Abstract:
A speech and musical signal codec employing a band splitting technique encodes sound source signals of each of a plurality of bands using a small number of bits. The codec includes a second pulse position generating circuit, to which an index output by a minimizing circuit and a first pulse position vector P{overscore ( )}=(P 1 , P 2 , . . . , P M ) are input, for revising the first pulse position vector using a pulse position revision quantity d{overscore ( )} i =(d i1 , d i2 , . . . , d iM ) specified by the index and outputting the revised vector to a second sound source generating circuit as a second pulse position vector P{overscore ( )} t =(P 1 +d i1 , P 2 +d i2 , . . . , P M +d iM ).

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is a continuation of application Ser. No. 09/258,900, filed Mar. 1, 1999, U.S. Pat. No. 6,401,062, and based on Japanese Patent Application No. 10-64721, filed Feb. 27, 1998, by Atsushi MURASHIMA. This application claims only subject matter disclosed in the parent application and therefore presents no new matter. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to an apparatus for encoding and an apparatus for decoding speech and musical signals. More particularly, the invention relates to a coding apparatus and a decoding apparatus for transmitting speech and musical signals at a low bit rate. 
     SUMMARY OF THE INVENTION 
     A method of encoding a speech signal by separating the speech signal into a linear prediction filter and its driving sound source signal is used widely as a method of encoding a speech signal efficiently at medium to low bit rates. 
     One such method that is typical is CELP (Code-Excited Linear Prediction). With CELP, a linear prediction filter for which linear prediction coefficients obtained by subjecting input speech to linear prediction analysis have been decided is driven by a sound source signal represented by the sum of a signal that represents the speech pitch period and a noise signal, whereby there is obtained a synthesized speech signal (i.e., a reconstructed signal). For a discussion of CELP, see the paper (referred to as “Reference 1”) “Code excited linear prediction: High quality speech at very low bit rates” by M. Schroeder et. al (Proc. ICASSP, pp. 937-940, 1985). 
     A method using a higher-order linear prediction filter representing the complicated spectrum of music is known as a method of improving music encoding performance by CELP. According to this method, the coefficients of a higher-order linear prediction filter are found by applying linear prediction analysis at a high order of from 50 to 100 to a signal obtained by inverse filtering a past reconstructed signal using a linear prediction filter. A signal obtained by inputting a musical signal to the higher-order linear prediction filter is applied to a linear prediction filter to obtain the reconstructed signal. 
     As an example of an apparatus for encoding speech and musical signals using a higher-order prediction linear filter, see the paper (referred to as “Reference 2”) “Improving the Quality of Musical Signals in CELP Coding”, by Sasaki et al. (Acoustical Society of Japan, Spring, 1996 Meeting for Reading Research Papers, Collected Papers, pp. 263-264, 1996) and the paper (referred to as “Reference 3”) “A 16 Kbit/s Wideband CELP Coder with a High-Order Backward Predictor and its Fast Coefficient Calculation” by M Serizawa et al. 
     (IEEE Workshop on Speech Coding for Telecommunications, pp. 107-108, 1997). 
     A known method of encoding a sound source signal in CELP involves expressing a sound source signal efficiently by a multipulse signal comprising a plurality of pulses and defined by the positions of the pulses and pulse amplitudes. 
     For a discussion of encoding of a sound source signal using a multipulse signal, see the paper (referred to as “Reference 4”) “MP-CELP Speech Coding based upon a Multipulse Spectrum Quantized Sound Source and High-Speed Searching” by Ozawa et. al (Collected Papers A of the Society of Electronic Information Communications, pp. 1655-1663, 1996). Further, by adopting a band splitting arrangement using a sound source signal found for each band and a higher-order backward linear prediction filter in an apparatus for encoding speech and musical signals based upon CELP, the ability to encode music is improved. 
     With regard to CELP using band splitting, see the paper (referred to as “Reference 5”) “Multi-band CELP Coding of Speech and Music” by A. Ubale et al. (IEEE Workshop on Speech Coding for Telecommunications, pp.101-102, 1997). 
     FIG. 10 is a block diagram showing an example of the construction of an apparatus for encoding speech and music according to the prior art. For the sake of simplicity, it is assumed here that the number of bands is two. 
     As shown in FIG. 10, an input signal (input vector) enters from an input terminal  10 . The input signal is generated by sampling a speech or musical signal and gathering a plurality of the samples into a single vector as one frame. 
     A first linear prediction coefficient calculation circuit  140  receives the input vector as an input from the input terminal  10 . This circuit subjects the input vector to linear prediction analysis, obtains a linear prediction coefficient and quantizes the coefficient. 
     The first linear prediction coefficient calculation circuit  140  outputs the linear prediction coefficient to a weighting filter  160  and outputs an index, which corresponds to a quantized value of the linear prediction coefficient, to a linear prediction filter  150  and to a code output circuit  690 . 
     A known method of quantizing a linear prediction coefficient involves converting the coefficient to a line spectrum pair (referred to as an “LSP”) to effect quantization. For a discussion of the conversion of a linear prediction coefficient to an LSP, see the paper (referred to as “Reference 6”) “Speech Information Compression by Line Spectrum Pair (LSP) Speech Analysis Synthesis” by Sugamura et al. (Collected Papers A of the Society of Electronic Information Communications, Vol. J64-A, No. 8, pp. 599-606, 1981). In regard to quantization of an LSP, see the paper (referred to as “Reference 7”) “Vector Quantization of LSP Parameter Using Running-Mean Interframe Prediction” by Omuro et al. (Collected Papers A of the Society of Electronic Information Communications, Vol. J77-A, No. 3, pp. 303-312, 1994). 
     A first pulse position generating circuit  610  receives as an input an index that is output by a minimizing circuit  670 , generates a first pulse position vector using the position of each pulse specified by the index and outputs this vector to a first sound source generating circuit  20 . 
     Let M represent the number of pulses and let P 1 , P 2 , . . . , PM represent the positions of the pulses. The vector P, therefore, is written as follows: 
     
       
         =(P{overscore ( )} 1 , P 2 , . . . , P M )  
       
     
     (It should be noted that the bar over P indicates that P is a vector.) 
     A first pulse amplitude generating circuit  120  has a table in which M-dimensional vectors A{overscore ( )}j, j=1, . . . , NA have been stored, where NA represents the size of the table. The index output by the minimizing circuit  670  enters the first pulse amplitude generating circuit  120 , which proceeds to read an M-dimensional vector A{overscore ( )}i corresponding to this index out of the above-mentioned table and outputs this vector to the first sound source generating circuit  20  as a first pulse amplitude vector. 
     Letting A i1 , A i2 , . . . , A iM  represent the amplitude values of the pulses, we have 
     
       
         A{overscore ( )} i =(A i1 , A i2 , . . . , A iM )  
       
     
     A second pulse position generating circuit  611  receives as an input the index that is output by the minimizing circuit  670 , generates a second pulse position vector using the position of each pulse specified by the index and outputs this vector to a second sound source generating circuit  21 . 
     A second pulse amplitude generating circuit  121  has a table in which M-dimensional vectors B{overscore ( )} j , j=1, . . . , N B  have been stored, where N B  represents the size of the table. 
     The index output by the minimizing circuit  670  enters the second pulse amplitude generating circuit  121 , which proceeds to read an M-dimensional vector B{overscore ( )} j  corresponding to this index out of the above-mentioned table and outputs this vector to the second sound source generating circuit  21  as a second pulse amplitude vector. 
     The first pulse position vector P{overscore ( )}=(P 1 , P 2 , . . . , P M ) output by the first pulse position generating circuit  610  and the first pulse amplitude vector A{overscore ( )} i =(A i1 , A i2 , . . . , A iM ) output by the first pulse amplitude generating circuit  120  enter the first sound source generating circuit  20 . The first sound source generating circuit  20  outputs an N-dimensional vector for which the values of the P 1 st, P 2 nd, . . . , P M th elements are A i1 , A i2 , . . . , A iM , respectively, and the values of the other elements are zero to a first gain circuit  30  as a first sound source signal (sound source vector). 
     A second pulse position vector Q{overscore ( )}=(Q 1 , Q 2 , . . . , Q M ) output by the second pulse position generating circuit  611  and a second pulse amplitude vector B{overscore ( )}=(B i1 , B i2 , . . . , B iM ) output by the second pulse amplitude generating circuit  121  enter the second sound source generating circuit  21 . The second sound source generating circuit  21  outputs an N-dimensional vector for which the values of the Q 1 st, Q 2 nd, . . . Q M th elements are B i1 , B i2 , . . . , B iM , respectively, and the values of the other elements are zero to a second gain circuit  31  as a second sound source signal. 
     The first gain circuit  30  has a table in which gain values have been stored. The index output by the minimizing circuit  670  and the first sound source vector output by the first sound source generating circuit  20  enter the first gain circuit  30 , which proceeds to read a first gain corresponding to the index out of the table, multiply the first gain by the first sound source vector to thereby generate a third sound source vector, and output the generated third sound source vector to a first higher-order linear prediction filter  130 . 
     The second gain circuit  31  has a table in which gain values have been stored. The index output by the minimizing circuit  670  and the second sound source vector output by the second sound source generating circuit  21  enter the second gain circuit  31 , which proceeds to read a second gain corresponding to the index out of the table, multiply the second gain by the second sound source vector to thereby generate a fourth sound source vector, and output the generated fourth sound source vector to a second higher-order linear prediction filter  131 . 
     A third higher-order linear prediction coefficient output by a higher-order linear prediction coefficient calculation circuit  180  and a third sound source vector output by the first gain circuit  30  enter the first higher-order linear prediction filter  130 . The filter thus set to the third higher-order linear prediction coefficient is driven by the third sound source vector, whereby a first excitation vector is obtained. The first excitation vector is output to a first band-pass filter  135 . 
     A fourth higher-order linear prediction coefficient output by the higher-order linear prediction coefficient calculation circuit  180  and a fourth sound source vector output by the second gain circuit  31  enter the second higher-order linear prediction filter  131 . The filter thus set to the fourth higher-order linear prediction coefficient is driven by the fourth sound source vector, whereby a second excitation vector is obtained. The second excitation vector is output to a second band-pass filter  136 . 
     The first excitation vector output by the first higher-order linear prediction filter  130  enters the first band-pass filter  135 . The first excitation vector has its band limited by the filter  135 , whereby a third excitation vector is obtained. The first band-pass filter  135  outputs the third excitation vector to an adder  40 . 
     The second excitation vector output by the second higher-order linear prediction filter  131  enters the second band-pass filter  136 . The second excitation vector has its band limited by the filter  136 , whereby a fourth excitation vector is obtained. The fourth excitation vector is output to the adder  40 . 
     The adder  40  adds the inputs applied thereto, namely the third excitation vector output by the first band-pass filter  135  and the fourth excitation vector output by the second band-pass filter  136 , and outputs a fifth excitation vector, which is the sum of the third and fourth excitation vectors, to the linear prediction filter  150 . 
     The linear prediction filter  150  has a table in which quantized values of linear prediction coefficients have been stored. The fifth excitation vector output by the adder  40  and an index corresponding to a quantized value of a linear prediction coefficient output by the first linear prediction coefficient calculation circuit  140  enter the linear prediction filter  150 . The quantized value of the linear prediction coefficient corresponding to this index is read out of this table and the filter thus set to this quantized linear prediction coefficient is driven by the fifth excitation vector, whereby a reconstructed signal (reconstructed vector) is obtained. This vector is output to a subtractor  50  and to the higher-order linear prediction coefficient calculation circuit  180 . 
     The reconstructed vector output by the linear prediction filter  150  enters the higher-order linear prediction coefficient calculation circuit  180 , which proceeds to calculate the third higher-order linear prediction coefficient and the fourth higher-order linear prediction coefficient. The third higher-order linear prediction coefficient is output to the first higher-order linear prediction filter  130 , and the fourth higher-order linear prediction coefficient is output to the second higher-order linear prediction filter  131 . The details of construction of the higher-order linear prediction coefficient calculation circuit  180  will be described later. 
     The input vector enters the subtractor  50  via the input terminal  10 , and the reconstructed vector output by the linear prediction filter  150  also enters the subtractor  50 . The subtractor  50  calculates the difference between these two inputs. The subtractor  50  outputs a difference vector, which is the difference between the input vector and the reconstructed vector, to the weighting filter  160 . 
     The difference vector output by the subtractor  50  and the linear prediction coefficient output by the first linear prediction coefficient calculation circuit  140  enter the weighting filter  160 . The latter uses this linear prediction coefficient to produce a weighting filter corresponding to the characteristic of the human sense of hearing and drives this weighting filter by the difference vector, whereby there is obtained a weighted difference vector. The weighted difference vector is output to the minimizing circuit  670 . For a discussion of a weighting filter, see Reference 1. 
     Weighted difference vectors output by the weighting filter  160  successively enter the minimizing circuit  670 , which proceeds to calculate the norms. 
     Indices corresponding to all values of the elements of the first pulse position vector in the first pulse position generating circuit  610  are output successively from the minimizing circuit  670  to the first pulse position generating circuit  610 . Indices corresponding to all values of the elements of the second pulse position vector in the second pulse position generating circuit  611  are output successively from the minimizing circuit  670  to the second pulse position generating circuit  611 . Indices corresponding to all first pulse amplitude vectors that have been stored in the first pulse amplitude generating circuit  120  are output successively from the minimizing circuit  670  to the first pulse amplitude generating circuit  120 . Indices corresponding to all second pulse amplitude vectors that have been stored in the second pulse amplitude generating circuit  121  are output successively from the minimizing circuit  670  to the second pulse amplitude generating circuit  121 . Indices corresponding to all first gains that have been stored in the first gain circuit  30  are output successively from the minimizing circuit  670  to the first gain circuit  30 . Indices corresponding to all second gains that have been stored in the second gain circuit  31  are output successively from the minimizing circuit  670  to the second gain circuit  31 . Further, the minimizing circuit  670  selects the value of each element in the first pulse position vector, the value of each element in the second pulse position vector, the first pulse amplitude vector, the second pulse amplitude vector and the first gain and second gain that will result in the minimum norm and outputs the indices corresponding to these to the code output circuit  690 . 
     With regard to a method of obtaining the position of each pulse that is an element of a pulse position vector as well as the amplitude value of each pulse that is an element of a pulse amplitude vector, see Reference 4, by way of example. 
     The index corresponding to the quantized value of the linear prediction coefficient output by the first linear prediction coefficient calculation circuit  140  enters the code output circuit  690  and so do the indices corresponding to the value of each element in the first pulse position vector, the value of each element in the second pulse position vector, the first pulse amplitude vector, the second pulse amplitude vector and the first gain and second gain. The code output circuit  690  converts these indices to a bit-sequence code and outputs the code via an output terminal  60 . 
     The higher-order linear prediction coefficient calculation circuit  180  will now be described with reference to FIG.  11 . 
     As shown in FIG. 11, the reconstructed vector output by the linear prediction filter  150  enters a second linear prediction coefficient calculation circuit  910  via an input terminal  900 . The second linear prediction coefficient calculation circuit  910  subjects this reconstructed vector to linear prediction analysis, obtains a linear prediction coefficient and outputs this coefficient to a residual signal calculation circuit  920  as a second linear prediction coefficient. 
     The second linear prediction coefficient output by the second linear prediction coefficient calculation circuit  910  and the reconstructed vector output by the linear prediction filter  150  enter the residual signal calculation circuit  920 , which proceeds to use a filter, in which the second linear prediction coefficient has been set, to subject the reconstructed vector to inverse filtering, whereby a first residual vector is obtained. The first residual vector is output to an FFT (Fast-Fourier Transform) circuit  930 . 
     The FFT circuit  930 , to which the first residual vector output by the residual signal calculation circuit  920  is applied, subjects this vector to a Fourier transform and outputs the Fourier coefficients thus obtained to a band splitting circuit  940 . 
     The band splitting circuit  940 , to which the Fourier coefficients output by the FFT circuit  930  are applied, equally partitions these Fourier coefficients into high- and low-frequency regions, thereby obtaining low-frequency Fourier coefficients and high-frequency Fourier coefficients. The low-frequency coefficients are output to a first downsampling circuit  950  and the high-frequency coefficients are output to a second downsampling circuit  951 . 
     The first downsampling circuit  950  downsamples the low-frequency Fourier coefficients output by the band splitting circuit  940 . Specifically, the first downsampling circuit  950  removes bands corresponding to high frequency in the low-frequency Fourier coefficients and generates first Fourier coefficients the band whereof is half the full band. The first Fourier coefficients are output to a first inverse FFT circuit  960 . 
     The second downsampling circuit  951  downsamples the high-frequency Fourier coefficients output by the band splitting circuit  940 . Specifically, the second downsampling circuit  951  removes bands corresponding to low frequency in the high-frequency Fourier coefficients and loops back the high-frequency coefficients to the low-frequency side, thereby generating second Fourier coefficients the band whereof is half the full band. The second Fourier coefficients are output to a second inverse FFT circuit  961 . 
     The first Fourier coefficients output by the first downsampling circuit  950  enter the first inverse FFT circuit  960 , which proceeds to subject these coefficients to an inverse FFT, thereby obtaining a second residual vector that is output to a first higher-order linear prediction coefficient calculation circuit  970 . 
     The second Fourier coefficients output by the second downsampling circuit  951  enter the second inverse FFT circuit  961 , which proceeds to subject these coefficients to an inverse FFT, thereby obtaining a third residual vector that is output to a second higher-order linear prediction coefficient calculation circuit  971 . 
     The second residual vector output by the first inverse FFT circuit  960  enters the first higher-order linear prediction coefficient calculation circuit  970 , which proceeds to subject the second residual vector to higher-order linear prediction analysis, thereby obtaining the first higher-order linear prediction coefficient. This is output to a first upsampling circuit  980 . 
     The third residual vector output by the second inverse FFT circuit  961  enters the second higher-order linear prediction coefficient calculation circuit  971 , which proceeds to subject the third residual vector to higher-order linear prediction analysis, thereby obtaining the second higher-order linear prediction coefficient. This is output to a second upsampling circuit  981 . 
     The first higher-order linear prediction coefficient output by the first higher-order linear prediction coefficient calculation circuit  970  enters the first upsampling circuit  980 . By inserting zeros in alternation with the first higher-order linear prediction coefficient, the first upsampling circuit  980  obtains an upsampled prediction coefficient. This is output as the third higher-order linear prediction coefficient to the first higher-order linear prediction filter  130  via an output terminal  901 . 
     The second higher-order linear prediction coefficient output by the second higher-order linear prediction coefficient calculation circuit  971  enters the second upsampling circuit  981 . By inserting zeros in alternation with the second higher-order linear prediction coefficient, the second upsampling circuit  981  obtains an upsampled prediction coefficient. This is output as the fourth higher-order linear prediction coefficient to the second higher-order linear prediction filter  131  via an output terminal  902 . 
     FIG. 12 is a block diagram showing an example of the construction of an apparatus for decoding speech and music according to the prior art. Components in FIG. 12 identical with or equivalent to those of FIG. 10 are designated by like reference characters. 
     As shown in FIG. 12, a code in the form of a bit sequence enters from an input terminal  200 . A code input circuit  720  converts the bit-sequence code that has entered from the input terminal  200  to an index. 
     The code input circuit  720  outputs an index corresponding to each element in the first pulse position vector to a first pulse position generating circuit  710 , outputs an index corresponding to each element in the second pulse position vector to a second pulse position generating circuit  711 , outputs an index corresponding to the first pulse amplitude vector to the first pulse amplitude generating circuit  120 , outputs an index corresponding to the second pulse amplitude vector to the second pulse amplitude generating circuit  121 , outputs an index corresponding to the first gain to the first gain circuit  30 , outputs an index corresponding to the second gain to the second gain circuit  31 , and outputs an index corresponding to the quantized value of a linear prediction coefficient to the linear prediction filter  150 . 
     The index output by the code input circuit  720  enters the first pulse position generating circuit  710 , which proceeds to generate the first pulse position vector using the position of each pulse specified by the index and output the vector to the first sound source generating circuit  20 . 
     The first pulse amplitude generating circuit  120  has a table in which M-dimensional vectors A{overscore ( )} j , j=1, . . . , N A  have been stored. The index output by the code input circuit  720  enters the first pulse amplitude generating circuit  120 , which proceeds to read an M-dimensional vector A{overscore ( )} i  corresponding to this index out of the above-mentioned table and to output this vector to the first sound source generating circuit  20  as a first pulse amplitude vector. 
     The index output by the code input circuit  720  enters the second pulse position generating circuit  711 , which proceeds to generate the second pulse position vector using the position of each pulse specified by the index and output the vector to the second sound source generating circuit  21 . 
     The second pulse amplitude generating circuit  121  has a table in which M-dimensional vectors B{overscore ( )} j , j=1, . . . , N B  have been stored. The index output by the code input circuit  720  enters the second pulse amplitude generating circuit  121 , which proceeds to read an M-dimensional vector B{overscore ( )} j  corresponding to this index out of the above-mentioned table and to output this vector to the second sound source generating circuit  21  as a second pulse amplitude vector. 
     The first pulse position vector P{overscore ( )}=(P{overscore ( )} 1 , P 2 , . . . , P M ) output by the first pulse position generating circuit  710  and the first pulse amplitude vector A{overscore ( )} i =(A i1 , A i2 , . . . , A iM ) output by the first pulse amplitude generating circuit  120  enter the first sound source generating circuit  20 . The first sound source generating circuit  20  outputs an N-dimensional vector for which the values of the P 1 st, P 2 nd, . . . , P  M th elements are A i1 , A i2 , . . . , A iM , respectively, and the values of the other elements are zero to the first gain circuit  30  as a first sound source signal vector. 
     The second pulse position vector Q{overscore ( )}=(Q 1 , Q 2 , . . . , Q M ) output by the second pulse position generating circuit  711  and the second pulse amplitude vector B{overscore ( )} j =(B i1 , B i2 , . . . , B iM ) output by the second pulse amplitude generating circuit  121  enter the second sound source generating circuit  21 . The second sound source generating circuit  21  outputs an N-dimensional vector for which the values of the Q 1 st, Q 2 nd, . . . , Q M th elements are B i1 , B i2 , . . . , B iM , respectively, and the values of the other elements are zero to the second gain circuit  31  as a second sound source signal. 
     The first gain circuit  30  has a table in which gain values have been stored. The index output by the code input circuit  720  and the first sound source vector output by the first sound source generating circuit  20  enter the first gain circuit  30 , which proceeds to read a first gain corresponding to the index out of the table, multiply the first gain by the first sound source vector to thereby generate a third sound source vector and output the generated third sound source vector to the first higher-order linear prediction filter  130 . 
     The first gain circuit  31  has a table in which gain values have been stored. The index output by the code input circuit  720  and the second sound source vector output by the second sound source generating circuit  21  enter the second gain circuit  31 , which proceeds to read a second gain corresponding to the index out of the table, multiply the second gain by the second sound source vector to thereby generate a fourth sound source vector and output the generated fourth sound source vector to a second higher-order linear prediction filter  131 . 
     The third higher-order linear prediction coefficient output by the higher-order linear prediction coefficient calculation circuit  180  and the third sound source vector output by the first gain circuit  30  enter the first higher-order linear prediction filter  130 . The filter thus set to the third higher-order linear prediction coefficient is driven by the third sound source vector, whereby a first excitation vector is obtained. The first excitation vector is output to the first band-pass filter  135 . 
     The fourth higher-order linear prediction coefficient output by the higher-order linear prediction coefficient calculation circuit  180  and the fourth sound source vector output by the second gain circuit  31  enter the second higher-order linear prediction filter  131 . The filter thus set to the fourth higher-order linear prediction coefficient is driven by the fourth sound source vector, whereby a second excitation vector is obtained. The second excitation vector is output to the second band-pass filter  136 . 
     The first excitation vector output by the first higher-order linear prediction filter  130  enters the first band-pass filter  135 . The first excitation vector has its band limited by the filter  135 , whereby a third excitation vector is obtained. The first band-pass filter  135  outputs the third excitation vector to the adder  40 . 
     The second excitation vector output by the second higher-order linear prediction filter  131  enters the second band-pass filter  136 . The second excitation vector has its band limited by the filter  136 , whereby a fourth excitation vector is obtained. The fourth excitation vector is output to the adder  40 . 
     The adder  40  adds the inputs applied thereto, namely the third excitation vector output by the first band-pass filter  135  and the fourth excitation vector output by the second band-pass filter  136 , and outputs a fifth excitation vector, which is the sum of the third and fourth excitation vectors, to the linear prediction filter  150 . 
     The linear prediction filter  150  has a table in which quantized values of linear prediction coefficients have been stored. The fifth excitation vector output by the adder  40  and an index corresponding to a quantized value of a linear prediction coefficient output by the code input circuit  720  enter the linear prediction filter  150 . The latter reads the quantized value of the linear prediction coefficient corresponding to this index out of the table and drives the filter thus set to this quantized linear prediction coefficient by the fifth excitation vector, whereby a reconstructed vector is obtained. 
     The reconstructed vector obtained is output to an output terminal  201  and to the higher-order linear prediction coefficient calculation circuit  180 . 
     The reconstructed vector output by the linear prediction filter  150  enters the higher-order linear prediction coefficient calculation circuit  180 , which proceeds to calculate the third higher-order linear prediction coefficient and the fourth higher-order linear prediction coefficient. The third higher-order linear prediction is output to the first higher-order linear prediction filter  130 , and the fourth higher-order linear prediction coefficient is output to the second higher-order linear prediction filter  131 . 
     The reconstructed vector calculated by the linear prediction filter  150  is output via the output terminal  201 . 
     SUMMARY OF THE DISCLOSURE 
     In the course of investigations toward the present invention, the following problem has been encountered. Namely, a problem with the conventional apparatus for encoding and decoding speech and musical signals by the above-described band splitting technique is that a large number of bits is required to encode the sound source signals. 
     The reason for this is that the sound source signals are encoded independently in each band without taking into consideration the correlation between bands of the input signals. 
     Accordingly, an object of the present invention is to provide an apparatus for encoding and decoding speech and musical signals, wherein the sound source signal of each band can be encoded using a small number of bits. 
     Another object of the present invention is to provide an apparatus for encoding or decoding speech and musical (i.e., sound) signals with simplified structure and/or high efficiency. Further objects of the present invention will become apparent in the entire disclosure. Generally, the present invention contemplates to utilize the correlation between bands of the input signals upon encoding/decoding in such a fashion to reduce the entire bit number. 
     According to a first aspect of the present invention, the foregoing object is attained by providing a speech and musical signal encoding apparatus which, when encoding an input signal upon splitting the input signal into a plurality of bands, generates a reconstructed signal using a multipulse sound source signal that corresponds to each band, wherein a position obtained by shifting the position of each pulse which defines the multipulse signal in the band(s) is used when defining a multipulse signal in the other band(s). 
     According to a second aspect of the present invention, the foregoing object is attained by providing a speech and musical signal decoding apparatus for generating a reconstructed signal using a multipulse sound source signal corresponding to each of a plurality of bands, wherein a position obtained by shifting the position of each pulse which defines the multipulse signal in the band(s) is used when defining a multipulse signal in the other band(s). 
     According to a third aspect of the present invention, the foregoing object is attained by providing a speech and musical signal encoding apparatus which, when encoding an input signal upon splitting the input signal into a plurality of bands, generates a reconstructed signal by exciting a synthesis filter by a full-band sound source signal, which is obtained by summing, over all bands, multipulse sound source signals corresponding to respective ones of the plurality of bands, wherein a position obtained by shifting the position of each pulse which defines the multipulse signal in the band(s) is used when defining a multipulse signal in the other band(s). 
     According to a fourth aspect of the present invention, the foregoing object is attained by providing a speech and musical signal decoding apparatus for generating a reconstructed signal by exciting a synthesis filter by a full-band sound source signal, which is obtained by summing, over all bands, multipulse sound source signals corresponding to respective ones of a plurality of bands, wherein a position obtained by shifting the position of each pulse which defines the multipulse signal in the band(s) is used when defining a multipulse signal in the other band(s). 
     According to a fifth aspect of the present invention, the foregoing object is attained by providing a speech and musical signal encoding apparatus which, when encoding an input signal upon splitting the input signal into a plurality of bands, generates a reconstructed signal by exciting a synthesis filter by a full-band sound source signal, which is obtained by summing, over all bands, signals obtained by exciting a higher-order linear prediction filter, which represents a microspectrum relating to the input signal of each band, by a multipulse sound source signal corresponding to each band, wherein a position obtained by shifting the position of each pulse which defines the multipulse signal in the band(s) is used when defining a multipulse signal in the other band(s). 
     According to a sixth aspect of the present invention, the foregoing object is attained by providing a speech and musical signal decoding apparatus for generating a reconstructed signal by exciting a synthesis filter by a full-band sound source signal, which is obtained by summing, over all bands, signals obtained by exciting a higher-order linear prediction filter, which represents a microspectrum relating to an input signal of each of a plurality of bands, by a multipulse sound source signal corresponding to each band, wherein a position obtained by shifting the position of each pulse which defines the multipulse signal in the band(s) is used when defining a multipulse signal in the other band(s). 
     According to a seventh aspect of the present invention, the foregoing object is attained by providing a speech and musical signal encoding apparatus which, when encoding an input signal upon splitting the input signal into a plurality of bands, generates a reconstructed signal by exciting a synthesis filter by a full-band sound source signal, which is obtained by summing, over all bands, signals obtained by exciting a higher-order linear prediction filter, which represents a microspectrum relating to the input signal of each band, by a multipulse sound source signal corresponding to each band, wherein a residual signal is found by inverse filtering of the reconstructed signal using a linear prediction filter for which linear prediction coefficients obtained from the reconstructed signal have been decided, conversion coefficients obtained by converting the residual signal are split into bands, and the higher-order linear prediction filter uses coefficients obtained from a residual signal of each band generated in each band by back-converting the conversion coefficients that have been split into the bands. 
     According to an eighth aspect of the present invention, the foregoing object is attained by providing a speech and musical signal decoding apparatus for generating a reconstructed signal by exciting a synthesis filter by a full-band sound source signal, which is obtained by summing, over all bands, signals obtained by exciting a higher-order linear prediction filter, which represents a microspectrum relating to an input signal of each of a plurality of bands, by a multipulse sound source signal corresponding to each band, wherein a residual signal is found by inverse filtering of the reconstructed signal using a linear prediction filter for which linear prediction coefficients obtained from the reconstructed signal have been decided, conversion coefficients obtained by converting the residual signal are split into bands, and the higher-order linear prediction filter uses coefficients obtained from a residual signal of each band generated in each band by back-converting the conversion coefficients that have been split into the bands. 
     According to a ninth aspect of the present invention, in the fifth aspect of the invention a residual signal is found by inverse filtering of the reconstructed signal using a linear prediction filter for which linear prediction coefficients obtained from the reconstructed signal have been decided, conversion coefficients obtained by converting the residual signal are split into bands, and the higher-order linear prediction filter uses coefficients obtained from a residual signal of each band generated in each band by back-converting the conversion coefficients that have been split into the bands. 
     According to a tenth aspect of the present invention, in the sixth aspect of the invention a residual signal is found by inverse filtering of the reconstructed signal using a linear prediction filter for which linear prediction coefficients obtained from the reconstructed signal have been decided, conversion coefficients obtained by converting the residual signal are split into bands, and the higher-order linear prediction filter uses coefficients obtained from a residual signal of each band generated in each band by back-converting the conversion coefficients that have been split into the bands. 
    
    
     Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating the construction of a first embodiment of an apparatus for encoding speech and musical signals according to the present invention; 
     FIG. 2 is a block diagram illustrating the construction of a first embodiment of an apparatus for decoding speech and musical signals according to the present invention; 
     FIG. 3 is a block diagram illustrating the construction of a second embodiment of an apparatus for encoding speech and musical signals according to the present invention; 
     FIG. 4 is a block diagram illustrating the construction of a second embodiment of an apparatus for decoding speech and musical signals according to the present invention; 
     FIG. 5 is a block diagram illustrating the construction of a third embodiment of an apparatus for encoding speech and musical signals according to the present invention; 
     FIG. 6 is a block diagram illustrating the construction of a higher-order linear prediction coefficient calculation circuit according to the third embodiment; 
     FIG. 7 is a block diagram illustrating the construction of a third embodiment of an apparatus for decoding speech and musical signals according to the present invention; 
     FIG. 8 is a block diagram illustrating the construction of a fourth embodiment of an apparatus for encoding speech and musical signals according to the present invention; 
     FIG. 9 is a block diagram illustrating the construction of a fourth embodiment of an apparatus for decoding speech and musical signals according to the present invention; 
     FIG. 10 is a block diagram illustrating the construction of an apparatus for encoding speech and musical signals according to the prior art prior art; FIG. 11 is a block diagram illustrating the construction of a higher-order linear prediction coefficient calculation circuit according to the prior art; and 
     FIG. 12 is a block diagram illustrating the construction of a fourth embodiment of an apparatus for decoding speech and musical signals according to the prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred modes of practicing the present invention will now be described. An apparatus for encoding speech and musical signals according to the present invention in a first preferred mode thereof generates a reconstructed signal using a multipulse sound source signal that corresponds to each of a plurality of bands when a speech input signal is encoded upon being split into a plurality of bands, wherein some of the information possessed by a sound source signal encoded in a certain band is used to encode a sound source signal in another band. More specifically, the encoding apparatus has means (a first pulse position generating circuit  110 , a second pulse position generating circuit  111  and a minimizing circuit  170  shown in FIG. 1) for using a position obtained by shifting the position of each pulse, which defines the multipulse signal in the band or bands, when a multipulse signal in the other band(s) is defined. 
     More specifically, in regard to a case where the number of bands is two, for example, an index output by the minimizing circuit  170  in FIG. 1 and a first pulse position vector P{overscore ( )}=(P 1 , P 2 , . . . , P M ) output by the minimizing circuit  170  enter the second pulse position generating circuit  111 . The latter revises the first pulse position vector using a pulse position revision quantity d{overscore ( )} i =(d i1 , d i2 , . . . , d iM ) specified by the index and outputs the revised vector to the second sound source generating circuit  21  in FIG. 1 as a second pulse position vector P{overscore ( )} t =(P 1 +d i1 , P 2 +d i2 , . . . P M +d iM ). 
     An apparatus for decoding speech and musical signals according to the present invention in the first preferred mode thereof uses some of the information possessed by a sound source signal decoded in certain band or bands to decode a sound source signal in another band or the other bands. 
     More specifically, the decoding apparatus has means (a first pulse position generating circuit  210 , a second pulse position generating circuit  211  and a code input circuit  220  shown in FIG. 2) for using a position obtained by shifting the position of each pulse, which defines the multipulse signal in the band, when a multipulse signal in another band is defined. 
     An apparatus for encoding speech and musical signals according to the present invention in a second preferred mode thereof generates a reconstructed signal by exciting a synthesis filter by a full-band sound source signal, which is obtained by summing, over all bands, multipulse sound source signals corresponding to respective ones of the plurality of bands. More specifically, the encoding apparatus has means ( 110 ,  111 ,  170  in FIG. 1) for using a position obtained by shifting the position of each pulse, which defines the multipulse signal in the band(s), when a multipulse signal in the other band(s) is defined, means (adder  40  in FIG. 1) for obtaining the full-band sound source signal by summing, over all bands, multipulse sound source signals corresponding to respective ones of the bands, and means (linear prediction filter  150  in FIG. 1) for generating the reconstructed signal by exciting the synthesis filter by the full-band sound source signal. 
     An apparatus for decoding speech and musical signals according to the present invention in the second preferred mode thereof generates a reconstructed signal by exciting a synthesis filter by a full-band sound source signal, which is obtained by summing, over all bands, multipulse sound source signals corresponding to respective ones of the plurality of bands. More specifically, the decoding apparatus has means ( 210 ,  211  and  220  in FIG. 2) for using a position obtained by shifting the position of each pulse, which defines the multipulse signal in the band(s), when a multipulse signal in the other band(s) is defined; means (adder  40  in FIG. 2) for obtaining the full-band sound source signal by summing, over all bands, multipulse sound source signals corresponding to respective ones of the bands; and means (linear prediction filter  150  in FIG. 1) for generating the reconstructed signal by exciting the synthesis filter by the full-band sound source signal. 
     An apparatus for encoding speech and musical signals according to the present invention in a third preferred mode thereof generates a reconstructed signal by exciting a synthesis filter by a full-band sound source signal, which is obtained by summing, over all bands, signals obtained by exciting a higher-order linear prediction filter, which represents a microspectrum relating to the input signal of each band, by a multipulse sound source signal corresponding to each band. More specifically, the encoding apparatus has means (the first pulse position generating circuit  110 , second pulse position generating circuit  111  and minimizing circuit  170  shown in FIG. 1) for using a position obtained by shifting the position of each pulse, which defines the multipulse signal in the band(s), when a multipulse signal in the other band(s) is defined; means (first and second higher-order linear prediction filters  130 ,  131  in FIG. 3) for exciting the higher-order linear prediction filter by the multipulse sound source signal corresponding to each band; means (adder  40  in FIG. 3) for obtaining the full-band sound source signal by summing, over all bands, signals obtained by exciting the higher-order linear prediction filter; and means (linear prediction filter  150  in FIG. 3) for generating the reconstructed signal by exciting the synthesis filter by the full-band sound source signal. 
     An apparatus for decoding speech and musical signals according to the present invention in the third preferred mode thereof generates a reconstructed signal by exciting a synthesis filter by a full-band sound source signal, which is obtained by summing, over all bands, signals obtained by exciting a higher-order linear prediction filter, which represents a microspectrum relating to the input signal of each band, by a multipulse sound source signal corresponding to each band. More specifically, the decoding apparatus has means (first pulse position generating circuit  210 , second pulse position generating circuit  211  and code input circuit  220  shown in FIG. 4) for using a position obtained by shifting the position of each pulse, which defines the multipulse signal in the band(s), when a multipulse signal in the other band(s) is defined; means (first and second higher-order linear prediction filters  130 ,  131  in FIG. 4) for exciting the higher-order linear prediction filter by the multipulse sound source signal corresponding to each band; means (adder  40  in FIG. 4) for obtaining the full-band sound source signal by summing, over all bands, signals obtained by exciting the higher-order linear prediction filter; and means (linear prediction filter  150  in FIG. 4) for generating the reconstructed signal by exciting the synthesis filter by the full-band sound source signal. 
     In a fourth preferred mode of the present invention, the apparatus for encoding speech and musical signals of the third mode is characterized in that a higher-order linear prediction calculation circuit is implemented by a simple arrangement. More specifically, the encoding apparatus has means (second linear prediction coefficient calculation circuit  910  and residual signal calculation circuit  920  in FIG. 6) for obtaining a residual signal by inverse filtering of the reconstructed signal using a linear prediction filter for which linear prediction coefficients obtained from the reconstructed signal have been decided and set; means (FFT circuit  930  and band splitting circuit  540  in FIG. 6) for splitting, into bands, conversion coefficients obtained by converting the residual signal; and means (first zerofill circuit  550 , second zerofill circuit  551 , first inverse FFT circuit  560 , second inverse FFT circuit  561 , first higher-order linear prediction coefficient calculation circuit  570  and second higher-order linear prediction coefficient calculation circuit  571  in FIG. 6) for outputting, to the higher-order linear prediction filter, coefficients obtained from a residual signal of each band generated in each band by back-converting the conversion coefficients that have been split into the bands. 
     In a fourth preferred mode of the present invention, the apparatus for decoding speech and musical signals of the third mode is characterized in that a higher-order linear prediction calculation circuit is implemented by a simple arrangement. More specifically, the encoding apparatus has means ( 910 ,  920  in FIG. 6) for obtaining a residual signal by inverse filtering of the reconstructed signal using a linear prediction filter for which linear prediction coefficients obtained from the reconstructed signal have been decided; means ( 930 ,  540  in FIG. 6) for splitting, into bands, conversion coefficients obtained by converting the residual signal; and means ( 550 ,  551 ,  560 ,  561 ,  570 ,  571  in FIG. 6) for outputting, to the higher-order linear prediction filter, coefficients obtained from a residual signal of each band generated in each band by back-converting the conversion coefficients that have been split into the bands. 
     In a fifth preferred mode of the present invention, the apparatus for encoding speech and musical signals of the fourth mode is further characterized in that the sound source signal of each band is encoded independently. More specifically, the encoding apparatus has means (first pulse position generating circuit  510 , second pulse position generating circuit  511  and minimizing circuit  670  in FIG. 8) for separately obtaining, in each band, the position of each pulse defining the multipulse signal. 
     In the fifth preferred mode of the present invention, the apparatus for decoding speech and musical signals of the fourth mode is further characterized in that the sound source signal of each band is decoded independently. More specifically, the decoding apparatus has means (first pulse position generating circuit  710 , second pulse position generating circuit  711  and code input circuit  720  in FIG. 9) for separately (individually) obtaining, in each band, the position of each pulse defining the multipulse signal. 
     In the modes of the present invention described above, some of the information possessed by a sound source signal that has been encoded in a certain band or bands is used to encode a sound source signal in the other band or bands. That is, encoding is performed taking into account the correlation between bands possessed by the input signal. More specifically, the position of each pulse obtained by uniformly shifting the positions of the pulses obtained when a multipulse sound source signal is encoded in a first band is used when encoding a sound source signal in a second band. 
     As a consequence, in relation to the sound source signal in the second band, the number of bits necessary in the conventional method to separately represent the position of each pulse is reduced to a number of bits necessary solely for representing the amount of shift. 
     As a result, it is possible to reduce the number of bits needed to encode the sound source signal in the second band. 
     Embodiments of the present invention will now be described with reference to the drawings in order to explain further the modes of the invention set forth above. 
     [First Embodiment] 
     FIG. 1 is a block diagram illustrating the construction of a first embodiment of an apparatus for encoding speech and musical signals according to the present invention. Here it is assumed for the sake of simplicity that the number of bands is two. 
     As shown in FIG. 1, an input vector enters from the input terminal  10 . The first linear prediction coefficient calculation circuit  140  receives the input vector as an input from the input terminal  10  and this circuit subjects the input vector to linear prediction analysis, obtains a linear prediction coefficient and quantizes the coefficient. The first linear prediction coefficient calculation circuit  140  outputs the linear prediction coefficient to the weighting filter  160  and outputs an index, which corresponds to a quantized value of the linear prediction coefficient, to the linear prediction filter  150  and to a code output circuit  190 . 
     The first pulse position generating circuit  110  receives as an input an index that is output by the minimizing circuit  170 , generates a first pulse position vector P{overscore ( )} using the position of each pulse specified by the index and outputs this vector to the first sound source generating circuit  20  and to the second pulse position generating circuit  111 . 
     Let M represent the number of pulses and let P 1 , P 2 , . . . , P M  represent the positions of the pulses. The vector P{overscore ( )}, therefore, is written as follows: 
      P{overscore ( )}=(P 1 , P 2 , . . . , P M ) 
     The first pulse amplitude generating circuit  120  has a table in which M-dimensional vectors A{overscore ( )} j , j=1, . . . , N A  have been stored, where N A  represents the size of the table. The index output by the minimizing circuit  170  enters the first pulse amplitude generating circuit  120 , which proceeds to read an M-dimensional vector A{overscore ( )} i  corresponding to this index out of the above-mentioned table and to output this vector to the first sound source generating circuit  20  as a first pulse amplitude vector. 
     Letting A i1 , A i2 , . . . , A iM  represent the amplitude values of the pulses, we have A{overscore ( )} i =(A i1 , A i2 , . . . , A iM ). 
     The second pulse position generating circuit  111  receives as inputs the index that is output by the minimizing circuit  170  and the first pulse position vector P{overscore ( )}=(P 1 , P 2 , . . . , P M ) output by the first pulse position generating circuit  110 , revises the first pulse position vector using the pulse position revision quantity d{overscore ( )} i =(d i1 , d i2 , . . . , d iM ) specified by the index and outputs the revised vector to the second sound source generating circuit  21  as a second pulse position vector Q{overscore ( )} t =(P 1 +d i1 , P 2 +d i2 , . . . , P M +d iM ). 
     The second pulse amplitude generating circuit  121  has a table in which M-dimensional vectors B{overscore ( )} j , j=1, . . . , N B  have been stored, where N B  represents the size of the table. 
     The index output by the minimizing circuit  170  enters the second pulse amplitude generating circuit  121 , which proceeds to read an M-dimensional vector B{overscore ( )} i  corresponding to this index out of the above-mentioned table and to output this vector to the second sound source generating circuit  21  as a second pulse amplitude vector. 
     The first pulse position vector P{overscore ( )}=(P 1 , P 2 , . . . , P M ) output by the first pulse position generating circuit  110  and the first pulse amplitude vector A{overscore ( )} i =(A i1 , A i2 , . . . , A iM ) output by the first pulse amplitude generating circuit  120  enter the first sound source generating circuit  20 . The first sound source generating circuit  20  outputs an N-dimensional vector for which the values of the P 1 st, P 2 nd, . . . , P M th elements are A i1 , A i2 , . . . , A iM , respectively, and the values of the other elements are zero to the first gain circuit  30  as a first sound source vector. 
     A second pulse position vector Q{overscore ( )} t =(Q t   1 , Q t   2 , . . . , Q t   M ) output by the second pulse position generating circuit  111  and a second pulse amplitude vector B{overscore ( )} i =(B i1 , B i2 , . . . , B iM ) output by the second pulse amplitude generating circuit  121  enter the second sound source generating circuit  21 . The second sound source generating circuit  21  outputs an N-dimensional vector for which the values of the Q t   1 st, Q t   2 nd, . . . , Q t   M th elements are B i1 , B i2 , . . . , B iM , respectively, and the values of the other elements are zero to a second gain circuit  31  as a second sound source vector. 
     The first gain circuit  30  has a table in which gain values have been stored. The index output by the minimizing circuit  170  and the first sound source vector output by the first sound source generating circuit  20  enter the first gain circuit  30 , which proceeds to read a first gain corresponding to the index out of the table, multiply the first gain by the first sound source vector to thereby generate a third sound source vector, and output the generated third sound source vector to the first band-pass filter  135 . 
     The second gain circuit  31  has a table in which gain values have been stored. The index output by the minimizing circuit  170  and the second sound source vector output by the second sound source generating circuit  21  enter the second gain circuit  31 , which proceeds to read a second gain corresponding to the index out of the table, multiply the second gain by the second sound source vector to thereby generate a fourth sound source vector, and output the generated fourth sound source vector to the second band-pass filter  136 . 
     The third sound source vector output by the first gain circuit  30  enters the first band-pass filter  135 . The third sound source vector has its band limited by the filter  135 , whereby a fifth sound source vector is obtained. The first band-pass filter  135  outputs the fifth sound source vector to the adder  40 . 
     The fourth sound source vector output by the second gain circuit  31  enters the second band-pass filter  136 . The fourth sound source vector has its band limited by the filter  136 , whereby a sixth sound source vector is obtained. The second band-pass filter  136  outputs the sixth sound source vector to the adder  40 . 
     The adder  40  adds the inputs applied thereto, namely the fifth sound source vector output by the first band-pass filter  135  and the sixth sound source vector output by the second band-pass filter  136 , and outputs an excitation vector, which is the sum of the fifth and sixth sound source vectors, to the linear prediction filter  150 . 
     The linear prediction filter  150  has a table in which quantized values of linear prediction coefficients have been stored. The excitation vector output by the adder  40  and an index corresponding to a quantized value of a linear prediction coefficient output by the first linear prediction coefficient calculation circuit  140  enter the linear prediction filter  150 . The linear prediction filter  150  reads the quantized value of the linear prediction coefficient corresponding to this index out of the table and drives the filter thus set to this quantized linear prediction coefficient by the excitation vector, whereby a reconstructed vector is obtained. The linear prediction filter  150  outputs this reconstructed vector to the subtractor  50 . 
     The input vector enters the subtractor  50  via the input terminal  10 , and the reconstructed vector output by the linear prediction filter  150  also enters the subtractor  50 . The subtractor  50  calculates the difference between these two inputs. The subtractor  50  outputs a difference vector, which is the difference between the input vector and the reconstructed vector, to the weighting filter  160 . 
     The difference vector output by the subtractor  50  and the linear prediction coefficient output by the first linear prediction coefficient calculation circuit  140  enter the weighting filter  160 . The latter uses this linear prediction coefficient to produce a weighting filter corresponding to the characteristic of the human sense of hearing and drives this weighting filter by the difference vector, whereby there is obtained a weighted difference vector. The weighted difference vector is output to the minimizing circuit  170 . 
     The weighted difference vector output by the weighting filter  160  enters the minimizing circuit  170 , which proceeds to calculate the norm. Indices corresponding to all values of the elements of the first pulse position vector in the first pulse position generating circuit  110  are output successively from the minimizing circuit  170  to the first pulse position generating circuit  110 . Indices corresponding to all values of the elements of the second pulse position vector in the second pulse position generating circuit  111  are output successively from the minimizing circuit  170  to the second pulse position generating circuit  111 . Indices corresponding to all first pulse amplitude vectors that have been stored in the first pulse amplitude generating circuit  120  are output successively from the minimizing circuit  170  to the first pulse amplitude generating circuit  120 . Indices corresponding to all second pulse amplitude vectors that have been stored in the second pulse amplitude generating circuit  121  are output successively from the minimizing circuit  170  to the second pulse amplitude generating circuit  121 . Indices corresponding to all first gains that have been stored in the first gain circuit  30  are output successively from the minimizing circuit  170  to the first gain circuit  30 . Indices corresponding to all second gains that have been stored in the second gain circuit  31  are output successively from the minimizing circuit  170  to the second gain circuit  31 . Further, the minimizing circuit  170  selects the value of each element in the first pulse position vector, the amount of pulse position revision, the first pulse amplitude vector, the second pulse amplitude vector and the first gain and second gain that will result in the minimum norm and outputs the indices corresponding to these to the code output circuit  190 . 
     The index corresponding to the quantized value of the linear prediction coefficients output by the first linear prediction coefficient calculation circuit  140  enters the code output circuit  190  and so do the indices corresponding to the value of each element in the first pulse position vector, the amount of pulse position revision, the first pulse amplitude vector, the second pulse amplitude vector and the first gain and second gain. The code output circuit  190  converts each index to a bit-sequence code and outputs the code via the output terminal  60 . 
     FIG. 2 is a block diagram illustrating the construction of a first embodiment of an apparatus for encoding speech and musical signals according to the present invention. Components in FIG. 2 identical with or equivalent to those of FIG. 1 are designated by like reference characters. 
     As shown in FIG. 2, a code in the form of a bit sequence enters from the input terminal  200 . A code input circuit  220  converts the bit-sequence code that has entered from the input terminal  200  to an index. 
     The code input circuit  220  outputs an index corresponding to each element in the first pulse position vector to the first pulse position generating circuit  210 ; outputs an index corresponding to the amount of pulse position revision to the second pulse position generating circuit  211 ; outputs an index corresponding to the first pulse amplitude vector to the first pulse amplitude generating circuit  120 ; outputs an index corresponding to the second pulse amplitude vector to the second pulse amplitude generating circuit  121 ; outputs an index corresponding to the first gain to the first gain circuit  30 ; outputs an index corresponding to the second gain to the second gain circuit  31 ; and outputs an index corresponding to the quantized value of a linear prediction coefficient to the linear prediction filter  150 . 
     The index output by the code input circuit  220  enters the first pulse position generating circuit  210 , which proceeds to generate the first pulse position vector using the position of each pulse specified by the index and output the vector to the first sound source generating circuit  20  and to the second pulse position generating circuit  211 . 
     The first pulse amplitude generating circuit  120  has a table in which M-dimensional vectors A{overscore ( )} j , j=1, . . . , N A  have been stored. The index output by the code input circuit  220  enters the first pulse amplitude generating circuit  120 , which reads an M-dimensional vector A{overscore ( )} j  corresponding to this index out of the above-mentioned table and outputs this vector to the first sound source generating circuit  20  as a first pulse amplitude vector. 
     The index output by the code input circuit  220  and the first pulse position vector P{overscore ( )}=(P 1 , P 2 , . . . , P M ) output by the first pulse position generating circuit  210  enter the second pulse position generating circuit  211 . The latter revises the first pulse position vector using the pulse position revision quantity d{overscore ( )} i =(d i1 , d i2 , . . . , d iM ) specified by the index and outputs the revised vector to the second sound source generating circuit  21  as a second pulse position vector Q{overscore ( )} t =(P 1 +d i1 , P 2 +d i2 , . . . , P M +d iM ). 
     The second pulse amplitude generating circuit  121  has a table in which M-dimensional vectors B{overscore ( )} j , j=1, . . . , N B  have been stored. The index output by the code input circuit  220  enters the second pulse amplitude generating circuit  121 , which reads an M-dimensional vector B{overscore ( )} i  corresponding to this index out of the above-mentioned table and outputs this vector to the second sound source generating circuit  21  as a second pulse amplitude vector. 
     The first pulse position vector P{overscore ( )}=(P 1 , P 2 , . . . , P M ) output by the first pulse position generating circuit  210  and the first pulse amplitude vector A{overscore ( )} i =(A i1 , A i2 , . . . , A iM ) output by the first pulse amplitude generating circuit  120  enter the first sound source generating circuit  20 . The first sound source generating circuit  20  outputs an N-dimensional vector for which the values of the P 1 st, P 2 nd . . . , P M th elements are A i1 , A i2 , . . . , A iM , respectively, and the values of the other elements are zero to the first gain circuit  30  as a first sound source vector. 
     A second pulse position vector Q{overscore ( )} t =(Q t   1 , Q t   2 , . . . , Q t   M ) output by the second pulse position generating circuit  211  and a second pulse amplitude vector B{overscore ( )} i =(B i1 , B i2 , . . . , B iM ) output by the second pulse amplitude generating circuit  121  enter the second sound source generating circuit  21 . The second sound source generating circuit  21  outputs an N-dimensional vector for which the values of the Q t   1 st, Q t   2 nd, . . . , Q t   M th elements are B i1 , B i2 , . . . , B iM , respectively, and the values of the other elements are zero to the second gain circuit  31  as a second sound source vector. 
     The first gain circuit  30  has a table in which gain values have been stored. The index output by the code input circuit  220  and the first sound source vector output by the first sound source generating circuit  20  enter the first gain circuit  30 , which reads a first gain corresponding to the index out of the table, multiplies the first gain by the first sound source vector to thereby generate a third sound source vector, and outputs the generated third sound source vector to the first band-pass filter  135 . 
     The second gain circuit  31  has a table in which gain values have been stored. The index output by the code input circuit  220  and the second sound source vector output by the second sound source generating circuit  21  enter the second gain circuit  31 , which reads a second gain corresponding to the index out of the table, multiplies the second gain by the second sound source vector to thereby generate a fourth sound source vector, and outputs the generated fourth sound source vector to the second band-pass filter  136 . 
     The third sound source vector output by the first gain circuit  30  enters the first band-pass filter  135 . The third sound source vector has its band limited by the filter  135 , whereby a fifth sound source vector is obtained. The first band-pass filter  135  outputs the fifth sound source vector to the adder  40 . 
     The fourth sound source vector output by the second gain circuit  31  enters the second band-pass filter  136 . The fourth sound source vector has its band limited by the filter  136 , whereby a sixth sound source vector is obtained. The second band-pass filter  136  outputs the sixth sound source vector to the adder  40 . 
     The adder  40  adds the inputs applied thereto, namely the fifth sound source vector output by the first band-pass filter  135  and the sixth sound source vector output by the second band-pass filter  136 , and outputs an excitation vector, which is the sum of the fifth and sixth sound source vectors, to the linear prediction filter  150 . 
     The linear prediction filter  150  has a table in which quantized values of linear prediction coefficients have been stored. The excitation vector output by the adder  40  and an index corresponding to a quantized value of a linear prediction coefficient output by the code input circuit  220  enter the linear prediction filter  150 . The linear prediction filter  150  reads the quantized value of the linear prediction coefficient corresponding to this index out of the table and drives the filter thus set to this quantized linear prediction coefficient by the excitation vector, whereby a reconstructed vector is obtained. The linear prediction filter  150  outputs this reconstructed vector via the output terminal  201 . 
     [Second Embodiment] 
     FIG. 3 is a block diagram illustrating the construction of a second embodiment of an apparatus for encoding speech and musical signals according to the present invention. Here also it is assumed for the sake of simplicity that the number of bands is two. 
     Components in FIG. 3 identical with or equivalent to those of the prior art illustrated in FIG. 10 are designated by like reference characters and are not described again in order to avoid prolixity. 
     As shown in FIG. 3, the first pulse position generating circuit  110  receives as an input an index that is output by the minimizing circuit  170 , generates a first pulse position vector using the position of each pulse specified by the index and outputs this vector to the first sound source generating circuit  20  and to the second pulse position generating circuit  111 . 
     The second pulse position generating circuit  111  receives as inputs the index that is output by the minimizing circuit  170  and the first pulse position vector P{overscore ( )}=(P 1 , P 2 , . . . , P M ) output by the first pulse position generating circuit  110 , revises the first pulse position vector using the pulse position revision quantity d{overscore ( )} i =(d i1 , d i2 , . . . , d iM ) specified by the index and outputs the revised vector to the second sound source generating circuit  21  as a second pulse position vector Q{overscore ( )} t =(P 1 +d i1 , P 2 +d i2 , . . . , P M +d iM ). 
     The weighted difference vector output by the weighting filter  160  enters the minimizing circuit  170 , which proceeds to calculate the norm. Indices corresponding to all values of the elements of the first pulse position vector in the first pulse position generating circuit  110  are output successively from the minimizing circuit  170  to the first pulse position generating circuit  110 . Indices corresponding to all values of the elements of the second pulse position vector in the second pulse position generating circuit  111  are output successively from the minimizing circuit  170  to the second pulse position generating circuit  111 . Indices corresponding to all first pulse amplitude vectors that have been stored in the first pulse amplitude generating circuit  120  are output successively from the minimizing circuit  170  to the first pulse amplitude generating circuit  120 . Indices corresponding to all second pulse amplitude vectors that have been stored in the second pulse amplitude generating circuit  121  are output successively from the minimizing circuit  170  to the second pulse amplitude generating circuit  121 . Indices corresponding to all first gains that have been stored in the first gain circuit  30  are output successively from the minimizing circuit  170  to the first gain circuit  30 . Indices corresponding to all second gains that have been stored in the second gain circuit  31  are output successively from the minimizing circuit  170  to the second gain circuit  31 . Further, the minimizing circuit  170  selects the value of each element in the first pulse position vector, the amount of pulse position revision, the first pulse amplitude vector, the second pulse amplitude vector and the first gain and second gain that will result in the minimum norm and outputs the indices corresponding to these to the code output circuit  190 . 
     The index corresponding to the quantized value of the linear prediction coefficient output by the first linear prediction coefficient calculation circuit  140  enters the code output circuit  190  and so do the indices corresponding to the value of each element in the first pulse position vector, the amount of pulse position revision, the first pulse amplitude vector, the second pulse amplitude vector and the first gain and second gain. The code output circuit  190  converts these indices to a bit-sequence code and outputs the code via the output terminal  60 . 
     FIG. 4 is a block diagram illustrating the construction of the second embodiment of an apparatus for decoding speech and musical signals according to the present invention. Components in FIG. 4 identical with or equivalent to those of FIGS. 3 and 12 are designated by like reference characters and are not described again in order to avoid prolixity. 
     As shown in FIG. 4, the code input circuit  220  converts the bit-sequence code that has entered from the input terminal  200  to an index. The code input circuit  220  outputs an index corresponding to each element in the first pulse position vector to the first pulse position generating circuit  210 , outputs an index corresponding to the amount of pulse position revision to the second pulse position generating circuit  211 , outputs an index corresponding to the first pulse amplitude vector to the first pulse amplitude generating circuit  120 , outputs an index corresponding to the second pulse amplitude vector to the second pulse amplitude generating circuit  121 , outputs an index corresponding to the first gain to the first gain circuit  30 , outputs an index corresponding to the second gain to the second gain circuit  31 , and outputs an index corresponding to the quantized value of a linear prediction coefficient to the linear prediction filter  150 . 
     The index output by the code input circuit  220  enters the first pulse position generating circuit  210 , which generates the first pulse position vector using the position of each pulse specified by the index and outputs the vector to the first sound source generating circuit  20  and to the second pulse position generating circuit  211 . 
     The index output by the code input circuit  220  and the first pulse position vector P{overscore ( )}=(P 1 , P 2 , . . . , P M ) output by the first pulse position generating circuit  210  enter the second pulse position generating circuit  211 . The latter revises the first pulse position vector using the pulse position revision quantity d{overscore ( )} i =(d i1 , d i2 , . . . , d iM ) specified by the index and outputs the revised vector to the second sound source generating circuit  21  as a second pulse position vector Q{overscore ( )} t =(P 1 +d i1 , P 2 +d i2 , . . . , P M +d iM ). 
     [Third Embodiment] 
     FIG. 5 is a block diagram illustrating the construction of a third embodiment of an apparatus for encoding speech and musical signals according to the present invention. As shown in FIG. 5, the apparatus for encoding speech and musical signals according to the third embodiment of the present invention has a higher-order linear prediction coefficient calculation circuit  380  substituted for the higher-order linear prediction coefficient calculation circuit  180  of the second embodiment shown in FIG.  3 . Moreover, the first band-pass filter  135  and second band-pass filter  136  are eliminated. 
     FIG. 6 is a diagram illustrating an example of the construction of the higher-order linear prediction coefficient calculation circuit  380  in the apparatus for encoding speech and musical signals according to the third embodiment depicted in FIG.  5 . Components in FIG. 6 identical with or equivalent to those of FIG. 11 are designated by like reference characters and are not described again in order to avoid prolixity. Only the features that distinguish this higher-order linear prediction coefficient calculation circuit will be discussed. 
     Fourier coefficients output by the FFT circuit  930  enter the band splitting circuit  540 . The latter equally partitions these Fourier coefficients into high- and low-frequency regions, thereby obtaining low-frequency Fourier coefficients and high-frequency (region) Fourier coefficients. The low-frequency coefficients are output to the first zerofill circuit  550  and the high-frequency coefficients are output to the second zerofill circuit  551 . 
     The low-frequency Fourier coefficients output by the band splitting circuit  540  enter the first zerofill circuit  550 , which fills the band corresponding to the high-frequency region with zeros, generates first full-band Fourier coefficients and outputs these coefficients to the first inverse FFT circuit  560 . 
     The high-frequency Fourier coefficients output by the band splitting circuit  540  enter the second zerofill circuit  551 , which fills the band corresponding to the low-frequency region with zeros, generates second full-band Fourier coefficients and outputs these coefficients to the second inverse FFT circuit  561 . 
     The first full-band Fourier coefficients output by the first zerofill circuit  550  enter the first inverse FFT circuit  560 , which proceeds to subject these coefficients to an inverse FFT, thereby obtaining a first residual signal that is output to the first higher-order linear prediction coefficient calculation circuit  570 . 
     The second full-band Fourier coefficients output by the second zerofill circuit  551  enter the second inverse FFT circuit  561 , which proceeds to subject these coefficients to an inverse FFT, thereby obtaining a second residual signal that is output to the second higher-order linear prediction coefficient calculation circuit  571 . 
     The first residual signal output by the first inverse FFT circuit  560  enters the first higher-order linear prediction coefficient calculation circuit  570 , which proceeds to subject the first residual signal to higher-order linear prediction analysis, thereby obtaining the first higher-order linear prediction coefficient. This is output to the first higher-order linear prediction filter  130  via the output terminal  901 . 
     The second residual signal output by the second inverse FFT circuit  561  enters the second higher-order linear prediction coefficient calculation circuit  571 , which proceeds to subject the second residual signal to higher-order linear prediction analysis, thereby obtaining the second higher-order linear prediction coefficient. This is output to the second higher-order linear prediction filter  131  via the output terminal  902 . 
     FIG. 7 is a block diagram illustrating the construction of the third embodiment of an apparatus for decoding speech and musical signals according to the present invention. As shown in FIG. 7, the apparatus for decoding speech and musical signals according to the third embodiment of the present invention has the higher-order linear prediction coefficient calculation circuit  380  substituted for the higher-order linear prediction coefficient calculation circuit  180  of the second embodiment shown in FIG.  4 . 
     Moreover, the first band-pass filter  135  and second band-pass filter  136  are eliminated. 
     [Fourth Embodiment] 
     FIG. 8 is a block diagram illustrating the construction of a fourth embodiment of an apparatus for encoding speech and musical signals according to the present invention. As shown in FIG. 8, the apparatus for encoding speech and musical signals according to the fourth embodiment of the present invention has the higher-order linear prediction coefficient calculation circuit  380  substituted for the higher-order linear prediction coefficient calculation circuit  180  shown in FIG.  10 . Moreover, the first band-pass filter  135  and second band-pass filter  136  are eliminated. 
     FIG. 9 is a block diagram illustrating the construction of the fourth embodiment of an apparatus for decoding speech and musical signals according to the present invention. As shown in FIG. 9, the apparatus for decoding speech and musical signals according to the fourth embodiment of the present invention has the higher-order linear prediction coefficient calculation circuit  380  substituted for the higher-order linear prediction coefficient calculation circuit  180  shown in FIG.  12 . Moreover, the first band-pass filter  135  and second band-pass filter  136  are eliminated. 
     Though the number of bands is limited to two in the foregoing description for the sake of simplicity, the present invention is applicable in similar fashion to cases where the number of bands is three or more. 
     Further, it goes without saying that the present invention may be so adapted that the first pulse position vector is used as the second pulse position vector. Further, it is possible to use all or part of the first pulse amplitude vector as the second pulse amplitude vector. 
     Thus, in accordance with the present invention, as described above, the sound source signal of each of a plurality of bands can be encoded using a small number of bits in a band-splitting-type apparatus for encoding speech and musical signals. The reason for this is that the correlation between bands possessed by the input signal is taken into consideration some of the information possessed by a sound source signal that has been encoded in a certain band or bands is used to encode a sound source signal in the other band(s). 
     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.