 . 
         [0096]    Next, details of the processing in characteristic parameter encoding section  302  of encoding apparatus  300  shown in  FIG. 7  will be described.  FIG. 8  is a block diagram showing an internal configuration of characteristic parameter encoding section  302 . In characteristic parameter encoding section  302  in  FIG. 8 , components common to those of characteristic parameter encoding section  106  shown in  FIG. 3  will be assigned the same reference numerals as those in  FIG. 3  and descriptions thereof will be omitted. 
         [0097]    In characteristic parameter encoding section  302  shown in  FIG. 8 , threshold calculation section  311  calculates a threshold to classify the input transform coefficient into peak components and floor components using the input transform coefficient after the removal of the envelope component inputted from envelope component removing section  111  and the pitch gain inputted from CELP decoding section  301  ( FIG. 7 ). 
         [0098]    Here, Embodiment 1 has described the case where threshold calculation section  112  ( FIG. 3 ) multiplies the statistic value of the input transform coefficient after the removal of the envelope component (standard deviation of the absolute value of the input transform coefficient) by coefficient c (equation 1). By contrast, threshold calculation section  311  according to the present embodiment adjusts, using the pitch gain, the value of a coefficient by which the statistic value of the above-described input transform coefficient is multiplied. 
         [0099]    To be more specific, threshold calculation section  311  stores a table of coefficients corresponding to the pitch gain and uses a candidate corresponding to the inputted pitch gain of the candidate group of coefficients stored in the table. For example, when the pitch gain is assumed to be g, threshold calculation section  311  calculates threshold Th according to following equation 10. 
         [0000]      [10] 
         [0000]        Th=c [INT( N·g/g _max)]·σ  (Equation 10)
 
         [0100]    Here, c[ ] represents a table that stores a candidate group of coefficients and table c[ ] stores coefficients in order from a minimum value to a maximum value in such a way that a greater coefficient is selected for a greater value of pitch gain g. Furthermore, N represents the number of coefficients (candidates) stored in the table and g_max represents a maximum value that the pitch gain can take. Furthermore, function INT(x) represents a function that outputs an integer value of argument x. 
         [0101]    Thus, threshold calculation section  311  increases the value of a coefficient used for a threshold calculation as pitch gain g increases (as the periodicity becomes stronger), and thereby sets high threshold Th to classify the transform coefficient as peak components. This allows only transform coefficients of strong peak performance to be selected as peak components and makes it possible to calculate a more accurate characteristic parameter. 
         [0102]    Threshold calculation section  312  calculates a threshold to classify the CELP decoded transform coefficient into peak components and floor components using the CELP decoded transform coefficient after the removal of the envelope component inputted from envelope component removing section  114  and the pitch gain inputted from CELP decoding section  301  ( FIG. 7 ) as in the case of threshold calculation section  311 . 
         [0103]      FIG. 9  is a block diagram showing a configuration of main parts of the decoding apparatus according to the present embodiment. In decoding apparatus  400  in  FIG. 9 , components common to those of decoding apparatus  200  shown in  FIG. 4  will be assigned the same reference numerals as those in  FIG. 4  and descriptions thereof will be omitted. 
         [0104]    In decoding apparatus  400  shown in  FIG. 9 , CELP decoding section  401  decodes CELP encoded data, generates a CELP decoded signal, decodes a pitch gain generated during decoding processing and outputs the decoded pitch gain to transform coefficient emphasizing section  402  as in the case of CELP decoding section  301  ( FIG. 7 ). 
         [0105]    Transform coefficient emphasizing section  402  emphasizes peak performance of the CELP decoded transform coefficient inputted from T/F transform section  203  using the decoded characteristic parameter inputted from characteristic parameter decoding section  204  and the pitch gain inputted from CELP decoding section  401 . 
         [0106]    Next, details of the processing of transform coefficient emphasizing section  402  in decoding apparatus  400  shown in  FIG. 9  will be described.  FIG. 10  is a block diagram showing an internal configuration of transform coefficient emphasizing section  402 . In transform coefficient emphasizing section  402  in  FIG. 10 , components common to those of transform coefficient emphasizing section  205  shown in  FIG. 5  will be assigned the same reference numerals as those in  FIG. 5  and descriptions thereof will be omitted. 
         [0107]    In transform coefficient emphasizing section  402  shown in  FIG. 10 , threshold calculation section  411  calculates a threshold (threshold Th shown in equation 10) to classify peak components from the CELP decoded transform coefficient using the CELP decoded transform coefficient after the removal of the envelope component and the pitch gain inputted from CELP decoding section  401  ( FIG. 9 ) as in the case of threshold calculation section  312  ( FIG. 8 ). 
         [0108]    In this way, encoding apparatus  300  and decoding apparatus  400  estimate encoding performance with respect to peak components by CELP encoding using a pitch gain corresponding to strength of periodicity of an input signal and control calculation processing of the characteristic parameter (to be more specific, a threshold) based on the estimation result. In this case, it is also possible to reduce noiseness in the CELP decoded signal and improve the quality of the decoded signal as in the case of Embodiment 1. 
         [0109]    Furthermore, hi the present embodiment, encoding apparatus  300  calculates a characteristic parameter using the pitch gain in CELP encoding. This allows decoding apparatus  400  to adjust the intensity of peak performance of the spectrum of the CELP decoded signal according to the coding performance of CELP encoding with respect to peak components of the spectrum, and can thereby obtain a further sound quality improvement effect of the CELP decoded signal. 
         [0110]    Thus, when encoding a music signal using CELP encoding, the present embodiment can further improve the quality of the decoded signal compared to Embodiment 1. 
         [0111]    A case has been described in the present embodiment where a pitch gain is used to measure the strength of periodicity of an input signal, but a correlation value obtained by correlation-analyzing an input signal may also be used instead of the pitch gain when measuring the strength of periodicity of the input signal. Alternatively, the pitch gain and the above-described correlation value may be combined to calculate the strength of periodicity of the input signal. 
       Embodiment 3 
       [0112]    A case has been described in Embodiment 1 and Embodiment 2 where the encoding apparatus uses one threshold when classifying a transform coefficient (input transform coefficient or CELP decoded transform coefficient) into peak components and floor components. By contrast, the present embodiment will describe a case where the encoding apparatus uses two thresholds; a threshold to classify a transform coefficient as peak components and a threshold to classify a transform coefficient as floor components. 
         [0113]    Hereinafter, the present embodiment will be described more specifically.  FIG. 11  is a block diagram showing an internal configuration of a characteristic parameter encoding section of encoding apparatus  100  ( FIG. 2 ) according to the present embodiment. In characteristic parameter encoding section  106   a  in  FIG. 11 , components common to those of characteristic parameter encoding section  106  shown in  FIG. 3  will be assigned the same reference numerals as those in  FIG. 3  and descriptions thereof will be omitted. 
         [0114]    In characteristic parameter encoding section  106   a  shown in  FIG. 11 , threshold calculation section  112   a  calculates a first threshold to classify the input transform coefficient as peak components (first transform coefficient) and a second threshold to classify the input transform coefficient as floor components (second transform coefficient) using the input transform coefficient after the removal of the envelope component inputted from envelope component removing section  111 . 
         [0115]    For example, threshold calculation section  112   a  calculates first threshold Th 1  and second threshold Th 2  using standard deviation σ of the absolute value of the input transform coefficient after the removal of the envelope component as shown in following equations 11 and 12 in the same way as in equation 1. 
         [0000]      [11] 
         [0000]        Th   1   =c   1 ·σ  (Equation 11)
 
         [0000]      [12] 
         [0000]        Th   2   =c   2 ·σ  (Equation 12)
 
         [0116]    Here, c 1  and c 2  represent coefficients to calculate first threshold Th 1  and second threshold Th 2  and have a relationship shown in following equation 13. 
         [0000]      [13] 
         [0000]      0 &lt;c   2   &lt;c   1   (Equation 13)
 
         [0117]    Transform coefficient classification section  113   a  classifies the input transform coefficient after the removal of the envelope component inputted from envelope component removing section  111  into peak components (first transform coefficient) and floor components (second transform coefficient) using first threshold Th 1  and second threshold Th 2  calculated in threshold calculation section  112   a  and classifies components that belong to neither component as other components, classifying them as neither component. To be more specific, when the absolute value of input transform coefficient S R (k) after the removal of the envelope component is equal to or above first threshold Th 1  (that is, when |S R (k)k|≧Th 1 ), transform coefficient classification section  113   a  classifies input transform coefficient S R (k) as peak components (first transform coefficient). Furthermore, when the absolute value of input transform coefficient S R (k) after the removal of the envelope component is equal to or less than second threshold Th 2  (that is, when |S R (k)|≦Th 2 ), transform coefficient classification section  113   a  classifies input transform coefficient S R (k) as floor components (second transform coefficient). On the other hand, when the absolute value of input transform coefficient S R (k) after the removal of the envelope component is less than first threshold Th 1  and greater than second threshold Th 2  (that is, when Th 2 &lt;| R (k)|&lt;Th 1 ), transform coefficient classification section  113   a  classifies input transform coefficient S R (k) as other components (components belonging to neither peak components nor floor components), classifying it as neither component. 
         [0118]    Furthermore, threshold calculation section  115   a  calculates a third threshold to classify peak components (third transform coefficient) of the CELP decoded transform coefficient and a fourth threshold to classify floor components (fourth transform coefficient) of the CELP decoded transform coefficient as in the case of threshold calculation section  112   a . Furthermore, transform coefficient classification section  116   a  classifies the CELP decoded transform coefficient after the removal of the envelope component into peak components (third transform coefficient) and floor components (fourth transform coefficient) using the third threshold and fourth threshold as in the case of transform coefficient classification section  113   a  and classifies components that belong to neither component as other components, classifying them as neither component. 
         [0119]      FIG. 12  is a block diagram showing an internal configuration of a transform coefficient emphasizing section of decoding apparatus  200  ( FIG. 4 ) according to the present embodiment. In transform coefficient emphasizing section  205   a  in  FIG. 12 , components common to those of transform coefficient emphasizing section  205  shown in  FIG. 5  will be assigned the same reference numerals as those in  FIG. 5  and descriptions thereof will be omitted. 
         [0120]    In transform coefficient emphasizing section  205   a  shown in  FIG. 12 , threshold calculation section  212   a  calculates the third threshold to classify peak components (third transform coefficient) of the CELP decoded transform coefficient as in the case of threshold calculation section  115   a  ( FIG. 11 ). Furthermore, transform coefficient classification section  213   a  classifies peak components (third transform coefficient) from the CELP decoded transform coefficient using the third threshold inputted from threshold calculation section  212   a  as in the case of transform coefficient classification section  116   a.    
         [0121]    In this way, in the present embodiment, encoding apparatus  100  (characteristic parameter encoding section  106   a ) uses two thresholds, and can thereby calculate a characteristic parameter by excluding components which cannot be clearly judged to belong to which of peak components or floor components (e.g., components that satisfy Th 2 &lt;|S R (k)|&lt;Th 1 ). In this way, encoding apparatus  100  can calculate the ratio of peak components and floor components of the transform coefficient (input transform coefficient or CELP decoded transform coefficient) more accurately than Embodiment 1. That is, encoding apparatus  100  according to the present embodiment can calculate the characteristic parameter more accurately than Embodiment 1 and further improve the sound quality improvement effect on a music signal decoded in decoding apparatus  200 . 
         [0122]    Thus, when encoding a music signal using CELP encoding, the present embodiment can further improve the quality of a decoded signal compared to Embodiment 1. 
       Embodiment 4 
       [0123]    The present embodiment will describe a case where scalable encoding using CELP encoding for a low layer (or basic layer) and using transform encoding for a high layer (or enhanced layer) is performed. 
         [0124]    Hereinafter, the present embodiment will be described more specifically.  FIG. 13  is a block diagram showing a configuration of main parts of an encoding apparatus according to the present embodiment. In encoding apparatus  500  in  FIG. 13 , components common to those of encoding apparatus  100  shown in  FIG. 2  will be assigned the same reference numerals as those in  FIG. 2  and descriptions thereof will be omitted. 
         [0125]    Encoding apparatus  500  shown in  FIG. 13  is an encoding apparatus that performs scalable encoding having at least a low layer and a high layer. Here, encoding apparatus  500  CELP-encodes an input signal in the low layer to generate CELP encoded data (first encoded data). Furthermore, in a high layer, encoding apparatus  500  encodes (transform-encodes) an error signal which is a difference between a decoded signal of CELP encoded data and an input signal in a frequency domain to generate transform encoded data (second encoded data). 
         [0126]    To be more specific, in encoding apparatus  500  in  FIG. 13 , subtractor  501  subtracts a CELP decoded signal inputted from CELP decoding section  102  from a delay-adjusted input signal inputted from delay section  104  to generate an error signal and outputs the generated error signal to T/F transform section  502 . 
         [0127]    T/F transform section  502  transforms the error signal inputted from subtractor  501  into a frequency domain signal, calculates an error transform coefficient and outputs the error transform coefficient to transform encoding section  503 . Here, MDCT (Modified Discrete Cosine Transform) is used for transforming to the frequency domain. 
         [0128]    Transform encoding section  503  performs encoding processing on the error transform coefficient inputted from T/F transform section  502  and generates transform encoded data. At this time, transform encoding section  503  which is an encoding section in a high layer encodes an error signal which is a difference between the CELP decoded signal and the input signal in part of the entire band of the input signal and generates transform encoded data. Transform encoding section  503  outputs the generated transform encoded data to multiplexing section  504 . 
         [0129]    Multiplexing section  504  multiplexes the CELP encoded data inputted from CELP encoding section  101  and transform encoded data inputted from transform encoding section  503 , generates a bit stream and outputs the bit stream to the decoding apparatus via a transmission channel (not shown). 
         [0130]      FIG. 14  is a block diagram showing a configuration of main parts of the decoding apparatus according to the present embodiment. In decoding apparatus  600  in  FIG. 14 , components common to those of decoding apparatus  200  shown in  FIG. 4  will be assigned the same reference numerals as those in  FIG. 4  and descriptions thereof will be omitted. 
         [0131]    In decoding apparatus  600  shown in  FIG. 14 , demultiplexing section  601  demultiplexes the bit stream inputted via a transmission channel (not shown) into CELP encoded data and transform encoded data. Demultiplexing section  601  outputs the CELP encoded data to CELP decoding section  202  and outputs the transform encoded data to transform decoding section  602 . 
         [0132]    Transform decoding section  602  performs decoding processing on the transform encoded data inputted from demultiplexing section  601 , generates a decoded error transform coefficient and outputs the generated decoded error transform coefficient to transform coefficient emphasizing section  603 . 
         [0133]    Transform coefficient emphasizing section  603  calculates the amount of improvement of the band with quality improved in a high layer using the CELP decoded transform coefficient inputted from T/F transform section  203  and the decoded error transform coefficient inputted from transform decoding section  602 . To be more specific, transform coefficient emphasizing section  603  calculates a characteristic parameter indicating the amount of fluctuation in the ratio of the peak components and the floor components between the spectra of the CELP decoded signal and the decoded transform coefficient obtained using the CELP decoded signal and error signal in part of the band in which the quality of the CELP decoded signal is improved in a high layer. Transform coefficient emphasizing section  603  emphasizes the CELP decoded transform coefficient based on the calculation result of the amount of improvement (that is, characteristic parameter). To be more specific, transform coefficient emphasizing section  603  adjusts the amplitude of peak components of the spectrum of the CELP decoded signal in the band other than the above-described part (band in which the quality of the CELP decoded signal is not improved in the high layer) using the characteristic parameter. Transform coefficient emphasizing section  603  outputs the emphasized CELP decoded transform coefficient to F/T transform section  206  as the emphasized transform coefficient. 
         [0134]    Next, details of the processing in transform coefficient emphasizing section  603  of decoding apparatus  600  shown in  FIG. 14  will be described.  FIG. 15  is a block diagram showing an internal configuration of transform coefficient emphasizing section  603 . In transform coefficient emphasizing section  603  in  FIG. 15 , components common to those of characteristic parameter encoding section  106  shown in  FIG. 3  and transform coefficient emphasizing section  205  shown in  FIG. 5  will be assigned the same reference numerals as those in  FIG. 3  and  FIG. 5 , and descriptions thereof will be omitted. 
         [0135]    In transform coefficient emphasizing section  603  shown in  FIG. 15 , adder  611  adds up the CELP decoded transform coefficient inputted from T/F transform section  203  and the decoded error transform coefficient inputted from transform decoding section  602  to generate a decoded transform coefficient. This decoded transform coefficient corresponds to the input transform coefficient in  FIG. 3  (spectrum of the input signal). This addition processing improves the quality of the band corresponding to the decoded error transform coefficient in the CELP decoded transform coefficient. Adder  611  outputs the generated decoded transform coefficient to envelope component removing section  612  and energy adjusting section  216 . 
         [0136]    Envelope component removing section  612  removes an envelope component (outline component of the spectrum) of the decoded transform coefficient inputted from adder  611  in the same way as in envelope component removing section  111  ( FIG. 3 ). Envelope component removing section  612  outputs the decoded transform coefficient after the removal of the envelope component to emphasized transform coefficient generation section  616 . Furthermore, envelope component removing section  612  outputs the decoded transform coefficient after the removal of the envelope component included in a band with quality improved in a high layer (enhanced layer) (hereinafter referred to as “improved band”) to threshold calculation section  112  and transform coefficient classification section  113 . On the other hand, envelope component removing section  612  outputs the decoded transform coefficient after the removal of the envelope component included in a band with quality not improved in a high layer (enhanced layer) (hereinafter referred to as “non-improved band”) to threshold calculation section  613  and transform coefficient classification section  614 . A certain value is stored as the decoded error transform coefficient of the band in which the quality of the CELP decoded transform coefficient has been improved in the high layer. Thus, envelope component removing section  612  checks components in each band of the decoded error transform coefficient, and can thereby determine in which band the quality of the CELP decoded transform coefficient has been improved. 
         [0137]    Thus, as shown in  FIG. 15 , characteristic parameter calculation section  117  receives peak components (first transform coefficient (improved band)) and floor components (second transform coefficient (improved band)) of the decoded transform coefficient in the improved band (corresponding to the input transform coefficient in  FIG. 3 ) from transform coefficient classification section  113 . 
         [0138]    Furthermore, threshold calculation section  115  and transform coefficient classification section  116  receive the CELP decoded transform coefficient after the removal of the envelope component in the improved band. Thus, as shown in  FIG. 15 , characteristic parameter calculation section  117  receives peak components (third transform coefficient (improved band)) and floor components (fourth transform coefficient (improved band)) of the CELP decoded transform coefficient in the improved band from transform coefficient classification section  116 . 
         [0139]    Thus, characteristic parameter calculation section  117  calculates a characteristic parameter using the first transform coefficient (improved band), the second transform coefficient (improved band), the third transform coefficient (improved band) and the fourth transform coefficient (improved band) as in the case of Embodiment 1. That is, characteristic parameter calculation section  117  calculates a characteristic parameter indicating the amount of fluctuation in the ratio of the peak components and the floor components between the spectra of the decoded transform coefficient (that is, decoded input signal) obtained using the CELP decoded transform coefficient (that is, CELP decoded signal) and the decoded error transform coefficient (that is, error signal) in the improved band (part of the band of the input signal) and the CELP decoded transform coefficient (CELP decoded signal). Characteristic parameter calculation section  117  outputs the calculated characteristic parameter to emphasizing section  615 . 
         [0140]    On the other hand, threshold calculation section  613  calculates a threshold corresponding to the decoded transform coefficient included in the non-improved band inputted from envelope component removing section  612  as in the case of threshold calculation section  112 . Furthermore, transform coefficient classification section  614  classifies the peak components from the decoded transform coefficient included in the non-improved band using the threshold inputted from threshold calculation section  613  as in the case of transform coefficient classification section  113  and outputs the first transform coefficient (non-improved band) which is the decoded transform coefficient corresponding to the peak components to emphasizing section  615 . 
         [0141]    Emphasizing section  615  emphasizes the first transform coefficient (non-improved band) inputted from transform coefficient classification section  614  using the characteristic parameter inputted from characteristic parameter calculation section  117 . That is, emphasizing section  615  adjusts the amplitude of the peak components of the spectrum (first transform coefficient (non-improved band)) of the CELP decoded signal in the non-improved band which is the part of the band other than the improved band of the entire band of the input signal using the characteristic parameter. 
         [0142]    That is, emphasizing section  615  emphasizes the peak components of the spectrum (CELP decoded transform coefficient) of the CELP decoded signal in the non-improved band using the characteristic parameter indicating the amount of fluctuation in the ratio of the peak components and the floor components of the spectrum of the CELP decoded signal in the improved band and the ratio of the peak components and the floor components of the spectrum of the input signal in the improved band (decoded transform coefficient in  FIG. 15 ). Emphasizing section  615  outputs the emphasized first transform coefficient (non-improved band) to emphasized transform coefficient generation section  616 . 
         [0143]    Emphasized transform coefficient generation section  616  substitutes the emphasized first transform coefficient inputted from emphasizing section  615  (non-improved band) (that is, amplitude-adjusted peak components) for the components included in the non-improved band of the decoded transform coefficient after the removal of the envelope component inputted from envelope component removing section  612  and judged as a peak component, and generates an emphasized transform coefficient. 
         [0144]    As in the case of Embodiment 1, envelope component adding section  215  adds an envelope component to the emphasized transform coefficient inputted from emphasized transform coefficient generation section  616  using the envelope component of the decoded transform coefficient inputted from envelope component removing section  612  and energy adjusting section  216  adjusts the energy of the emphasized transform coefficient. 
         [0145]    Next, a processing flow of transform coefficient emphasizing section  603  ( FIG. 15 ) will be described in detail using  FIG. 16 . 
         [0146]    To be more specific, adder  611  adds up the CELP decoded transform coefficient and the decoded error transform coefficient shown in  FIG. 16A  to generate a decoded transform coefficient and envelope component removing section  612  removes the envelope component of the decoded transform coefficient. Transform coefficient emphasizing section  603  checks the value of the decoded error transform coefficient as shown in  FIG. 16A , and can thereby decide which of the improved band or non-improved band each frequency band is. 
         [0147]    Next, transform coefficient classification section  113  classifies the decoded transform coefficient included in the improved band out of the decoded transform coefficient after the removal of the envelope component shown in  FIG. 16B  into peak components (first transform coefficient (improved band)) and floor components (second transform coefficient (improved band)) and outputs these components to characteristic parameter calculation section  117 . Similarly, transform coefficient classification section  116  classifies the CELP decoded transform coefficient included in the improved band out of the CELP decoded transform coefficient after the removal of the envelope component shown in  FIG. 16C  into peak components (third transform coefficient (improved band)) and floor components (fourth transform coefficient (improved band)) and outputs these components to characteristic parameter calculation section  117 . 
         [0148]    Characteristic parameter calculation section  117  calculates a characteristic parameter using the first transform coefficient (improved band) to the fourth transform coefficient (improved band). 
         [0149]    On the other hand, transform coefficient classification section  614  classifies the peak components (first transform coefficient (non-improved band)) of the decoded transform coefficient included in the non-improved band out of the decoded transform coefficient after the removal of the envelope component shown in  FIG. 16B  and outputs the peak components to emphasizing section  615 . Emphasizing section  615  then emphasizes the peak components of the decoded transform coefficient included in the non-improved band using the characteristic parameter calculated in characteristic parameter calculation section  117 . For example, emphasizing section  615  multiplies the peak components (first transform coefficient (non-improved band)) of the decoded transform coefficient included in the non-improved band by the characteristic parameter, and thereby performs emphasizing processing (amplitude adjustment) as in the case of equation 6 in Embodiment 1. 
         [0150]    Emphasized transform coefficient generation section  616  substitutes the first transform coefficient (non-improved band) emphasized in emphasizing section  615  for components included in the non-improved band of the decoded transform coefficient shown in  FIG. 16B  and corresponding to the peak components, and thereby generates an emphasized transform coefficient shown in  FIG. 16D . 
         [0151]    Envelope component adding section  215  then adds an envelope component to the emphasized transform coefficient shown in  FIG. 16D  and energy adjusting section  216  adjusts the energy of the emphasized transform coefficient, and an emphasized transform coefficient shown in  FIG. 16E  is thereby obtained. 
         [0152]    Thus, decoding apparatus  600  controls the ratio of the peak components and the floor components of the CELP decoded signal in the non-improved band using the characteristic parameter indicating the amount of fluctuation (fluctuation in the ratio of peak components and floor components) between the spectra of the CELP decoded signal and the input signal (decoded transform coefficient) in the improved band. That is, decoding apparatus  600  causes the ratio of the peak components and the floor components of the CELP decoded signal in the non-improved band to approximate to the ratio of the peak components and the floor components of the CELP decoded signal in the improved band. This allows decoding apparatus  600  to generate, even in the non-improved band, a CELP decoded signal having the intensity of peak performance similar to the intensity of peak performance of the spectrum of the CELP decoded signal in the improved band. 
         [0153]    Here, in scalable encoding, if bits are sufficiently distributed in a high layer, the encoding apparatus can encode the error transform coefficient in the entire band. However, in order to realize a low bit rate, when bits distributed in the high layer are insufficient, there is a constraint that the encoding apparatus can encode the error transform coefficient only in part of the band. 
         [0154]    By contrast, the present embodiment focuses attention on the difference in the amount of quality improvement between a band with quality improved in the high layer (improved band) and the rest of the band (non-improved band) and decoding apparatus  600  expresses the amount of improvement of the band with quality improved in the high layer (improved band) as the characteristic parameter. Decoding apparatus  600  then adjusts (emphasizes) the peak performance of the band with quality not improved in the high layer (non-improved band) based on the characteristic parameter. 
         [0155]    In the present embodiment, this allows decoding apparatus  600  to calculate the characteristic parameter and eliminates the necessity for transmitting the characteristic parameter from encoding apparatus  500  to decoding apparatus  600 . That is, when performing scalable encoding, it is possible to obtain a sound quality improvement effect without increasing the bit rate. 
         [0156]    In this way, according to the present embodiment, when scalable encoding having a low layer and a high layer is performed, it is possible to improve the quality of a decoded signal even when encoding a music signal using CELP encoding in the same way as in Embodiment 1. 
         [0157]    The embodiments of the present invention have been described so far. 
         [0158]    A case has been described in the above embodiments where calculation of a characteristic parameter in the entire band of an input signal, encoding and emphasizing processing on a transform coefficient are performed. However, the present invention is not limited to this, but a configuration may also be adopted in which the entire band of an input signal is divided into a plurality of subbands, and calculation of a characteristic parameter, encoding and emphasizing processing on a transform coefficient are performed in each subband. This allows the decoding apparatus to perform emphasizing processing on the transform coefficient in smaller units and thereby allows the sound quality of a music signal to be further improved. 
         [0159]    Furthermore, a case has been described in the above embodiments where when encoding the characteristic parameter and performing emphasizing processing on the transform coefficient, the input transform coefficient (or decoded transform coefficient) and CELP decoded transform coefficient are used as they are. However, when encoding the characteristic parameter and performing emphasizing processing on the transform coefficient, the present invention may also use an input transform coefficient and CELP decoded transform coefficient after smoothing processing such as moving average instead of using the input transform coefficient and CELP decoded transform coefficient as they are. When encoding the characteristic parameter and performing emphasizing processing on the transform coefficient for the input transform coefficient and CELP decoded transform coefficient, this makes it possible to reduce influences from an extremely large transform coefficient and perform more stable encoding processing and emphasizing processing. This makes it possible to further improve sound quality of music signals. 
         [0160]    Furthermore, the T/F transform section according to the above embodiments can use a DFT (Discrete Fourier Transform), FFT (Fast Fourier Transform), DCT (Discrete Cosine Transform), MDCT (Modified Discrete Cosine Transform), filter bank or the like. 
         [0161]    Also, although cases have been described with the above embodiments as examples where the present invention is configured by hardware, the present invention can also be implemented by software. 
         [0162]    Each function block employed in the description of each of the aforementioned embodiments may typically be implemented as an LSI constituted by an integrated circuit. These may be individual chips or partially or totally contained on a single chip. “LSI” is adopted here but this may also be referred to as “IC,” “system LSI,” “super LSI,” or “ultra LSI” depending on differing extents of integration. 
         [0163]    Further, the method of circuit integration is not limited to LSI&#39;s, and implementation using dedicated circuitry or general purpose processors is also possible. After LSI manufacture, utilization of a programmable FPGA (Field Programmable Gate Array) or a reconfigurable processor where connections and settings of circuit cells within an LSI can be reconfigured is also possible. 
         [0164]    Further, if integrated circuit technology comes out to replace LSI&#39;s as a result of the advancement of semiconductor technology or a derivative other technology, it is naturally also possible to carry out function block integration using this technology. Application of biotechnology is also possible. 
         [0165]    The disclosure of Japanese Patent Application No. 2010-006260, filed on Jan. 14, 2010, including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY 
       [0166]    The encoding apparatus, decoding apparatus, spectrum fluctuation calculation method and spectrum amplitude adjustment method or the like according to the present invention are suitable for use in codec of speech or music in particular. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           100 ,  300 ,  500  encoding apparatus 
           200 ,  400 ,  600  decoding apparatus 
           101  CELP encoding section 
           102 ,  202 ,  301 ,  401  CELP decoding section 
           103 ,  105 ,  203 ,  502  T/F transform section 
           104  delay section 
           106 ,  106   a ,  302  characteristic parameter encoding section 
           107 ,  504  multiplexing section 
           201 ,  601  demultiplexing section 
           204  characteristic parameter decoding section 
           205 ,  205   a ,  402 ,  603  transform coefficient emphasizing section 
           206  F/T transform section 
           111 ,  114 ,  211 ,  612  envelope component removing section 
           112 ,  112   a ,  115 ,  115   a ,  212 ,  212   a ,  311 ,  312 ,  411 ,  613  threshold calculation section 
           113 ,  113   a ,  116 ,  116   a ,  213 ,  213   a ,  614  transform coefficient classification section 
           117  characteristic parameter calculation section 
           118  characteristic parameter encoding section 
           214 ,  615  emphasizing section 
           215  envelope component adding section 
           216  energy adjusting section 
           501  subtractor 
           503  transform encoding section 
           602  transform decoding section 
           611  adder 
           616  emphasized transform coefficient generation section