Patent Publication Number: US-11043227-B2

Title: Coding method, device and recording medium

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims the benefit of priority under 35 U.S.C. § 120 from U.S. application Ser. No. 16/295,039 filed Mar. 7, 2019, which is a continuation of U.S. application Ser. No. 15/327,490 filed Jan. 19, 2017 (now U.S. Pat. No. 10,304,472 issued May 28, 2019), the entire contents of which are incorporated herein by reference. U.S. application Ser. No. 15/327,490 is a National Stage of PCT/JP2015/063989 filed May 15, 2015, which claims the benefit of priority under 35 U.S.C. § 119 from Japanese Application No. 2014-152958 filed Jul. 28, 2014. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a sound signal coding technology. More particularly, the present invention relates to a coding technology of coding a sound signal by converting the sound signal into the frequency domain. 
     BACKGROUND ART 
     In coding of sound signals such as speech and music, a method of coding an input sound signal in the frequency domain is widely used. As the method of coding a sound signal in the frequency domain, there are, for example, methods of Non-patent Literature 1 and Non-patent Literature 2. 
     The coding method described in Non-patent Literature 1 is a method that performs coding processing using a spectral envelope based on coefficients which are convertible into linear prediction coefficients. Specifically, the coding method described in Non-patent Literature 1 is a method that obtains a linear prediction coefficient code by coding coefficients which are obtained from an input sound signal and are convertible into linear prediction coefficients and obtains a normalized coefficient code by coding a normalized coefficient sequence which is obtained by normalizing a frequency domain coefficient sequence corresponding to the input sound signal by a spectral envelope coefficient sequence corresponding to coefficients which are convertible into quantized linear prediction coefficients corresponding to the linear prediction coefficient code. The coefficients which are convertible into linear prediction coefficients are, for example, linear prediction coefficients themselves, PARCOR coefficients (partial autocorrelation coefficients), LSP parameters, or the like. 
     The coding method described in Non-patent Literature 2 is a method that performs coding processing involving differential value variable-length coding by obtaining a differential between the logarithmic value of the average energy of coefficients in each frequency domain obtained by division and the logarithmic value of the average energy of an adjacent frequency domain. Specifically, the coding method described in Non-patent Literature 2 is a method that divides a frequency domain coefficient sequence corresponding to an input sound signal into frequency domains such that the lower the frequencies are, the smaller the number of samples of the frequency domain becomes; the higher the frequencies are, the greater the number of samples of the frequency domain becomes, obtains the average energy of each frequency domain obtained by division, and quantizes the average energy on the logarithmic axes; performs variable-length coding on a differential between the value obtained by quantization and the value obtained by quantizing the average energy of an adjacent frequency domain on the logarithmic axes in a similar manner; and adaptively determines, by using the average energy, which was quantized on the logarithmic axes, of each frequency domain obtained by division, a quantization bit number of each frequency domain coefficient and the quantization step width of each frequency domain coefficient, quantizes each frequency domain coefficient in accordance therewith, and further performs variable-length coding thereon. 
     PRIOR ART LITERATURE 
     Non-Patent Literature 
     
         
         Non-patent Literature 1: Anthony Vetro, “MPEG Unified Speech and Audio Coding”, Industry and Standards, IEEE MultiMedia, April-June, 2013. 
         Non-patent Literature 2: M. Bosi and R. E. Goldberg, “Introduction to Digital Audio Coding and Standards”, Kluwer Academic Publishers, 2003. 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     By the coding method of Non-patent Literature 2, since it is possible to reduce the code amount of an average energy code by performing variable-length coding on an average energy differential if the ascents and descents of a spectral envelope of an input sound signal are not steep and the degree of concentration of a spectrum is not high, it is possible to code the input sound signal efficiently. However, if the ascents and descents of a spectral envelope of an input sound signal are steep and the degree of concentration of a spectrum is high, the code amount of an average energy code which is obtained by performing variable-length coding on an average energy differential becomes large. 
     On the other hand, by the coding method of Non-patent Literature 1, since it is possible to code a spectral envelope efficiently by using coefficients which are convertible into linear prediction coefficients, it is possible to code an input sound signal more efficiently than the coding method of Non-patent Literature 2 if the ascents and descents of a spectral envelope of the input sound signal are steep and the degree of concentration of a spectrum is high. However, if the ascents and descents of a spectral envelope of an input sound signal are not steep and the degree of concentration of a spectrum is not high, it is not possible to perform coding as efficiently as the coding method of Non-patent Literature 2. 
     As described above, the existing coding methods sometimes cannot perform coding efficiently depending on the characteristics of an input sound signal. 
     An object of the present invention is to provide a coding method, a device, a program, and a recording medium that can perform coding efficiently irrespective of the characteristics of an input sound signal and obtain a decoded sound signal that sounds less artificial to a listener. 
     Means to Solve the Problems 
     A coding method according to one aspect of the present invention is a coding method that codes an input sound signal frame by frame of a predetermined time segment by a selected coding processing from a plurality of types of coding processing in the frequency domain, the coding method including: a selection step of making possible a selection to select coding processing which is different from coding processing of the preceding frame as coding processing of the present frame if at least one of the magnitude of the energy of high frequency components of an input sound signal of the preceding frame and the magnitude of the energy of high frequency components of an input sound signal of the present frame is smaller than or equal to a predetermined threshold value. 
     A coding method according to one aspect of the present invention is a coding method that codes an input sound signal frame by frame of a predetermined time segment by a selected coding processing from a plurality of types of coding processing in the frequency domain, the coding method including: a selection step of making possible a selection to select coding processing which is different from coding processing of the preceding frame as coding processing of the present frame if at least one of the magnitude of the energy of high frequency components of an input sound signal of the preceding frame and the magnitude of the energy of high frequency components of an input sound signal of the present frame is smaller than or equal to a predetermined threshold value; otherwise, deciding whether to make possible a selection to select coding processing which is different from the coding processing of the preceding frame as the coding processing of the present frame or select the same coding processing as the coding processing of the preceding frame as the coding processing of the present frame in accordance with a state in which the high frequency components of the input sound signal are sparse. 
     A coding method according to one aspect of the present invention is a coding method that codes an input sound signal frame by frame of a predetermined time segment by a selected coding processing from a plurality of types of coding processing in the frequency domain, the coding method including: a first coding step of coding a coefficient sequence in the frequency domain corresponding to the input sound signal by using a spectral envelope based on coefficients which are convertible into linear prediction coefficients corresponding to the input sound signal; a second coding step of coding the coefficient sequence in the frequency domain corresponding to the input sound signal, involving variable-length coding which is performed on a differential between the logarithmic value of the average energy of coefficients in each frequency domain obtained by division and the logarithmic value of the average energy of an adjacent frequency domain; and a selection step of making possible a selection to code the present frame in the first coding step if the ascents and descents of a spectrum of the input sound signal of the present frame are steep or the degree of concentration of the spectrum is high and making possible a selection to code the present frame in the second coding step if the ascents and descents of the spectrum of the input sound signal of the present frame are gentle or the degree of concentration of the spectrum is low. 
     Effects of the Invention 
     With a configuration that allows any one of a plurality of types of coding processing performing coding frame by frame in the frequency domain to be selected, it is possible to obtain a decoded sound signal that sounds less artificial to a listener. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating the configuration of a coding device. 
         FIG. 2  is a block diagram illustrating the configuration of a decoding device. 
         FIG. 3  is a diagram depicting an example of the flow of processing of a coding method. 
         FIG. 4  is a diagram depicting an example of the flow of processing of a selection unit  380 . 
         FIG. 5  is a diagram depicting an example of the flow of processing of a suitable coding processing judgment unit  382 . 
         FIG. 6  is a diagram depicting an example of the flow of processing of a switching selection unit  383  of a second embodiment. 
         FIG. 7  is a diagram depicting an example of the flow of processing of a suitable coding processing judgment unit  382  of a third embodiment. 
         FIG. 8  is a conceptual diagram of first coding processing and second coding processing. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     Hereinafter, a first embodiment of the present invention will be described. The first embodiment is configured such that, in a configuration in which a coefficient sequence in the frequency domain corresponding to an input sound signal of each frame is coded by any one of a plurality of different types of coding processing that performs coding processing in the frequency domain, switching of coding processing is performed only when the energy of high frequency components of an input sound signal or/and a coefficient sequence in the frequency domain corresponding to the input sound signal is small. The energy of high frequency components of an input sound signal is the magnitude itself of the energy of high frequency components of an input sound signal, the magnitude of the energy of high frequency components in an input sound signal, or the like. 
     &lt;Coding Device  300 &gt; 
     The configuration of a coding device  300  is depicted in  FIG. 1 . The coding device  300  includes a frequency domain conversion unit  110 , a selection unit  380 , a first coding unit  101 , and a second coding unit  201 . The first coding unit  101  includes, for example, a linear prediction analysis coding unit  120 , a spectral envelope coefficient sequence generating unit  130 , an envelope normalization unit  140 , and a normalized coefficient coding unit  150 . The second coding unit  201  includes, for example, a region division unit  220 , an average logarithmic energy differential variable-length coding unit  240 , and a coefficient coding unit  250 . To the coding device  300 , a speech sound digital signal (hereinafter referred to as an input sound signal) in the time domain is input frame by frame which is a predetermined time segment, and the following processing is performed on a frame-by-frame basis. Hereinafter, specific processing of each unit will be described based on the premise that the present input sound signal is an f-th frame. An input sound signal of the f-th frame is assumed to be x f (n) (n=1, . . . , Nt). Here, Nt represents the number of samples per frame. 
     Hereinafter, the operation of the coding device  300  will be described. By the coding device  300 , processing in each step of a coding method illustrated in  FIG. 3  is performed. 
     &lt;Frequency Domain Conversion Unit  110 &gt; 
     The frequency domain conversion unit  110  converts the input sound signal x f (n) (n=1, . . . , Nt) into a coefficient sequence in the frequency domain, for example, an MDCT coefficient sequence X f (n) (n=1, . . . , N) at point N and outputs the MDCT coefficient sequence X f (n) (n=1, . . . , N) (Step S 110 ). However, N is the number of samples in the frequency domain and is a positive integer. Conversion into the frequency domain may be performed by a publicly known conversion method which is not MDCT. 
     Moreover, if coefficient sequences in the frequency domain obtained at a plurality of degrees of accuracy and by a plurality of methods are necessary in the first coding unit  101 , the second coding unit  201 , and the selection unit  380 , it is possible to obtain coefficient sequences in the frequency domain at a plurality of degrees of accuracy and by a plurality of methods in the frequency domain conversion unit  110 . For example, when the first coding unit  101  and the second coding unit  201  use an MDCT coefficient sequence as a coefficient sequence in the frequency domain and the selection unit  380  uses a power spectral series as a coefficient sequence in the frequency domain, the frequency domain conversion unit  110  simply has to obtain an MDCT coefficient sequence and a power spectral series from an input sound signal. Furthermore, for example, when the first coding unit  101  and the second coding unit  201  use an MDCT coefficient sequence as a coefficient sequence in the frequency domain and the selection unit  380  uses a series of energy of each frequency band as a coefficient sequence in the frequency domain, the frequency domain conversion unit  110  simply has to obtain an MDCT coefficient sequence and a series of energy of each frequency band from an input sound signal. In addition, for example, when the first coding unit  101  and the second coding unit  201  use an MDCT coefficient sequence as a coefficient sequence in the frequency domain, a switching permission judgment unit  381  of the selection unit  380  uses a series of energy of each frequency band as a coefficient sequence in the frequency domain, and a suitable coding processing judgment unit  382  of the selection unit  380  uses a power spectral series as a coefficient sequence in the frequency domain, the frequency domain conversion unit  110  simply has to obtain an MDCT coefficient sequence, a series of energy of each frequency band, and a power spectral series from an input sound signal. 
     &lt;Selection Unit  380 &gt; 
     The selection unit  380  makes it possible to select coding processing which is different from the coding processing of the preceding frame as coding processing of the present frame if at least one of the magnitude of the energy of high frequency components of the input sound signal of the preceding frame and the magnitude of the energy of high frequency components of the input sound signal of the present frame is smaller than a predetermined threshold value (Step S 380 ). 
     In other words, the selection unit  380  makes a judgment to the effect that, if at least the energy of high frequency components of an input sound signal is small, the selection unit  380  allows a coefficient sequence in the frequency domain of the present frame to be coded by coding processing which is different from the coding processing by which the coefficient sequence in the frequency domain of the preceding frame was coded; otherwise, the selection unit  380  does not allow the coefficient sequence in the frequency domain of the present frame to be coded by coding processing which is different from the coding processing by which the coefficient sequence in the frequency domain of the preceding frame was coded. The selection unit  380  then performs switching control such that the coefficient sequence in the frequency domain of the present frame is coded in accordance with the judgment result. 
     The selection unit  380  includes, for example, the switching permission judgment unit  381 , the suitable coding processing judgment unit  382 , a switching selection unit  383 , and a switching unit  384 . Hereinafter, an example of the selection unit  380  will be described. The selection unit  380  performs processing in each step illustrated in  FIG. 4 . 
     &lt;Switching Permission Judgment Unit  381 &gt; 
     The switching permission judgment unit  381  judges that, if at least one of the magnitude of the energy of high frequency components of the input sound signal of the preceding frame and the magnitude of the energy of high frequency components of the input sound signal of the present frame is smaller than a predetermined threshold value, switching is permitted, that is, judges that the switching permission judgment unit  381  makes it possible to code the coefficient sequence in the frequency domain of the present frame by coding processing which is different from the coding processing by which the coefficient sequence in the frequency domain of the preceding frame was coded; otherwise, the switching permission judgment unit  381  judges that switching is not permitted, that is, the switching permission judgment unit  381  does not allow the coefficient sequence in the frequency domain of the present frame to be coded by coding processing which is different from the coding processing by which the coefficient sequence in the frequency domain of the preceding frame was coded; the switching permission judgment unit  381  then outputs the judgment result (Step S 381 ). 
     Hereinafter, an example of the operation of the switching permission judgment unit  381  will be described. First, an example in which the high frequency energy of an MDCT coefficient sequence is used as the magnitude of the energy of high frequency components of an input sound signal will be described. 
     The switching permission judgment unit  381  first obtains the high frequency energy Eh f-1  of an MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame by the following formula (1) and the high frequency energy Eh f  of an MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame by the following formula (2) (Step S 3811 ). In formula (1) and formula (2), M is a predetermined positive integer which is smaller than N. 
     
       
         
           
             
               
                 
                   
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     The switching permission judgment unit  381  then judges that, if at least one of the high frequency energy Eh f-1  of the preceding frame and the high frequency energy Eh f  of the present frame is smaller than a predetermined threshold value TH1, that is, Eh f-1 &lt;TH1 and/or Eh f &lt;TH1, switching is permitted; otherwise, the switching permission judgment unit  381  judges that switching is not permitted; the switching permission judgment unit  381  then outputs information on whether or not switching is permitted (Step S 3812 ). 
     Incidentally, the high frequency energy Eh f-1  of the preceding frame which is obtained in Step S 3811  of the present frame is the same as the high frequency energy Eh f  of the present frame obtained in Step S 3811  of the preceding frame. Thus, by storing the calculated high frequency energy Eh f  in the switching permission judgment unit  381  until at least a frame immediately following the frame, there is no need to calculate the high frequency energy Eh f-1  of the preceding frame. 
     Next, an example in which the ratio of high frequency energy to the total energy of an MDCT coefficient sequence is used as the magnitude of the energy of high frequency components of an input sound signal will be described. 
     The switching permission judgment unit  381  first obtains the ratio Eh f-1  of high frequency energy to the total energy of an MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame by the following formula (1A) and the ratio Eh f  of high frequency energy to the total energy of the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame by the following formula (2A) (Step S 3811 ). In formula (1A) and formula (2A), M is a predetermined positive integer. 
     
       
         
           
             
               
                 
                   
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     The switching permission judgment unit  381  then judges that, if at least one of the ratio Eh f-1  of high frequency energy to the total energy of the preceding frame and the ratio Eh f  of high frequency energy to the total energy of the present frame is smaller than the predetermined threshold value TH1, that is, Eh f-1 &lt;TH1 and/or Eh f &lt;TH1, switching is permitted; otherwise, the switching permission judgment unit  381  judges that switching is not permitted; the switching permission judgment unit  381  then outputs information on whether or not switching is permitted (Step S 3812 ). 
     Incidentally, the ratio Eh f-1  of high frequency energy to the total energy of the preceding frame which is obtained in Step S 3811  of the present frame is the same as the ratio Eh f  of high frequency energy to the total energy of the present frame obtained in Step S 3811  of the preceding frame. Thus, by storing the calculated ratio Eh f  of high frequency energy to the total energy in the switching permission judgment unit  381  until at least a frame immediately following the frame, there is no need to calculate the ratio Eh f-1  of high frequency energy to the total energy of the preceding frame. 
     Incidentally, in the above-described two examples, it is judged that switching is permitted if Eh f-1 &lt;TH1 and/or Eh f &lt;TH1; otherwise, it is judged that switching is not permitted, but it may be judged that switching is permitted if Eh f-1 &lt;TH1 and Eh f &lt;TH1; otherwise, it may be judged that switching is not permitted. In other words, it may be judged that, if both the magnitude of the energy of high frequency components of the input sound signal of the preceding frame and the magnitude of the energy of high frequency components of the input sound signal of the present frame are smaller than a predetermined threshold value, switching is permitted, that is, coding of the coefficient sequence in the frequency domain of the present frame by coding processing which is different from the coding processing by which the coefficient sequence in the frequency domain of the preceding frame was coded is made possible; otherwise, it may be judged that switching is not permitted, that is, coding of the coefficient sequence in the frequency domain of the present frame by coding processing which is different from the coding processing by which the coefficient sequence in the frequency domain of the preceding frame was coded is not allowed. 
     Moreover, in the above-described examples, high frequency energy and the ratio of high frequency energy to the total energy are obtained by using an MDCT coefficient sequence, but high frequency energy and the ratio of high frequency energy to the total energy may be obtained by using a power spectral series or a series of energy of each frequency band. 
     &lt;Suitable Coding Processing Judgment Unit  382 &gt; 
     The suitable coding processing judgment unit  382  makes a judgment as to whether a coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for coding processing of the first coding unit  101  or coding processing of the second coding unit  201  and outputs the judgment result (Step S 382 ). 
     Hereinafter, an example of the operation of the suitable coding processing judgment unit  382  will be described. The suitable coding processing judgment unit  382  performs processing in each step illustrated in  FIG. 5 . In the following example, the coding processing of the first coding unit  101  is coding processing using a spectral envelope based on coefficients which are convertible into linear prediction coefficients, which is illustrated in Non-patent Literature 1, and the coding processing of the second coding unit  201  is coding processing involving variable-length coding which is performed on a differential between the logarithmic value of the average energy of coefficients in each frequency domain obtained by division and the logarithmic value of the average energy of an adjacent frequency domain, which is illustrated in Non-patent Literature 2. 
     In this example, if the ascents and descents of a spectral envelope of an input sound signal are steep or/and the degree of concentration of the spectral envelope is high, it is judged that a coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the first coding unit  101 ; if the ascents and descents of a spectral envelope of an input sound signal are gentle or/and the degree of concentration of the spectral envelope is low, it is judged that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the second coding unit  201 . The judgment result is then output. 
     As a method of estimating the ascents and descents and the degree of concentration of a spectrum, any method may be adopted; in the following example, a configuration in which the depth of the valley of a spectrum or the envelope thereof is estimated will be described. In this configuration, it is judged that the ascents and descents of a spectrum are gentle and the degree of concentration of the spectrum is low if the valley of the spectrum or the envelope thereof is shallow and that the ascents and descents of a spectrum are steep and the degree of concentration of the spectrum is high if the valley of the spectrum or the envelope thereof is deep. The shallow valley of a spectrum or the envelope thereof translates into a high noise floor. Moreover, the deep valley of a spectrum or the envelope thereof translates into a low noise floor. 
     The suitable coding processing judgment unit  382  first divides the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame into Q partial coefficient sequences XS f (1)(n) (n=1, . . . , P), XS f (2)(n) (n=1, . . . , P), . . . , XS f (Q)(n) (n=1, . . . , P), each having P samples (Step S 3821 ). P and Q are positive integers that satisfy the relationship P×Q=N. P may be 1. Moreover, here, a configuration in which an MDCT coefficient sequence X f (n) (n=1, . . . , N) which is a coefficient sequence in the frequency domain that is to be subjected to coding processing in the first coding unit  101  or in the second coding unit  201  is also used in the suitable coding processing judgment unit  382  is adopted, but a coefficient sequence in the frequency domain obtained by conversion into the frequency domain at a different degree of accuracy and by a different method from those adopted for the MDCT coefficient sequence X f (n) (n=1, . . . , N), for example, a power spectral series may be used as an object on which processing by the suitable coding processing judgment unit  382  is to be performed. 
     The suitable coding processing judgment unit  382  then obtains a series AVE XS (q) (q=1, . . . , Q) formed of the mean value of power or the logarithmic value thereof of each of the partial coefficient sequences XS f (1)(n) (n=1, . . . , P), XS f (2)(n) (n=1, . . . , P), . . . , XS f (Q)(n) (n=1, . . . , P) (Step S 3822 ). The mean value of power is AVE XS (q) obtained by formula (3). Moreover, the logarithmic value of the mean value of power is AVE XS (q) obtained by formula (3A). 
     
       
         
           
             
               
                 
                   
                     
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     The suitable coding processing judgment unit  382  then judges, for each element of the series AVE XS (1), AVE XS (2), . . . , AVE XS (Q) formed of the mean value of power or the logarithmic value of the mean value of power, whether or not the element is smaller than both of two adjacent elements and obtains the number of elements judged to be smaller than both of two adjacent elements (Step S 3823 ). That is, the suitable coding processing judgment unit  382  obtains the number Vally of q that satisfies formula (4).
 
AVE XS ( q )−min(AVE XS ( q− 1),AVE XS ( q+ 1))&lt;0  (4)
 
     The suitable coding processing judgment unit  382  then obtains the mean value E V  of Vally AVE XS (q) corresponding to q that satisfies formula (4), that is, the mean value E V  of the partial regions of the valley (Step S 3824 ). If AVE XS (q) is the mean value of power, E V  obtained in Step S 3824  is the mean value of power of the partial regions in a portion of the valley. If AVE XS (q) is the logarithmic value of the mean value of power, E V  obtained in Step S 3824  is the mean value of the logarithmic value of the mean value of power of the partial regions in a portion of the valley. Moreover, the suitable coding processing judgment unit  382  obtains the mean value of power or the logarithmic value of the mean value of power of all the partial regions (Step S 3825 ). The mean value of power of all the partial regions is the mean value of power of the MDCT coefficient sequence X f (n) (n=1, . . . , N) and is E obtained by formula (11). The logarithmic value of the mean value of power of all the partial regions is the logarithmic value of the mean value of power of the MDCT coefficient sequence X f (n) (n=1, . . . , N) and is E obtained by formula (11A). 
     
       
         
           
             
               
                 
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     The suitable coding processing judgment unit  382  then judges that, since it is estimated that the valley of a spectrum is shallow and the spectrum is a spectrum whose spectral envelope has gentle ascents and descents or whose degree of concentration is low if a difference between the mean value E of AVE XS (q) of all the partial regions and the mean value E V  of AVE XS (q) of the partial regions of the valley is smaller than or equal to a predetermined threshold value TH2, a coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the second coding unit  201 . Conversely, since it is estimated that the valley of a spectrum is deep and the spectrum is a spectrum whose spectral envelope has steep ascents and descents or whose degree of concentration is high if a difference between the mean value E of AVE XS (q) of all the partial regions and the mean value E V  of AVE XS (q) of the partial regions of the valley is greater than the threshold value TH2, the suitable coding processing judgment unit  382  judges that a coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the first coding unit  101 . The suitable coding processing judgment unit  382  outputs information on which coding processing is suitable (Step S 3826 ). The information on suitable coding processing is also referred to as suitability information. 
     Moreover, in Step S 3821 , different sample numbers may be adopted for different partial coefficient sequences. For example, the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame may be divided into Q partial coefficient sequences XS f (1)(n) (n=1, . . . , P 1 ), XS f (2)(n) (n=1, . . . , P 2 ), . . . , XS f (Q)(n) (n=1, . . . , P Q ). P 1 , P 2 , . . . , P Q  are positive integers that satisfy P 1 +P 2 + . . . +P Q =N. Furthermore, it is preferable that P 1 , P 2 , . . . , P Q  satisfy P 1 ≤P 2 ≤ . . . ≤P Q . In addition, Q is a positive integer. 
     &lt;Switching Selection Unit  383 &gt; 
     Based on the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  and the information on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382 , the switching selection unit  383  selects whether to code the coefficient sequence in the frequency domain of the present frame in the first coding unit  101  or in the second coding unit  201  and outputs a switching code that is a code by which the selected coding processing can be identified (Step S 383 ). The output switching code is input to a decoding device  400 . Here, if switching is not permitted, the switching selection unit  383  selects to code the coefficient sequence in the frequency domain of the present frame by the same coding processing as the coding processing of the preceding frame irrespective of the coding processing for which the present frame is suitable. Moreover, if switching is permitted, the switching selection unit  383  selects to code the coefficient sequence in the frequency domain of the present frame by the coding processing for which the present frame is suitable irrespective of the coding processing of the preceding frame. However, there may be a case where, even when switching is permitted, the switching selection unit  383  selects to code the coefficient sequence in the frequency domain of the present frame by the same coding processing as the coding processing of the preceding frame, not by the coding processing for which the present frame is suitable. 
     Hereinafter, an example of the operation of the switching selection unit  383  will be described. In the following example, the coding processing of the first coding unit  101  is coding processing using a spectral envelope based on coefficients which are convertible into linear prediction coefficients, which is illustrated in Non-patent Literature 1, and the coding processing of the second coding unit  201  is coding processing involving variable-length coding which is performed on a differential between the logarithmic value of the average energy of coefficients in each frequency domain obtained by division and the logarithmic value of the average energy of an adjacent frequency domain, which is illustrated in Non-patent Literature 2. 
     If the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  indicates that switching is not permitted and/or the information on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382  indicates that the same coding processing as the coding processing performed on the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame, the switching selection unit  383  selects the same coding processing as the coding processing performed on the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame as the coding processing which is performed on the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame. 
     That is, if the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the first coding unit  101  and the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  indicates that switching is not permitted, the switching selection unit  383  selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame also in the first coding unit  101 . Moreover, if the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the first coding unit  101  and the information on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382  indicates the coding processing of the first coding unit  101 , the switching selection unit  383  also selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame also in the first coding unit  101 . 
     Moreover, if the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the second coding unit  201  and the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  indicates that switching is not permitted, the switching selection unit  383  selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame also in the second coding unit  201 . Furthermore, if the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the second coding unit  201  and the information on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382  indicates the coding processing of the second coding unit  201 , the switching selection unit  383  also selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame also in the second coding unit  201 . 
     If the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  indicates that switching is permitted and the information on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382  indicates coding processing which is different from the coding processing performed on the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame, the switching selection unit  383  selects the coding processing which is different from the coding processing performed on the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame as coding processing which is performed on the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame. That is, if the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the first coding unit  101  and the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  indicates that switching is permitted and the information on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382  indicates the coding processing of the second coding unit  201 , the switching selection unit  383  selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame in the second coding unit  201 . Moreover, if the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the second coding unit  201  and the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  indicates that switching is permitted and the information on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382  indicates the coding processing of the first coding unit  101 , the switching selection unit  383  selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame in the first coding unit  101 . 
     &lt;Switching Unit  384 &gt; 
     The switching unit  384  performs control so as to input the MDCT coefficient sequence X f (n) (n=1, . . . , N) output from the frequency domain conversion unit  110  to the first coding unit  101  or the second coding unit  201  such that the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame is coded by the coding processing selected by the switching selection unit  383  (Step S 384 ). Moreover, if the input sound signal x f (n) (n=1, . . . , Nt) of the present frame is also necessary for coding of the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame, the switching unit  384  also inputs the input sound signal x f (n) (n=1, . . . , Nt) of the present frame to the first coding unit  101  or/and the second coding unit  201 . 
     For example, if the coding processing of the first coding unit  101  is coding processing using a spectral envelope based on coefficients which are convertible into linear prediction coefficients, which is illustrated in Non-patent Literature 1, and the coding processing of the second coding unit  201  is coding processing involving variable-length coding which is performed on a differential between the average energy of coefficients in each frequency domain obtained by division and the average energy of an adjacent frequency domain, which is illustrated in Non-patent Literature 2, since the input sound signal x f (n) (n=1, . . . , Nt) of the present frame is necessary only in the first coding unit  101 , the switching unit  384  also inputs the input sound signal x f (n) (n=1, . . . , Nt) of the present frame to the first coding unit  101  when inputting the MDCT coefficient sequence X f (n) (n=1, . . . , N) to the first coding unit  101 . 
     &lt;First Coding Unit  101 , Second Coding Unit  201 &gt; 
     Both the first coding unit  101  and the second coding unit  201  perform coding processing of coding a coefficient sequence in the frequency domain, but the first coding unit  101  and the second coding unit  201  perform different types of coding processing. That is, the first coding unit  101  codes a coefficient sequence in the frequency domain of the present frame by coding processing which is different from the coding processing of the second coding unit  201  and outputs a first code which is a code thus obtained (Step S 101 ). Moreover, the second coding unit  201  codes a coefficient sequence in the frequency domain of the present frame by coding processing which is different from the coding processing of the first coding unit  101  and outputs a second code which is a code thus obtained (Step S 201 ). For example, the first coding unit  101  performs coding processing using a spectral envelope based on coefficients which are convertible into linear prediction coefficients, and the second coding unit  201  performs coding processing using the average energy of coefficients in each frequency domain obtained by division. 
     Hereinafter, an example of the operation of the first coding unit  101  and the second coding unit  201  will be described. In the following example, the coding processing of the first coding unit  101  is coding processing using a spectral envelope based on coefficients which are convertible into linear prediction coefficients, which is illustrated in Non-patent Literature 1, and the coding processing of the second coding unit  201  is coding processing involving variable-length coding which is performed on a differential between the average energy of coefficients in each frequency domain obtained by division and the average energy of an adjacent frequency domain, which is illustrated in Non-patent Literature 2. 
     In this example, as illustrated on the left side of  FIG. 8 , first coding processing by the first coding unit  101  expresses a spectral envelope shape in the frequency domain by coefficients which are convertible into linear prediction coefficients. On the other hand, as illustrated on the right side of  FIG. 8 , second coding processing by the second coding unit  201  expresses an envelope shape by a scale factor band (division of a frequency domain coefficient sequence into a plurality of regions). It can be said that the second coding processing is very efficient if the mean value changes smoothly because the second coding processing uses variable-length coding of the differential value of the average height of each region. 
     Based on the result of decision or selection made by the selection unit  380 , one of the processing of the first coding unit  101  and the processing of the second coding unit  201 , which are a plurality of types of coding processing in the frequency domain, is performed. 
     &lt;First Coding Unit  101 &gt; 
     The first coding unit  101  includes the linear prediction analysis coding unit  120 , the spectral envelope coefficient sequence generating unit  130 , the envelope normalization unit  140 , and the normalized coefficient coding unit  150 . To the first coding unit  101 , the MDCT coefficient sequence X f (n) (n=1, . . . , N) and the input sound signal x f (n) (n=1, . . . , Nt) of the present frame are input, and a first code containing a linear prediction coefficient code CL f  and a normalized coefficient code CN f  is output therefrom. The output first code is input to the decoding device  400 . Incidentally, the first coding unit  101  is what is obtained by removing, from the coding processing described in Non-patent Literature 1, a portion converting an input sound signal into a coefficient sequence in the frequency domain. That is, the coding processing which is performed in the frequency domain conversion unit  110  and the first coding unit  101  is similar to the coding processing described in Non-patent Literature 1. 
     &lt;Linear Prediction Analysis Coding Unit  120 &gt; 
     The linear prediction analysis coding unit  120  obtains coefficients which are convertible into linear prediction coefficients by performing linear prediction analysis on the input sound signal x f (n) (n=1, . . . , Nt), and obtains a linear prediction coefficient code CL f  and coefficients which are convertible into quantized linear prediction coefficients corresponding to the linear prediction coefficient code CL f  by coding the coefficients which are convertible into linear prediction coefficients and outputs the linear prediction coefficient code CL f  and the coefficients (Step S 120 ). The coefficients which are convertible into linear prediction coefficients are linear prediction coefficients themselves, PARCOR coefficients (partial autocorrelation coefficients), LSP parameters, or the like. 
     &lt;Spectral Envelope Coefficient Sequence Generating Unit  130 &gt; 
     The spectral envelope coefficient sequence generating unit  130  obtains a power spectral envelope coefficient sequence W f (n) (n=1, . . . , N) corresponding to the coefficients which are convertible into the quantized linear prediction coefficients obtained by the linear prediction analysis coding unit  120  and outputs the power spectral envelope coefficient sequence W f (n) (n=1, . . . , N) (Step S 130 ). 
     &lt;Envelope Normalization Unit  140 &gt; 
     The envelope normalization unit  140  normalizes each coefficient X f (n) (n=1, . . . , N) of the MDCT coefficient sequence obtained by the frequency domain conversion unit  110  by using the power spectral envelope coefficient sequence W f (n) (n=1, . . . , N) obtained by the spectral envelope coefficient sequence generating unit  130  and outputs a normalized MDCT coefficient sequence XN f (n) (n=1, . . . , N) (Step S 140 ). That is, the envelope normalization unit  140  obtains, as the normalized MDCT coefficient sequence XN f (n) (n=1, . . . , N), a series formed of values obtained by dividing each coefficient of the MDCT coefficient sequence X f (n) (n=1, . . . , N) by a corresponding coefficient contained in the power spectral envelope coefficient sequence W f (n) (n=1, . . . , N). 
     &lt;Normalized Coefficient Coding Unit  150 &gt; 
     The normalized coefficient coding unit  150  obtains the normalized coefficient code CN f  by coding the normalized MDCT coefficient sequence XN f (n) (n=1, . . . , N) obtained by the envelope normalization unit  140  (Step S 150 ). 
     &lt;Second Coding Unit  201 &gt; 
     Moreover, the second coding unit  201  includes the region division unit  220 , the average logarithmic energy differential variable-length coding unit  240 , and the coefficient coding unit  250 . To the second coding unit  201 , the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame is input, and a second code containing an average energy code CA f  and a coefficient code CD f  is output therefrom. The output second code is input to the decoding device  400 . Incidentally, the second coding unit  201  is what is obtained by removing, from the coding processing described in Non-patent Literature 2, a portion converting an input sound signal into a coefficient sequence in the frequency domain. That is, the coding processing which is performed in the frequency domain conversion unit  110  and the second coding unit  201  is similar to the coding processing described in Non-patent Literature 2. 
     &lt;Region Division Unit  220 &gt; 
     The region division unit  220  divides the MDCT coefficient sequence X f (n) (n=1, . . . , N) obtained by the frequency domain conversion unit  110  into a plurality of partial regions such that the lower the frequencies of the partial regions are, the smaller the number of samples of the partial regions becomes; the higher the frequencies of the partial regions are, the greater the number of samples of the partial regions becomes (Step S 220 ). If the number of partial regions is assumed to be R and the numbers of samples contained in the partial regions are assumed to be S 1 , . . . , S R , each coefficient X f (n) (n=1, . . . , N) of the MDCT coefficient sequence is divided into partial regions from the sample at the lowest frequency in order as follows: XB f (1)(n) (n=1, . . . , S 1 ), XB f (2)(n) (n=1, . . . , S 2 ), . . . , XB f (R)(n) (n=1, . . . , S R ). R and S 1 , . . . , S R  are positive integers. It is assumed that S 1 , . . . , S R  satisfy the relationship S 1 ≤S 2 ≤ . . . ≤S R . XB f (1)(n) (n=1, . . . , S 1 ), XB f (2)(n) (n=1, . . . , S 2 ), . . . , XB f (R)(n) (n=1, . . . , S R ) is referred to as a partial region coefficient sequence. 
     &lt;Average Logarithmic Energy Differential Variable-Length Coding Unit  240 &gt; 
     The average logarithmic energy differential variable-length coding unit  240  obtains, for each partial region obtained by the region division unit  220 , the average energy of coefficients contained in the partial region, quantizes each average energy of the partial region on the logarithmic axes, performs variable-length coding on a difference in the quantization value of the average energy on the logarithmic axes between the adjacent partial regions, and obtains an average energy code CA f  (Step S 240 ). 
     The average logarithmic energy differential variable-length coding unit  240  first obtains the average energy E XB (r) (r=1, . . . , R) of each partial region r (r=1, . . . , R) by formula (5) (Step S 2401 ). 
     
       
         
           
             
               
                 
                   
                     
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     The average logarithmic energy differential variable-length coding unit  240  then performs scalar quantization of the average energy E XB (r) (r=1, . . . , R) in the log domain for each partial region and obtains a quantization value Q(log(E XB (r))) (r=1, . . . , R) of the average energy in the log domain (Step S 2402 ). The average logarithmic energy differential variable-length coding unit  240  then obtains, for each partial region, a difference DiffE XB (r) between the quantization value Q(log(E XB (r))) of the average energy in the log domain and the quantization value Q(log(E XB (r−1))) of the average energy in the log domain, the average energy of the coefficients contained in an adjacent partial region (Step S 2403 ). However, when r=1, the scalar quantization value Q(log(E XB (r))) itself of the logarithmic value of the average energy E XB (1) is used as DiffE XB (1). DiffE XB (r) (r=1, . . . , R) is referred to as an average logarithmic energy differential. That is, DiffE XB (r) (r=1, . . . , R) is obtained by formula (6). However, Q( ) is assumed to be a scalar quantization function and is assumed to be a function that outputs an integer value which is obtained by rounding off a decimal fraction of a value obtained by normalizing (dividing) an input by a predetermined value.
 
Diff E   XB ( r )= Q (log( E   XB ( r )))− Q (log( E   XB ( r− 1)))( r≥ 2)
 
Diff E   XB (1)= Q (log( E   XB (1)))  (6)
 
     The average logarithmic energy differential variable-length coding unit  240  obtains an average energy code CA f  by performing variable-length coding on the average logarithmic energy differential DiffE XB (r) (r=1, . . . , R) (Step S 2404 ). Incidentally, since the statistical frequency of appearance is high when the absolute value of the average logarithmic energy differential DiffE XB (r) is small, the variable-length code is determined in advance such that the code amount becomes smaller than that in a case where the absolute value is large. That is, when fluctuations in the average logarithmic energy in each region are small, that is, the ascents and descents of a spectral envelope are gentle, there is a tendency to be capable of shortening the length of the code of the average energy code CA f  if the degree of concentration of the spectral envelope is low. 
     &lt;Coefficient Coding Unit  250 &gt; 
     The coefficient coding unit  250  obtains a coefficient code CD f  by performing, for example, scalar quantization on each coefficient of the partial region coefficient sequence XB f (1)(n) (n=1, . . . , S 1 ), XB f (2)(n) (n=1, . . . , S 2 ), . . . , XB f (R)(n) (n=1, . . . , S R ) obtained by the region division unit  220  by using the quantization value Q(log(E XB (r))) (r=1, . . . , R) of the average energy in the log domain obtained by the average logarithmic energy differential variable-length coding unit  240  (Step S 250 ). The quantization step width and the quantization bit number which are used for this scalar quantization are determined from the quantization value Q(E XB (r)) (r=1, . . . , R) of the average energy for each partial region coefficient sequence XB f (1)(n) (n=1, . . . , S 1 ), XB f (2)(n) (n=1, . . . , S 2 ), . . . , XB f (R)(n) (n=1, . . . , S R ) obtained by the region division unit  220 . Incidentally, the quantization value Q(E XB (r)) (r=1, . . . , R) of the average energy is obtained by converting the quantization value Q(log(E XB (r))) (r=1, . . . , R) of the average energy in the log domain into a value in the linear domain by formula (7).
 
 Q ( E   XB ( r ))= e   Q(log(E     XB     (r)))   (7)
 
     The coefficient coding unit  250  first distributes the bit number given as the code amount of the coefficient code CD f  to the coefficients of each partial region coefficient sequence with consideration given to the quantization value Q(log(E XB (r))) (r=1, . . . , R) of the average energy in the log domain corresponding to each region and a value of a difference between that value and the logarithmic value of energy at an auditorily indiscernible spectral level which is estimated by the frequency (Step S 2501 ). 
     The coefficient coding unit  250  then determines the step width of scalar quantization of each coefficient of each partial region coefficient sequence from the quantization value Q(E XB (r)) (r=1, . . . , R) of the average energy of each partial region and the distributed bit number (Step S 2502 ). 
     The coefficient coding unit  250  then obtains a coefficient code CD f  by quantizing each coefficient of each partial region coefficient sequence by the step width thus determined and the bit number and performing variable-length coding on the integer value of each quantized coefficient (Step S 2503 ). 
     &lt;Decoding Device  400 &gt; 
     The configuration of the decoding device  400  is depicted in  FIG. 2 . The decoding device  400  includes a switching unit  480 , a first decoding unit  401 , and a second decoding unit  501 . The first decoding unit  401  includes, for example, a linear prediction decoding unit  420 , a spectral envelope coefficient sequence generating unit  430 , a normalized coefficient decoding unit  450 , and an envelope inverse normalization unit  440 . The second decoding unit  501  includes, for example, an average logarithmic energy differential variable-length decoding unit  540  and a coefficient decoding unit  550 . To the decoding device  400 , a code containing a switching code and an input code is input frame by frame which is a predetermined time segment. In the case of the frame coded in the first coding unit  101 , the input code contains the linear prediction coefficient code CL f  and the normalized coefficient code CN f ; in the case of the frame coded in the second coding unit  201 , the input code contains the average energy code CA f  and the coefficient code CD f . Hereinafter, specific processing of each unit will be described based on the premise that the frame which is currently being processed is an f-th frame. 
     Hereinafter, the operation of the decoding device  400  will be described. 
     &lt;Switching Unit  480 &gt; 
     The switching unit  480  selects whether to decode the input code of the present frame in the first decoding unit  401  or in the second decoding unit  501  based on the input switching code and performs control in such a way as to input the input code to the first decoding unit  401  or the second decoding unit  501  such that the selected decoding processing is performed (Step S 480 ). 
     Specifically, if the input switching code is a code specifying the coding processing of the first coding unit  101 , that is, a code specifying the coding processing using a spectral envelope based on coefficients which are convertible into linear prediction coefficients, the switching unit  480  performs control in such a way as to input the input code to the first decoding unit  401  that performs decoding processing corresponding to the coding processing of the first coding unit  101 . Moreover, if the input switching code is a code specifying the coding processing of the second coding unit  201 , that is, a code specifying the coding processing involving variable-length coding which is performed on a differential between the average energy of coefficients in each frequency domain obtained by division and the average energy of an adjacent frequency domain, the switching unit  480  performs control in such a way as to input the input code to the second decoding unit  501  that performs decoding processing corresponding to the coding processing of the second coding unit  201 . 
     &lt;First Decoding Unit  401 &gt; 
     The first decoding unit  401  includes the linear prediction decoding unit  420 , the spectral envelope coefficient sequence generating unit  430 , the normalized coefficient decoding unit  450 , and the envelope inverse normalization unit  440 . To the first decoding unit  401 , the linear prediction coefficient code CL f  and the normalized coefficient code CN f  of the present frame are input, and a coefficient sequence X f (n) (n=1, . . . , N) in the frequency domain is output therefrom. 
     &lt;Linear Prediction Decoding Unit  420 &gt; 
     The linear prediction decoding unit  420  obtains coefficients which are convertible into decoded linear prediction coefficients by decoding the linear prediction coefficient code CL f  contained in the input code. The coefficients which are convertible into decoded linear prediction coefficients are the same as the coefficients which are convertible into the quantized linear prediction coefficients obtained by the linear prediction analysis coding unit  120  of the coding device  300 . Moreover, the decoding processing which is performed by the linear prediction decoding unit  420  corresponds to the coding processing which is performed by the linear prediction analysis coding unit  120  of the coding device  300 . Incidentally, the coefficients which are convertible into linear prediction coefficients are linear prediction coefficients themselves, PARCOR coefficients (partial autocorrelation coefficients), LSP parameters, or the like. 
     &lt;Spectral Envelope Coefficient Sequence Generating Unit  430 &gt; 
     The spectral envelope coefficient sequence generating unit  430  obtains a power spectral envelope coefficient sequence W f (n) (n=1, . . . , N) corresponding to the coefficients obtained by the linear prediction decoding unit  420 , which are convertible into decoded linear prediction coefficients, and outputs the power spectral envelope coefficient sequence W f (n) (n=1, . . . , N). However, N is a sample number in the frequency domain and is a positive integer. 
     &lt;Normalized Coefficient Decoding Unit  450 &gt; 
     The normalized coefficient decoding unit  450  obtains a decoded normalized MDCT coefficient sequence {circumflex over ( )}XN f (n) (n=1, . . . , N) by decoding the input normalized coefficient code CN f  (Step S 450 ). Here, the decoding processing which is performed by the normalized coefficient decoding unit  450  corresponds to the coding processing which is performed by the normalized coefficient coding unit  150  of the coding device  300 . That is, if the conversion processing into the frequency domain which is not MDCT is performed in the coding device  300 , {circumflex over ( )}XN f (n) (n=1, . . . , N) is a coefficient sequence in the frequency domain, which is not MDCT, corresponding to the conversion processing into the frequency domain performed in the coding device  300 . Incidentally, although the decoded normalized MDCT coefficient sequence {circumflex over ( )}XN f (n) (n=1, . . . , N) corresponds to the normalized MDCT coefficient sequence XN f (n) (n=1, . . . , N) input to the normalized coefficient coding unit  150  of the coding device  300 , since a quantization error is contained in each coefficient, {circumflex over ( )}XN f (n) obtained by adding “{circumflex over ( )}” to XN f (n) is used. 
     &lt;Envelope Inverse Normalization Unit  440 &gt; 
     The envelope inverse normalization unit  440  performs inverse normalization on each coefficient {circumflex over ( )}XN f (n) (n=1, . . . , N) of the decoded normalized MDCT coefficient sequence obtained by the normalized coefficient decoding unit  450  by using the power spectral envelope coefficient sequence W f (n) (n=1, . . . , N) obtained by the spectral envelope coefficient sequence generating unit  430 , and outputs a decoded MDCT coefficient sequence {circumflex over ( )}XN f (n) (n=1, . . . , N) (Step S 440 ). That is, the envelope inverse normalization unit  440  obtains a series formed of values obtained by multiplying corresponding coefficients of the coefficients of the decoded normalized MDCT coefficient sequence XN f (n) (n=1, . . . , N) and the coefficients of the power spectral envelope coefficient sequence W f (n) (n=1, . . . , N) as the decoded MDCT coefficient sequence {circumflex over ( )}X f (n) (n=1, . . . , N). 
     &lt;Second Decoding Unit  501 &gt; 
     The second decoding unit  501  includes the average logarithmic energy differential variable-length decoding unit  540  and the coefficient decoding unit  550 . To the second decoding unit  501 , the average energy code CA f  and the coefficient code CD f  of the present frame are input, and a coefficient sequence X f (n) (n=1, . . . , N) in the frequency domain is output therefrom. 
     &lt;Average Logarithmic Energy Differential Variable-Length Decoding Unit  540 &gt; 
     The average logarithmic energy differential variable-length decoding unit  540  obtains a decoded average energy Q(E XB (r)) (r=1, . . . , R) in a partial region by decoding the input average energy code CA f  (Step S 540 ). Incidentally, since the decoded average energy is the same as the quantization value of the average energy obtained in the coefficient coding unit  250  of the coding device  300 , the same symbol Q(E XB (r)) is used. 
     The average logarithmic energy differential variable-length decoding unit  540  first obtains a difference DiffE XB (r) (r=1, . . . , R) in energy in the log domain of each partial region by decoding the average energy code CA f  (Step S 5401 ). Here, the decoding processing which is performed by the average logarithmic energy differential variable-length decoding unit  540  corresponds to the coding processing which is performed by the average logarithmic energy differential variable-length coding unit  240  of the coding device  300 . Incidentally, since a difference in energy in the log domain of each partial region is the same as a difference in energy in the log domain of each partial region which is obtained in the average logarithmic energy differential variable-length coding unit  240  of the coding device  300 , the same symbol DiffE XB (r) is used. 
     The average logarithmic energy differential variable-length decoding unit  540  then obtains, for each partial region, a decoded value Q(log(E XB (r))) of the average energy in the log domain by adding the difference DiffE XB (r) (r=1, . . . , R) in energy in the log domain to a decoded value Q(log(E XB (r−1))) of the average energy in the log domain of an adjacent partial region (Step S 5402 ). Incidentally, since the decoded value of the average energy in the log domain is the same as the quantization value of the average energy in the log domain which is obtained in the average logarithmic energy differential variable-length coding unit  240  of the coding device  300 , the same symbol Q(log(E XB (r))) is used.
 
 Q (log( E   XB (1)))=Diff E   XB (1)
 
 Q (log( E   XB ( r )))=Diff E   XB ( r )+ Q (log( E   XB ( r− 1)))( r≥ 2)  (8)
 
     The average logarithmic energy differential variable-length decoding unit  540  then obtains what has the decoded value Q(log(E XB (r−1))) (r=1, . . . , R) of the average energy in the log domain as a value in the linear domain as decoded average energy Q(E XB (r)) (r=1, . . . , R) (Step S 5403 ). 
     &lt;Coefficient Decoding Unit  550 &gt; 
     The coefficient decoding unit  550  obtains a decoded coefficient sequence {circumflex over ( )}X f (n) (n=1, . . . , N) by decoding the coefficient code CD f  by using the decoded average energy Q(E XB (r)) (r=1, . . . , R) obtained in the average logarithmic energy differential variable-length decoding unit  540  (Step S 550 ). Here, the decoding processing which is performed by the coefficient decoding unit  550  corresponds to the coding processing which is performed by the coefficient coding unit  250  of the coding device  300 . Since the input coefficient code CD f  is what was obtained by performing variable-length coding on each coefficient of each partial region coefficient sequence in the coefficient coding unit  250  of the coding device  300 , the code length of a code portion of the coefficient code CD f  corresponding to each coefficient can be automatically reconstituted. Moreover, the quantization step width of each region is obtained from the decoded average energy Q(E XB (r)) obtained in the average logarithmic energy differential variable-length decoding unit  540 . As a result, it is possible to obtain the decoded MDCT coefficient sequence {circumflex over ( )}X f (n) (n=1, . . . , N) in the frequency domain from the coefficient code CD f . 
     &lt;Time Domain Conversion Unit  410 &gt; 
     The time domain conversion unit  410  obtains a decoded sound signal {circumflex over ( )}x f (n) (n=1, . . . , Nt) by converting a decoded MDCT coefficient sequence {circumflex over ( )}X f (n) (n=1, . . . , N) at point N into the time domain and outputs the decoded sound signal {circumflex over ( )}x f (n) (n=1, . . . , Nt) (Step S 410 ). However, Nt is a sample number in the time domain and is a positive integer. If conversion into the frequency domain which is not MDCT is performed in the frequency domain conversion unit  110  of the coding device  300 , it is necessary simply to perform conversion processing into the time domain corresponding to that conversion processing. 
     According to the first embodiment, since switching of coding processing and decoding processing can be performed only when the high frequency energy of an input sound signal is small, even when a plurality of types of coding processing and decoding processing which are different in the quantization characteristics of high frequency components are installed, it is possible to obtain a decoded sound signal that sounds less artificial to a listener. 
     According to the first embodiment, moreover, since it is possible to select coding processing suitable for an input sound signal, without performing actual coding, from coding processing using a spectral envelope based on coefficients which are convertible into linear prediction coefficients and coding processing using the average energy of coefficients in each frequency domain obtained by division, it is possible to perform coding processing suitable for an input sound signal with a small arithmetic processing amount. 
     According to the first embodiment, furthermore, since it is possible to perform coding by selecting coding processing from coding processing using a spectral envelope based on coefficients which are convertible into linear prediction coefficients and coding processing using the average energy of coefficients in each frequency domain obtained by division, irrespective of whether or not the ascents and descents of a spectrum of an input sound signal are steep and whether or not the degree of concentration of the spectrum is high, it is possible to perform efficient coding processing regardless of the characteristics of the input sound signal. 
     Second Embodiment 
     In the first embodiment, a coefficient sequence in the frequency domain of the present frame is always coded by the same coding processing as the coding processing of the preceding frame when the magnitude of the energy of high frequency components of an input sound signal is great; a second embodiment allows a coefficient sequence in the frequency domain of the present frame to be coded by coding processing which is different from the coding processing of the preceding frame depending on the state in which high frequency components of an input sound signal are sparse even when the magnitude of the energy of the high frequency components of the input sound signal is great. 
     A coding device of the second embodiment makes it possible to select coding processing which is different from the coding processing of the preceding frame as coding processing of the present frame if the energy of high frequency components of an input sound signal is small; otherwise, in accordance with the state in which the high frequency components of the input sound signal are sparse, the coding device selects whether to make it possible to select coding processing which is different from the coding processing of the preceding frame as coding processing of the present frame or select the same coding processing as the coding processing of the preceding frame as coding processing of the present frame. 
     The configuration of the coding device of the second embodiment is  FIG. 1 , which is the same as the first embodiment. A coding device  300  of the second embodiment is the same as the coding device  300  of the first embodiment except that processing of the switching permission judgment unit  381  and the switching selection unit  383  in the selection unit  380  is different from that of the coding device  300  of the first embodiment. The configuration of a decoding device of the second embodiment is  FIG. 2 , which is the same as the first embodiment, and processing of each unit is also the same as that of the decoding device of the first embodiment. Hereinafter, the switching permission judgment unit  381  and the switching selection unit  383  in the selection unit  380 , which perform processing different from the processing performed in the coding device  300  of the first embodiment, will be described. 
     &lt;Switching Permission Judgment Unit  381 &gt; 
     The switching permission judgment unit  381  judges that, if at least one of the magnitude of the energy of high frequency components of the input sound signal of the preceding frame and the magnitude of the energy of high frequency components of the input sound signal of the present frame is smaller than a predetermined threshold value, switching is permitted, that is, judges that the switching permission judgment unit  381  makes it possible to code a coefficient sequence in the frequency domain of the present frame by coding processing which is different from the coding processing by which the coefficient sequence in the frequency domain of the preceding frame was coded, and outputs the judgment result (Step S 381 ). Otherwise, the switching permission judgment unit  381  does not make any judgment such as switching is permitted or switching is not permitted, and outputs information indicating that any judgment such as switching is permitted or switching is not permitted has not been made as the judgment result, or does not output any judgment result. As the magnitude of the energy of high frequency components of an input sound signal, as is the case with the first embodiment, high frequency energy may be used or the ratio of high frequency energy to the total energy may be used. 
     &lt;Switching Selection Unit  383 &gt; 
     The switching selection unit  383  selects whether the coefficient sequence in the frequency domain of the present frame is coded in the first coding unit  101  or in the second coding unit  201  based on the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381 , the information on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382 , and the state, which is obtained from the input sound signal, indicating whether or not the high frequency components of the input sound signal are sparse, and outputs a switching code that is a code by which the selected coding processing can be identified (Step S 383 B). The output switching code is input to the decoding device  400 . 
     If the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  indicates that switching is permitted, that is, the energy of high frequency components of the input sound signal is small, the switching selection unit  383  performs the same processing as the switching selection unit  383  of the first embodiment. If the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  indicates that any judgment such as switching is permitted or switching is not permitted has not been made or the judgment result is not input to the switching permission judgment unit  381 , that is, if the energy of high frequency components of the input sound signal is great, the switching selection unit  383  selects whether or not to allow the coefficient sequence in the frequency domain of the present frame to be coded by coding processing which is different from the coding processing of the preceding frame based on the state, which is obtained from the input sound signal, indicating whether or not the high frequency components of the input sound signal are sparse. 
     Hereinafter, of the operation of the switching selection unit  383 , a difference from the operation of the switching selection unit  383  of the first embodiment, that is, an example of the operation of the switching selection unit  383  when the energy of the high frequency components of the input sound signal is great will be described. In the following example, as is the case with the first embodiment, the coding processing of the first coding unit  101  is coding processing using a spectral envelope based on coefficients which are convertible into linear prediction coefficients, which is illustrated in Non-patent Literature 1, and the coding processing of the second coding unit  201  is coding processing involving variable-length coding which is performed on a differential between the logarithmic value of the average energy of coefficients in each frequency domain obtained by division and the logarithmic value of the average energy of an adjacent frequency domain, which is illustrated in Non-patent Literature 2. The switching selection unit  383  performs processing from Steps S 3831 B to S 3836 B of  FIG. 6 , for example. 
     The switching selection unit  383  first divides the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame into Q partial coefficient sequences XS f (1)(n) (n=1, . . . , P), XS f (2)(n) (n=1, . . . , P), . . . , XS f (Q)(n) (n=1, . . . , P), each having P samples (Step S 3831 B). P and Q are positive integers that satisfy the relationship P×Q=N. P may be 1. Moreover, here, a configuration in which the MDCT coefficient sequence X f (n) (n=1, . . . , N) which is a coefficient sequence in the frequency domain that is to be subjected to coding processing in the first coding unit  101  or in the second coding unit  201  is also used in the switching selection unit  383  is adopted, but a coefficient sequence in the frequency domain obtained by conversion into the frequency domain at a different degree of accuracy and by a different method from those adopted for the MDCT coefficient sequence X f (n) (n=1, . . . , N), for example, a power spectral series may be used as an object on which processing by the switching selection unit  383  is to be performed. 
     The switching selection unit  383  then obtains a series AVE XS (q) (q=1, . . . , Q) formed of the logarithmic value of the mean value of power of each of the partial coefficient sequences XS f (1)(n) (n=1, . . . , P), XS f (2)(n) (n=1, . . . , P), . . . , XS f (Q)(n) (n=1, . . . , P) (Step S 3832 B). The logarithmic value of the mean value of power of each partial coefficient sequence is AVE XS (q) which is obtained by formula (3A). 
     Moreover, the switching selection unit  383  obtains the logarithmic value of the mean value of power of the MDCT coefficient sequence X f (n) (n=1, . . . , N) (Step S 3833 B). The logarithmic value of the mean value of power of the MDCT coefficient sequence is AVE Total  which is obtained by formula (9). 
     
       
         
           
             
               
                 
                   
                     AVE 
                     Total 
                   
                   = 
                   
                     log 
                     ( 
                     
                       
                         
                           ∑ 
                           
                             n 
                             = 
                             1 
                           
                           N 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           
                             ( 
                             
                               
                                 X 
                                 f 
                               
                               ⁡ 
                               
                                 ( 
                                 n 
                                 ) 
                               
                             
                             ) 
                           
                           2 
                         
                       
                       N 
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     The switching selection unit  383  then obtains the number of AVE XS (q) in which q satisfies formula (10) within the previously set range of Q Low  (1&lt;Q Low ) to Q High  (Q Low ≤Q High ≤Q), that is, the predetermined range of one or more than one partial region located on the high frequency side, in other words, the number of regions at the peak (Step S 3834 B). μ and λ are positive constants.
 
AVE XS ( q )≥μ·AVE Total +λ  (10)
 
     The switching selection unit  383  then judges that the high frequency components of the input sound signal of the present frame are sparse if the number of regions at the peak is smaller than or equal to a threshold value TH3 and judges that the high frequency components of the input sound signal of the present frame are not sparse if the number of regions at the peak exceeds the threshold value TH3 (Step S 3835 B). Here, the threshold value TH3 is a value that is determined by a predetermined rule such that, if the high frequency components of the input sound signal of a past frame close to the present frame are sparse, the value becomes a value greater than a value which is set when the high frequency components of the input sound signal of the past frame close to the present frame are not sparse. For example, if the high frequency components of the input sound signal of the past frame close to the present frame are sparse, predetermined TH3_1 is used as the threshold value TH3; if the high frequency components of the input sound signal of the past frame close to the present frame are not sparse, predetermined TH3_2 which is a value smaller than TH3_1 is used as the threshold value TH3. Here, the past frame close to the present frame is, for example, the preceding frame or the frame before the preceding frame. The judgment result as to whether or not the high frequency components of the input sound signal of the present frame are sparse is stored in the switching selection unit  383  until the end of at least two frames after the present frame. 
     The switching selection unit  383  then selects to code the coefficient sequence in the frequency domain of the present frame in either the first coding unit  101  or the second coding unit  201  based on the coding processing of the preceding frame and the judgment result on the present frame and the past frame close to the present frame as to whether or not the high frequency components of the input sound signal are sparse (Step S 3836 B). That is, the switching selection unit  383  selects whether or not to allow the coefficient sequence in the frequency domain of the present frame to be coded by coding processing which is different from the coding processing of the preceding frame. 
     For example, when the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the first coding unit  101 , if the high frequency components of the present frame are not sparse and the high frequency components are sparse in at least one of the preceding frame and the frame before the preceding frame, the switching selection unit  383  makes it possible to select to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame in the second coding unit  201 ; otherwise, the switching selection unit  383  selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame in the first coding unit  101 . That is, when the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the first coding unit  101 , if the high frequency components of the present frame are not sparse and the high frequency components are sparse in at least one of the preceding frame and the frame before the preceding frame, the switching selection unit  383  allows the coefficient sequence in the frequency domain of the present frame to be coded by coding processing which is different from the coding processing of the preceding frame; otherwise, the switching selection unit  383  does not allow the coefficient sequence in the frequency domain of the present frame to be coded by coding processing which is different from the coding processing of the preceding frame. 
     Moreover, when the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the second coding unit  201 , if (1) the high frequency components of the present frame are sparse and the high frequency components of the preceding frame are not sparse or (2) the high frequency components of the present frame are sparse, the high frequency components of the preceding frame are sparse, and the high frequency components of the frame before the preceding frame are not sparse, the switching selection unit  383  makes it possible to select to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame in the first coding unit  101 ; otherwise, the switching selection unit  383  selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame in the second coding unit  201 . That is, when the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the second coding unit  201 , if (1) the high frequency components of the present frame are sparse and the high frequency components of the preceding frame are not sparse or (2) the high frequency components of the present frame are sparse, the high frequency components of the preceding frame are sparse, and the high frequency components of the frame before the preceding frame are not sparse, the switching selection unit  383  allows the coefficient sequence in the frequency domain of the present frame to be coded by coding processing which is different from the coding processing of the preceding frame; otherwise, the switching selection unit  383  does not allow the coefficient sequence in the frequency domain of the present frame to be coded by coding processing which is different from the coding processing of the preceding frame. 
     Incidentally, if the switching selection unit  383  allows the coefficient sequence in the frequency domain of the present frame to be coded by coding processing which is different from the coding processing of the preceding frame, the switching selection unit  383  selects the coding processing of the coefficient sequence in the frequency domain of the present frame based on the information on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382 . For example, if the switching selection unit  383  allows the coefficient sequence in the frequency domain of the present frame to be coded by coding processing which is different from the coding processing of the preceding frame, even when the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the second coding unit  201 , if the information on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382  indicates the coding processing of the first coding unit  101 , the switching selection unit  383  selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame in the first coding unit  101 . Moreover, if the switching selection unit  383  allows the coefficient sequence in the frequency domain of the present frame to be coded by coding processing which is different from the coding processing of the preceding frame, even when the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the first coding unit  101 , if the information on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382  indicates the coding processing of the second coding unit  201 , the switching selection unit  383  selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame in the second coding unit  201 . 
     Incidentally, even when the switching selection unit  383  allows the coefficient sequence in the frequency domain of the present frame to be coded by coding processing which is different from the coding processing of the preceding frame, if it is judged that the coefficient sequence in the frequency domain of the present frame has to be coded by the same coding processing as the coding processing of the preceding frame based on the other information obtained by a means that is not depicted in the coding device  300 , the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame may be coded by the same coding processing as the coding processing of the preceding frame. 
     Moreover, in Step S 3831 B, different sample numbers may be adopted for different partial coefficient sequences. For example, the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame may be divided into Q partial coefficient sequences XS f (1)(n) (n=1, . . . , P 1 ), XS f (2)(n) (n=1, . . . , P 2 ), . . . , XS f (Q)(n) (n=1, . . . , P Q ). P 1 , P 2 , . . . , P Q  are positive integers that satisfy P 1 +P 2 + . . . +P Q =N. Furthermore, it is preferable that P 1 , P 2 , . . . , P Q  satisfy P 1 ≤P 2 ≤ . . . ≤P Q . Moreover, Q is a positive integer. 
     Furthermore, if the suitable coding processing judgment unit  382  performed the same processing as the processing in Step S 3831 B, Step S 3832 B, and Step S 3833 B, the switching selection unit  383  may use the result of the processing performed by the suitable coding processing judgment unit  382  without performing Step S 3831 B, Step S 3832 B, and Step S 3833 B. 
     Third Embodiment 
     In the first embodiment and the second embodiment, coding processing for which the present frame is suitable is judged by using one threshold value; in a third embodiment, a judgment using two threshold values is made. 
     The configuration of a coding device of the third embodiment is  FIG. 1 , which is the same as the first embodiment. A coding device  300  of the third embodiment is the same as the coding device  300  of the first embodiment or the second embodiment except that processing of the suitable coding processing judgment unit  382  and the switching selection unit  383  in the selection unit  380  is different from that of the coding device  300  of the first embodiment or the second embodiment. The configuration of a decoding device of the third embodiment is  FIG. 2 , which is the same as the first embodiment, and processing of each unit is also the same as that of the decoding device of the first embodiment. Hereinafter, the suitable coding processing judgment unit  382  and the switching selection unit  383  in the selection unit  380 , which perform processing different from the processing performed in the coding device  300  of the first embodiment, will be described. 
     &lt;Suitable Coding Processing Judgment Unit  382 &gt; 
     The suitable coding processing judgment unit  382  performs processing in each step illustrated in  FIG. 7 . The suitable coding processing judgment unit  382  judges whether the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the first coding unit  101  or the coding processing of the second coding unit  201 , in other words, which coding processing may be performed, and outputs the judgment result (Step S 382 A). 
     Hereinafter, an example of the operation of the suitable coding processing judgment unit  382  will be described. The suitable coding processing judgment unit  382  performs processing in each step illustrated in  FIG. 7 . In the following example, the coding processing of the first coding unit  101  is coding processing using a spectral envelope based on coefficients which are convertible into linear prediction coefficients, which is illustrated in Non-patent Literature 1, and the coding processing of the second coding unit  201  is coding processing involving variable-length coding which is performed on a differential between the logarithmic value of the average energy of coefficients in each frequency domain obtained by division and the logarithmic value of the average energy of an adjacent frequency domain, which is illustrated in Non-patent Literature 2. 
     In this example, if the ascents and descents of a spectral envelope of an input sound signal are steep or/and the degree of concentration of the spectral envelope is high, the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the first coding unit  101 ; if the ascents and descents of the spectral envelope of the input sound signal are gentle or/and the degree of concentration of the spectral envelope is low, the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the second coding unit  201 ; if the ascents and descents of the spectral envelope of the input sound signal are moderate or/and the degree of concentration of the spectral envelope is medium, the suitable coding processing judgment unit  382  judges that either the coding processing of the first coding unit  101  or the coding processing of the second coding unit  201  may be performed on the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame, that is, the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201 . The suitable coding processing judgment unit  382  then outputs the judgment result. 
     If the ascents and descents of the spectral envelope of the input sound signal are moderate or/and the degree of concentration of the spectral envelope is medium, as will be described later, the switching selection unit  383  selects to code the coefficient sequence in the frequency domain of the present frame by the same coding processing as the coding processing of the preceding frame. That is, in the switching selection unit  383 , the coding processing of the present frame is selected such that switching of the coding processing between the preceding frame and the present frame does not give the listener a strong feeling of artificiality. Thus, a case where the ascents and descents of the spectral envelope of the input sound signal are moderate or/and the degree of concentration of the spectral envelope is medium may include not only a case where either the coding processing of the first coding unit  101  or the coding processing of the second coding unit  201  may be performed on the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame and a case where the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201 , but also a case where it is difficult to say for which of the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201  the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable and a case where there is a possibility that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is not suitable for both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201 . That is, the above-described judgment “being suitable for both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201 ” may translate into a judgment “it is impossible to judge the suitability for the coding processing of the first coding unit  101  and the suitability for the coding processing of the second coding unit  201 ”. 
     As a method of estimating the ascents and descents and the degree of concentration of a spectrum, any method may be adopted, and a configuration in which the depth of the valley of a spectral envelope is estimated will be described. In this configuration, if the valley of a spectral envelope is shallow, it is judged that the ascents and descents of a spectrum are gentle and the degree of concentration of the spectrum is low; if the valley of a spectral envelope is deep, it is judged that the ascents and descents of a spectrum are steep and the degree of concentration of the spectrum is high; if the depth of the valley of a spectral envelope is medium, it is judged that the ascents and descents of a spectrum are moderate and the degree of concentration of the spectrum is medium. 
     The suitable coding processing judgment unit  382  performs Steps S 3821  to S 3825  which are the same as those of the suitable coding processing judgment unit  382  of the first embodiment and Step S 3826 A which is different from the corresponding step of the suitable coding processing judgment unit  382  of the first embodiment. Hereinafter, a difference from the suitable coding processing judgment unit  382  of the first embodiment will be described. 
     After Step S 3825 , the suitable coding processing judgment unit  382  performs the following judgment processing using threshold values TH2_1 and TH2_2 and output of suitability information which will be described later (Step S 3826 A). 
     If a difference between the mean value E of AVE XS (q) of all the partial regions and the mean value E V  of AVE XS (q) of the partial regions of the valley is smaller than a predetermined threshold value TH2_1, since it is estimated that the valley of a spectrum is shallow and the spectrum is a spectrum whose spectral envelope has gentle ascents and descents or whose degree of concentration is low, the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the second coding unit  201 . 
     Moreover, if a difference between the mean value E of AVE XS (q) of all the partial regions and the mean value E V  of AVE XS (q) of the partial regions of the valley is greater than a predetermined threshold value TH2_2 which is a value greater than the threshold value TH2_1, since it is estimated that the valley of a spectrum is deep and the spectrum is a spectrum whose spectral envelope has steep ascents and descents or whose degree of concentration is high, the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the first coding unit  101 . 
     Furthermore, if a difference between the mean value E of AVE XS (q) of all the partial regions and the mean value E V  of AVE XS (q) of the partial regions of the valley is greater than or equal to the threshold value TH2_1 but smaller than or equal to the threshold value TH2_2, since it is estimated that the depth of the valley of a spectrum is medium and the spectrum is a spectrum whose spectral envelope has moderate ascents and descents or whose degree of concentration is medium, the suitable coding processing judgment unit  382  may perform either the coding processing of the first coding unit  101  or the coding processing of the second coding unit  201  on the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame. That is, the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201 . 
     Then, the suitable coding processing judgment unit  382  outputs suitability information which is information on suitable coding processing. The suitability information is the judgment result of the suitable coding processing judgment unit  382  and it can be said that the suitability information is information on which of the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201  is suitable or information as to whether or not both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201  are suitable. 
     Incidentally, the suitable coding processing judgment unit  382  may output information indicating that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the first coding unit  101  or information indicating that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the second coding unit  201  only when it is judged that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for any one of the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201  and perform any one of the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201  on the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame. That is, a configuration may be adopted in which the judgment result is not output if it is judged that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201 . 
     &lt;Switching Selection Unit  383 &gt; 
     Based on the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  and the information, which was obtained by the suitable coding processing judgment unit  382 , on which of the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201  is suitable or the information as to whether or not both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201  are suitable, that is, the information (suitability information) on suitable coding processing, the switching selection unit  383  selects whether to code the coefficient sequence in the frequency domain of the present frame in the first coding unit  101  or in the second coding unit  201  and outputs a switching code that is a code by which the selected coding processing can be identified (Step S 383 A). The output switching code is input to the decoding device  400 . Here, if switching is not permitted, irrespective of the coding processing for which the present frame is suitable, the switching selection unit  383  selects to code the coefficient sequence in the frequency domain of the present frame by the same coding processing as the coding processing of the preceding frame. Moreover, if switching is permitted and the present frame is suitable for both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201 , the switching selection unit  383  selects to code the coefficient sequence in the frequency domain of the present frame by the same coding processing as the coding processing of the preceding frame. Furthermore, if switching is permitted and the present frame is suitable for any one of the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201 , irrespective of the coding processing of the preceding frame, the switching selection unit  383  selects to code the coefficient sequence in the frequency domain of the present frame by the coding processing for which the present frame is suitable. 
     Hereinafter, an example of the operation of the switching selection unit  383  will be described. In the following example, the coding processing of the first coding unit  101  is coding processing using a spectral envelope based on coefficients which are convertible into linear prediction coefficients, which is illustrated in Non-patent Literature 1, and the coding processing of the second coding unit  201  is coding processing involving variable-length coding which is performed on a differential between the logarithmic value of the average energy of coefficients in each frequency domain obtained by division and the logarithmic value of the average energy of an adjacent frequency domain, which is illustrated in Non-patent Literature 2. 
     If the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  indicates that switching is not permitted and/or the information (suitability information) on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382  indicates the same coding processing as the coding processing performed on the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame or indicates that both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201  are suitable, the switching selection unit  383  selects the same coding processing as the coding processing performed on the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame as the coding processing which is performed on the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame. 
     That is, if the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the first coding unit  101  and the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  indicates that switching is not permitted, the switching selection unit  383  selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame also in the first coding unit  101 . Moreover, if the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the first coding unit  101  and the information (suitability information) on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382  indicates the coding processing of the first coding unit  101 , the switching selection unit  383  selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame also in the first coding unit  101 . Furthermore, if the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the first coding unit  101  and the information (suitability information) on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382  indicates that both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201  are suitable, the switching selection unit  383  selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame also in the first coding unit  101 . 
     Moreover, if the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the second coding unit  201  and the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  indicates that switching is not permitted, the switching selection unit  383  selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame also in the second coding unit  201 . Furthermore, if the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the second coding unit  201  and the information (suitability information) on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382  indicates the coding processing of the second coding unit  201 , the switching selection unit  383  selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame also in the second coding unit  201 . In addition, if the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the second coding unit  201  and the information (suitability information) on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382  indicates that both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201  are suitable, the switching selection unit  383  selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame also in the second coding unit  201 . 
     If the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  indicates that switching is permitted and the information (suitability information) on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382  indicates coding processing which is different from the coding processing performed on the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame, the switching selection unit  383  selects the coding processing which is different from the coding processing performed on the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame as the coding processing which is performed on the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame. That is, if the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the first coding unit  101 , the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  indicates that switching is permitted, and the information (suitability information) on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382  indicates the coding processing of the second coding unit  201 , the switching selection unit  383  selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame in the second coding unit  201 . Moreover, if the MDCT coefficient sequence X f-1 (n) (n=1, . . . , N) of the preceding frame was coded in the second coding unit  201 , the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  indicates that switching is permitted, and the information (suitability information) on which coding processing is suitable that was obtained by the suitable coding processing judgment unit  382  indicates the coding processing of the first coding unit  101 , the switching selection unit  383  selects to code the MDCT coefficient sequence X f (n) (n=1, . . . , N) of the present frame in the first coding unit  101 . 
     Incidentally, if the suitable coding processing judgment unit  382  is configured so as not to output the judgment result if it is judged that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201 , the switching selection unit  383  simply has to perform, if the information on suitable coding processing is not input thereto, processing which is performed when the above-described information (suitability information) on which coding processing is suitable indicates that both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201  are suitable. 
     First Modification 
     As a judgment as to whether the input sound signal of the present frame is suitable for the coding processing using a spectral envelope based on coefficients which are convertible into linear prediction coefficients, which is illustrated in Non-patent Literature 1, or the coding processing involving variable-length coding which is performed on a differential between the logarithmic value of the average energy of coefficients in each frequency domain obtained by division and the logarithmic value of the average energy of an adjacent frequency domain, which is illustrated in Non-patent Literature 2, a judgment including not only the magnitude of the ascents and descents of a spectral envelope of the input sound signal and the degree of concentration of the spectral envelope, but also the other information may be made. 
     For example, even when the coefficient sequence in the frequency domain corresponding to the input sound signal of the preceding frame was coded by the first coding unit  101 , the switching permission judgment unit  381  judges that switching is permitted, and the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the second coding unit  201 , if it is judged that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame has to be coded by the coding processing of the first coding unit  101  based on the other information obtained by a means which is not depicted in the coding device  300 , the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame may be coded by the first coding unit  101 . That is, the coding device  300  simply has to be configured so as to make it possible to select to code the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame by the second coding unit  201  if the coefficient sequence in the frequency domain corresponding to the input sound signal of the preceding frame was coded by the first coding unit  101 , the switching permission judgment unit  381  judges that switching is permitted, and the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the second coding unit  201 . 
     Moreover, conversely, even when the coefficient sequence in the frequency domain corresponding to the input sound signal of the preceding frame was coded by the second coding unit  201 , the switching permission judgment unit  381  judges that switching is permitted, and the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the first coding unit  101 , if it is judged that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame has to be coded by the coding processing of the second coding unit  201  based on the other information obtained by a means which is not depicted in the coding device  300 , the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame may be coded by the second coding unit  201 . That is, the coding device  300  simply has to be configured so as to make it possible to select to code the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame by the first coding unit  101  if the coefficient sequence in the frequency domain corresponding to the input sound signal of the preceding frame was coded by the second coding unit  201 , the switching permission judgment unit  381  judges that switching is permitted, and the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the first coding unit  101 . 
     Furthermore, for example, in the coding device  300  of the third embodiment, even when the coefficient sequence in the frequency domain corresponding to the input sound signal of the preceding frame was coded by the first coding unit  101  and the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201 , if the switching permission judgment unit  381  judges that switching is permitted and it is judged that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame has to be coded by the coding processing of the second coding unit  201  based on the other information obtained by a means which is not depicted in the coding device  300 , the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame may be coded by the second coding unit  201 . 
     Moreover, conversely, even when the coefficient sequence in the frequency domain corresponding to the input sound signal of the preceding frame was coded by the second coding unit  201  and the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201 , if the switching permission judgment unit  381  judges that switching is permitted and it is judged that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame has to be coded by the coding processing of the first coding unit  101  based on the other information obtained by a means which is not depicted in the coding device  300 , the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame may be coded by the first coding unit  101 . 
     That is, the coding device  300  of the third embodiment simply has to be configured so as to make it possible to select to code the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame by the same coding processing as the coding processing of the preceding frame if the switching permission judgment unit  381  judges that switching is permitted and the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201 . 
     Second Modification 
     For a selection as to whether to code the coefficient sequence in the frequency domain of the present frame in the first coding unit  101  or in the second coding unit  201 , the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381  may not be used. In this case, the switching permission judgment unit  381  does not have to be provided in the selection unit  380 . 
     In this case, the switching selection unit  383  selects whether to code the coefficient sequence in the frequency domain of the present frame in the first coding unit  101  or in the second coding unit  201  based on the suitability information obtained by the suitable coding processing judgment unit  382  without using the information on whether or not switching is permitted that was obtained by the switching permission judgment unit  381 , and outputs a switching code that is a code by which the selected coding processing can be identified. 
     For example, it is necessary simply to code the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame by the first coding unit  101  if the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the first coding unit  101  and code the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame by the second coding unit  201  if the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the second coding unit  201 . 
     Also in this case, as is the case with the first modification, a judgment including the other information may be made. For example, even when the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the first coding unit  101 , if it is judged that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame has to be coded by the coding processing of the second coding unit  201  based on the other information obtained by a means which is not depicted in the coding device  300 , the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame may be coded by the second coding unit  201 . 
     Conversely, even when the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the second coding unit  201 , if it is judged that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame has to be coded by the coding processing of the first coding unit  101  based on the other information obtained by a means which is not depicted in the coding device  300 , the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame may be coded by the first coding unit  101 . 
     That is, a configuration simply has to be a configuration that makes it possible to select to code the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame by the coding processing of the first coding unit  101  if the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the first coding unit  101 . Moreover, a configuration simply has to be a configuration that makes it possible to select to code the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame by the coding processing of the second coding unit  201  if the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the second coding unit  201 . 
     Furthermore, for example, in the coding device  300  of the third embodiment, of cases where the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201 , in a case where it is judged that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame has to be coded by the coding processing of the first coding unit  101  based on the other information obtained by a means which is not depicted in the coding device  300 , the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame may be coded by the first coding unit  101 . 
     Moreover, of cases where the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201 , in a case where it is judged that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame has to be coded by the coding processing of the second coding unit  201  based on the other information obtained by a means which is not depicted in the coding device  300 , the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame may be coded by the second coding unit  201 . 
     That is, the coding device  300  of the third embodiment simply has to be configured so as to make it possible to select to code the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame by the same coding processing as the coding processing of the preceding frame if the suitable coding processing judgment unit  382  judges that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for both the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201 . 
     Incidentally, in a comparison between the calculated value and the threshold value in the above-described embodiments, settings simply have to be made such that, if the calculated value is the same value as the threshold value, the calculated value is classified into any one of two cases adjacent to each other with the threshold value placed therebetween as a boundary. That is, a condition that a value is greater than or equal to a certain threshold value may translate into a condition that a value is greater than that threshold value and a condition that a value is smaller than that threshold value may translate into a condition that a value is smaller than or equal to that threshold value. Moreover, a condition that a value is greater than a certain threshold value may translate into a condition that a value is greater than or equal to that threshold value and a condition that a value is smaller than or equal to that threshold value may translate into a condition that a value is smaller than that threshold value. 
     For example, in the first embodiment, the selection unit  380  may make it possible to select coding processing which is different from the coding processing of the preceding frame as the coding processing of the present frame if at least one of the magnitude of the energy of high frequency components of the input sound signal of the preceding frame and the magnitude of the energy of high frequency components of the input sound signal of the present frame is smaller than or equal to a predetermined threshold value (Step S 380 ). 
     Moreover, in the second embodiment, the switching permission judgment unit  381  may judge that switching is permitted, that is, the switching permission judgment unit  381  makes it possible to code the coefficient sequence in the frequency domain of the present frame by coding processing which is different from the coding processing by which the coefficient sequence in the frequency domain of the preceding frame was coded if at least one of the magnitude of the energy of high frequency components of the input sound signal of the preceding frame and the magnitude of the energy of high frequency components of the input sound signal of the present frame is smaller than or equal to a predetermined threshold value and output the judgment result. 
     Furthermore, in the first embodiment, if a difference between the mean value E of AVE XS (q) of all the partial regions and the mean value E V  of AVE XS (q) of the partial regions of the valley is smaller than the predetermined threshold value TH2, since it is estimated that the valley of a spectrum is shallow and the spectrum is a spectrum whose spectral envelope has gentle ascents and descents or whose degree of concentration is low, the suitable coding processing judgment unit  382  may judge that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the second coding unit  201 . Conversely, if a difference between the mean value E of AVE XS (q) of all the partial regions and the mean value E V  of AVE XS (q) of the partial regions of the valley is greater than or equal to the threshold value TH2, since it is estimated that the valley of a spectrum is deep and the spectrum is a spectrum whose spectral envelope has steep ascents and descents or whose degree of concentration is high, the suitable coding processing judgment unit  382  may judge that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the first coding unit  101 . 
     Moreover, in the third embodiment, if a difference between the mean value E of AVE XS (q) of all the partial regions and the mean value E V  of AVE XS (q) of the partial regions of the valley is greater than or equal to the predetermined threshold value TH2_2 which is a value greater than the threshold value TH2_1, since it is estimated that the valley of a spectrum is deep and the spectrum is a spectrum whose spectral envelope has steep ascents and descents or whose degree of concentration is high, the suitable coding processing judgment unit  382  may judge that the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame is suitable for the coding processing of the first coding unit  101 . In this case, if a difference between the mean value E of AVE XS (q) of all the partial regions and the mean value E V  of AVE XS (q) of the partial regions of the valley is greater than or equal to the threshold value TH2_1 but smaller than the threshold value TH2_2, since it is estimated that the depth of the valley of a spectrum is medium and the spectrum is a spectrum whose spectral envelope has moderate ascents and descents or whose degree of concentration is medium, the suitable coding processing judgment unit  382  may perform any one of the coding processing of the first coding unit  101  and the coding processing of the second coding unit  201  on the coefficient sequence in the frequency domain corresponding to the input sound signal of the present frame. 
     The processing described in the coding device and the coding method may be performed, in addition to being performed in chronological order in the order mentioned in the description, in parallel or individually depending on the processing power of a device that performs the processing or when needed. 
     Moreover, when the steps in the coding method are implemented by a computer, the processing details of the functions supposed to be provided in the coding method are described by a program. As a result of this program being executed by the computer, the steps are implemented on the computer. 
     The program describing the processing details can be recorded on a computer-readable recording medium. As the computer-readable recording medium, any computer-readable recording medium, such as a magnetic recording device, an optical disk, a magneto-optical recording medium, semiconductor memory, or the like, may be used. 
     Furthermore, each processing means may be configured as a result of a predetermined program being executed on the computer, and at least part of the processing details thereof may be implemented on the hardware. 
     It goes without saying that changes may be made as appropriate without departing from the spirit of this invention.