Abstract:
A speech coder utilizing previously stored sound source vectors to generate synthetic speech, a distortion computing circuit for computing a distortion of synthetic speech from input speech and a selection circuit for selecting the sound source vector that provides minimum distortion. Sound source vectors are stored within a plurality of reduced size code books rather than a single larger code book. A vector adder adds the sound source vectors respectively output from each of the reduced code books thereby generating a single sound source vector for comparison with the input speech. The distortion circuit computes the distortion for this sound source vector by analyzing the sound source vectors respectively output from each of the reduced code books in addition to the sound source vector output from the vector adder. The computational complexity required to determine the distortion is greatly reduced from the complexity required if a single larger code book of sound source vectors is utilized.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a speech coder which subjects a speech signal to data compression prior to digital transmission or storage. 
     2. Description of the Related Art 
     There are speech coding systems in which a speech signal is separated into a parameter representative of a synthesis filter and a parameter representative of a sound source to thereby effect data compression. One example of such coding is code-excited linear prediction (hereinafter referred to as CELP). 
     One example of CELP is shown in M. R. Schroeder, B. S. Atal, &#34;Code-Excited Linear Prediction (CELP): High-quality speech at very low bit rates&#34;, in Proceedings IEEE International Conference on Acoustics, Speech and Signal Processing, pp. 937-940 (1985). In this paper, a parameter representative of a synthesis filter is analytically obtained once every 10 msec, 40-point random noise time series, that is, 40-dimensional vectors (hereinafter referred to as &#34;sound source vectors&#34;), produced from random numbers, are employed as a parameter representative of a sound source time-corresponding to speech which is coded in blocks each consisting of 40 speech samples (5 msec in duration when the sampling frequency is 8 kHz). 
     I. M. Trancoso, B. S. Atal, &#34;Efficient procedures for finding the optimum innovation in stochastic coders&#34;, Proceedings IEEE International Conference on Acoustics, Speech and Signal Processing, pp. 2375-2378 (1986) disclose a speech coder performing the speech coding of the Schroeder et al. paper (see FIG. 3). 
     Referring to FIG. 3, the reference numeral 3 denotes N-dimensional Discrete Fourier transform (hereinafter referred to as DFT) vectors obtained by subjecting sound source vectors, which are J-point sampling value sequences, to 2·N-point DFT. The reference numeral 1 denotes a code book comprising L DFT sound source vectors 3. The reference numeral 5 denotes a change-over switch used to select the DFT sound source vectors 3 stored in the code book 1. Denominator term computing circuit 15 outputs a denominator term 17 for distortion computation on the basis of the squared value 16 (hereinafter referred to as &#34;squared evaluation weight&#34;) of the amplitude term of certain frequency characteristics (hereinafter referred to as &#34;evaluation weighting filter&#34;). Those frequency characteristics are obtained by subjecting the DFT sound source vector 3 and the impulse response of the synthesis filter to 2·N-point DFT. Vector product sum computing circuit 8 is supplied as its inputs, with the DFT sound source vector 3 and the weighted DFT input speech 7. Weighted DFT input speech 7 is the product of the evaluation weighting filter output and the conjugate complex number of what is obtained by subjecting input speech which is a J-point sampling value sequence to 2·N-point DFT. In response to those inputs, vector product sum computing circuit 8 outputs a numerator term vector product sum 10 for distortion computation. Final distortion computing circuit 12 computes a distortion 18 of the synthetic speech from the input speech in the frequency domain on the basis of the numerator term vector product sum 10 and the denominator term 17. Optimum sound source vector selecting circuit 19 selects a sound source vector code 20 corresponding to a sound source vector having the smallest distortion 18. The reference symbol A denotes a distortion computing means. 
     The operation will next be explained by use of the flowchart shown in FIG. 4. 
     When the k-th one of the L DFT sound source vectors 3 stored in the code book 1 is used, distortion 18 is generally known as follows: ##EQU1## where X(i) is the i-th component in the DFT input speech, H(i) the i-th component in the evaluation weighting filter, C(i, k) the i-th component in the k-th DFT sound source vector, and g(k) the gain coefficient that minimizes the distortion E(k). 
     First, the vector product sum computing circuit 8 is supplied, as its inputs, with the DFT sound source vector C(i, k) and the weighted DFT input speech Y(i). These inputs enable circuit 8 to output the following numerator term vector product sum P(k) (Step ST1): ##EQU2## where Y(i)* denotes the conjugate complex number of Y(i) which satisfies the relation of Y(i)=X(i)·H(i)*, and the symbols Re. and Im. denote the real and imaginary numbers, respectively, of the complex number. 
     The denominator term computing circuit 15 is supplied, as its inputs, with the DFT sound source vector C(i,k) and the squared evaluation weight a(i) 2 , to output the following denominator term 17 (Step ST2): ##EQU3## 
     Since a(i) 2  is the square of the evaluation weighting filter H(i), the relation of a(i) 2  =|H(i)| 2  is satisfied. 
     Next, the final distortion computing circuit 12 is supplied, as its inputs, with the numerator term vector product sum P(k) expressed by the equation (2) and the denominator term 17 expressed by the equation (3) to output the following distortion E(k) (Step ST3): ##EQU4## 
     It should be understood to be known that the equation (4) is obtained by selecting a gain coefficient g(k) that minimizes the distortion E(k) of the equation (1) and that the equation (4) is equivalent to the equation (1). 
     After the final distortion computing circuit 12 completes the computation of distortions 18 for all the L DFT sound source vectors 3 (Step ST4), the optimum sound source vector selecting circuit 19 selects as an optimum sound source vector code 20 the number of the DFT sound source vector 3 that gives the smallest value of the L distortions 18 (Step ST5). 
     The conventional speech coder described above carries out L numerator term vector multiply-add operations in the vector product sum computing circuit 8 to compute L distortions. This conventional speech decoder needs to increase the value of L (e.g., L=1024) in order to code speech in high quality (i.e., the synthetic speech includes no noise). However, if L is increased, the computational complexity, that is, the number of multiply-add operations, required for the distortion computation becomes enormous and, at the same time, the memory capacity needed for the code book increases enormously, resulting in an exceedingly large-scale speech coder. 
     SUMMARY OF THE INVENTION 
     In view of the above-described problems of the prior art, it is a primary object of the present invention to reduce the number of computational operations required for the distortion computation and thereby obtain a small-scale speech coder. 
     To this end, the present invention provides a speech coder comprising: a plurality of reduced code books extracted from a code book; a vector adder which adds sound source vectors respectively selected from the reduced code books to produce a single sound source vector; and a distortion computing means for computing a distortion on the basis of the sound source vector produced by the vector adder and the sound source vectors respectively selected from the reduced code books. 
     The vector adder in the present invention adds sound source vectors respectively selected from a plurality of reduced code books to produce a single sound source vector, and the distortion computing means in the present invention computes a distortion on the basis of the sound source vector produced in the vector adder and the sound source vectors respectively selected from the reduced code books. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following description of the preferred embodiment thereof, taken in conjunction with the accompanying drawings, in which like reference numerals denote like elements, and of which: 
     FIG. 1 is a block diagram showing the arrangement of one embodiment of the speech coder according to the present invention; 
     FIG. 2 is a flowchart showing the operation of the embodiment of the present invention; 
     FIG. 3 is a block diagram showing the arrangement of a conventional speech coder; and 
     FIG. 4 is a flowchart showing the operation of the prior art shown in FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One embodiment of the present invention will be described below with reference to the accompanying drawings. 
     FIG. 1 is a block diagram showing the arrangement of a speech coder for encoding in the frequency domain. The speech coder of FIG. 1 has two reduced code books. In FIG. 1, the same elements or portions as those in the prior art shown in FIG. 3 are denoted by the same reference numerals, and further description thereof is omitted. 
     Referring to FIG. 1, first reduced code book 2a and second reduced code book 2b each comprise M (L=M 2 ) DFT sound source vectors. First and second DFT sound source vectors 4a and 4b are stored in the first and second reduced code books 2a and 2b, respectively. The reference numerals 6a and 6b denote first and second change-over switches for selecting first and second DFT sound source vectors 4a and 4b from the first and second code books 2a and 2b, respectively. First and second vector product sum computing circuits 9a and 9b are respectively supplied with the selected first and second DFT sound source vectors 4a and 4b, as well as the weighted DFT input speech 7, to respectively output numerator term vector product sums 11a and 11b thereof. Vector adder 13 adds the first and second DFT sound source vectors 4a and 4b to produce a single DFT sound source vector 14. Distortion computing means B includes circuits 9a, 9b, 12 and 15. 
     The operation will next be explained by use of the flowchart shown in FIG. 2. 
     The operation conducted when the k 1  -th first DFT sound source vector in the first reduced code book, and the k 2  -th second DFT sound source vector in the second reduced code book, are used will first be explained. A(i,k 1 ) denotes the i-th component in the k 1  -th first DFT sound source vector, and B(i,k 2 ) denotes the i-th component in the k 2  -th second DFT sound source vector. Since the other parameters used in the following description are the same as those in the foregoing description of the prior art further, description thereof is omitted. 
     The first and second DFT sound source vectors A(i,k 1 ) and B(i,k 2 ), selected by the first and second change-over switches 6a and 6b, are input to the first and second vector product sum computing circuits 9a and 9b, respectively. First vector product sum computing circuit 9a outputs M first numerator term vector product sums P&#39;(k 1 ) in the same way as in the equation (2) (Steps ST6 and ST7): ##EQU5## Second vector product sum computing circuit 9b outputs M second numerator term vector product sums Q&#39;(k 2 ) in the same way as in the equation (2) (Steps ST8 and ST9): ##EQU6## 
     Vector adder 13 adds the first and second DFT sound source vectors A(i,k 1 ) and B(i,k 2 ) to produce a single DFT sound source vector C&#39;(i,k) as follows (Step ST10): 
     
         C&#39;(i,k)=A(i,k.sub.1)+B(i,k.sub.2)                          (7) 
    
     where 
     k=(k 1  -1)M+k 2   
     k 1  =1, 2, . . . , M 
     k 2  =1, 2, . . . , M 
     Since k 2  is changed M times per change of k 1 , there are given L k&#39;s, from 1 to L, and hence L C&#39;(i,k)&#39;s are produced. Denominator term computing circuit 15 outputs the denominator term 17 on the basis of the DFT sound source vector C&#39;(i,k) in the same way as in the equation (3) (Step ST11): ##EQU7## 
     Final distortion computing circuit 12 outputs the distortion E(k) on the basis of the first and second numerator vector product sums P&#39;(k 1 ) and Q&#39;(k 2 ) and the denominator term 17 in the same way as in the equation (4) (Step ST12): ##EQU8## 
     The numerator term vector product sum in the equation (4) is obtained from the relation of the equation (7) as follows: ##EQU9## 
     When all the L distortions E(k) have been obtained by the above-described computational operations (Step ST13), the optimum sound source vector selecting circuit 19 selects as an optimum sound source vector code 20 the number of the DFT sound source vector that gives the smallest value of the L distortions 18 (Step ST14). 
     The feature of the present invention resides in that the number of computational operations required for the whole numerator of the second term in the equation (4) can be reduced by a large margin. More specifically, the L vector multiply-add operations required for the equation (2) in the prior art are replaced by 2√L (L=M 2 ) vector multiply-add operations according to the equations (5) and (6) of the present invention and √L scalar additions required for the numerator of the second term in the equation (9) of the present invention. Thus, the computational complexity is reduced. 
     A comparison as to the computational complexity will next be made between the prior art and the present invention. The computational complexity in the prior art will first be described. 
     In actual practice, the computation of the equation (2) by the vector product sum computing circuit 8 is carried out in the manner expressed by the following equation (10), while the computation of the equation (3) by the denominator term computing circuit 15 is carried out in the manner expressed by the following equation (11): ##EQU10## 
     The number of computational operations required for the equation (10) is a total of 2·L·N multiplications and 2·L·N additions and subtractions, and the number of computational operations required for the equation (11) is a total of 3·L·N multiplications and 2·L·N additions. The final distortion computing circuit 12 carries out L multiplications for squaring the second term in the equation (4) and L divisions. It should be noted that, since the first term in the equation (4) is constant independently of k, it is not concerned with the selection of an optimum sound source vector and therefore no computation is carried out therefor. Assuming that the computational complexities required for a single multiplication, addition or subtraction and division are p, q and r, respectively, the overall computational complexity required for the whole second term in the equation (4) is (5·L·N+L)·p+4·L·N·q+L·r. 
     The computational complexity in the present invention will next be described. In actual practice, the computations of the equations (5) and (6) by the two vector product sum computing circuits 9a and 9b are carried out in the manner expressed by the following equations (12) and (13), and the computations of the equations (7) and (8) by the vector adder 13 and the denominator term computing circuit 15 are carried out in the manner expressed by the following equation (14): ##EQU11## 
     The number of computational operations required for the equations (12) and (13) is a total of 4·√L·N multiplications and 4·√L·N additions and substractions, and the number of computational operations required for the equation (14) is a total of 3·L·N multiplications and 4·L·N additions. Final distortion computing circuit 12 carries out a total of L additions for adding the first and second numerator term vector product sums, L multiplications for squaring and L divisions. Hence, the overall computational complexity required for the whole second term in the equation (9) is (4·√L·N+3·L·N+L)·p+(4.multidot.√L·N+4·L·N+L)·q+L.multidot.r. 
     Accordingly, when L satisfies the following condition and is the square of an integer, it is possible to reduce the computational complexity by the present invention: 
     
         L&gt;16·N.sup.2 ·(p+q).sup.2 /(2·N·p-q).sup.2 
    
     Although in the foregoing embodiment the numbers of sound source vectors in the two code books are equal to each other, these numbers may be different from each other. 
     Although in the foregoing embodiment DFT added sound source vectors are produced by a vector adder, these vectors may be stored in the code books; in such a case, the computation of the equation (7) becomes unnecessary and it is possible to further reduce the computational complexity. 
     Although in the foregoing embodiment the present invention has been described by way of a speech coder in the frequency domain, the present invention may be applied to a speech coder that employs the Walsh-Hadamard transform domain or the singular-value decomposition. Further, although in the foregoing embodiment two reduced code books are employed, three or more reduced code books may be employed to obtain the same advantageous effects. 
     Thus, according to the present invention, sound source vectors which are respectively selected from a plurality of reduced code books are added together by a vector adder to produce a single sound source vector, and a distortion is computed by a distortion computing means on the basis of the sound source vector produced in the vector adder and the sound source vectors respectively selected from the reduced code books. Therefore, it suffices only to carry out, 2√L numerator term vector multiply-add operations in the vector product sum computing circuit. This is advantageous because such operations require a high computational complexity. Accordingly, when L is relatively large, the computational complexity needed for the distortion computation is reduced by a large margin compared with the prior art. The number of DFT sound source vectors which need to be stored in each code book is 2√L, and the memory capacity required is reduced to 2L -1/2  times that in the prior art. By virtue of these two advantages, it is possible to set a satisfactorily large value for L and hence possible to code speech in high quality even by a small-scale speech coder. 
     Although the present invention has been described through specific terms, it should be noted here that the described embodiment is not necessarily exclusive and that various changes and modifications may be imparted thereto without departing from the scope of the invention which is limited solely by the appended claim.