Vocoder device for encoding and decoding speech signals

Abstract: The encoder unit of the vocoder device includes the AR code-book, the MA code-book, and the voice source code-book storing code words each corresponding to a set of the AR, the MA, and the voice source parameters, respectively, which are obtained beforehand by means of the "analysis by synthesis" of a multitude of speech waveform examples and then clustering the resulting respective parameters. The AR preliminary selector, the MA preliminary selector, and the voice source preliminary selector select from respective code-books a predetermined finite number of code words approximating the input speech signal, and in synchronism with the voice source position detected by the voice source position detector the speech synthesizer synthesizes a number of synthesized speech waveforms corresponding to the combinations of the selected AR, MA, and voice source parameters. Comparing the synthesized speech waveforms with the current input speech signal waveform, the optimal code word selector selects the combination of the AR, the MA, and the voice source code words having a minimum distance to the input speech signal waveform.

Description: This invention relates to a vocoder device for encoding and decoding speech signals for the purpose of digital signal transmission or storage, and more particularly to code-book driven vocoder devices provided with a voice source generator which are suitable to be used as component parts of on-board telephone equipment for automobiles. A vocoder device provided with a voice source generator using a waveform model is disclosed, for example, in an article by Mats Ljungqvist and Hiroya Fujisaki: "A Method for Estimating ARMA Parameters of Speech Using a Waveform Model of the Voice Source," Journal of Institute of Electronics and Communication Engineers of Japan, Vol. 86, No. 195, SP 86-49, pp. 39-45, 1986, where AR and MA parameters are used as spectral parameters of the speech signal and a waveform model of the voice source is defined as the derivative of a glottal flow waveform. This article uses the ARMA (auto-regressive moving-average) model of the voical tract, according to which the speech signal s(n), the voice source waveform (glottal flow derivative) g(n), and the error e(n) are related to each other by means of AR parameters a i and MA parameters b j : The model waveform of the voice source g(n) (glottal flow derivative) is shown in Fig. 9, where A is the slope at glottal opening; B is the slope prior to closure; C is the slope following closure; D is the glottal closure timing; W (= R + F) is the pulse width; and T is the fundamental period (pitch period). The voice source waveform g(n) is expressed using these voice source parameters as follows: g(n) = A - (2A+Rα)n R + (A+Rα)n 2 R (0≦n≦R) g(n) = α(n-R)+ (3B-2Fα)(n-R) 2 F 2 - (2B- Fα)(n-R) 3 F 3 (R>n≧W) g(n) = C - 2(C-β)(n-W) D + (C-β)(n-W) 2 D 2 (W<n≦W+D) g(n) = β where n represents the time and α and β are: α = (4AR - 6FB) / (F 2 - 2R 2 ) β = CD / {D - 3 (T - W)} Fig. 8a is a block diagram showing the structure of a speech analyzer unit of a conventional vocoder which operates in accordance with the method disclosed in the above article. A voice source generator 12 generates voice source waveforms 13 corresponding to the glottal flow derivative g(n), the first instance of which is selected arbitrarily. The instances of the voice source waveforms 13 are successively modified with a small perturbation as described below. In response to the input speech signal 1 corresponding to s(n) and the voice source waveforms 13 corresponding to g(n), an ARMA analyzer 44 determines the AR parameters 45 and MA parameters 46 corresponding to the a i 's and b j 's, respectively. Further, in response to the voice source waveforms 13, the AR parameters 45 and the MA parameters 46, a speech synthesizer 19 produces a synthesized speech waveforms 20. Then a distance evaluator 47 evaluates the distance E1 between the input speech signal 1 and the synthesized speech waveforms 20 by calculating the squared error: When the distance E1 is greater than a predetermined threshold value E0, one of the voice source parameters is given a small perturbation and the voice source parameters 48 are fed back to the voice source generator 12. In response thereto, the voice source generator 12 generates a new instance of the voice source waveform 13 in accordance with the perturbed voice source parameters, and the ARMA analyzer 44 generates new sets of AR parameters 45 and MA parameters 46 on the basis thereof, such that the speech synthesizer 19 produces a slightly modified synthesized speech waveforms 20. The above operations are repeated, where the magnitude of perturbation given to the voice source parameters are successively reduced. When the distance or error E1 finally becomes less than the threshold level E0, the voice source parameters 48, the AR parameters 49 and the MA parameters 50 encoding the input speech signal 1 are output from the distance evaluator 47. Fig. 8b is a block diagram showing the structure of a speech synthesizer unit of a conventional vocoder which synthesizes the speech from the voice source parameters 48, AR parameters 49 and the MA parameters 50 output from the analyzer of Fig. 8a. In response to the voice source parameters 48, a voice source generator 40 generates a voice source waveform 41. Further, a speech synthesizer 42 generates a synthesized speech 43 on the basis of the voice source waveform 41, the AR parameters 49 and the MA parameters 50. The above conventional vocoder device, however, has the following disadvantage. For each set of voice source parameters, the spectral parameters (i.e., the AR and the MA parameters) are calculated to produce a synthesized speech waveforms 20, such that the distance or squared error El between the input speech signal 1 and the synthesized speech waveforms 20 is determined. The voice source parameters are perturbed and the synthesis of the speech and the determination of the error E1 between the original and the synthesized speech are repeated until the error E1 finally becomes less than a threshold level E0. Since the spectral parameters and the voice source parameters are determined successively by the method of "analysis by synthesis," the calculation is quite complex. Further, the procedure for determining the parameters may become unstable. Furthermore, since the speech signal is processed in synchronism with the pitch period, a fixed or a low bit rate encoding of the speech signal is difficult to realize. EP 0 186 763 discloses a method of and a device for speech signal coding and decoding by vector qunatization techniques which provide a fltering of blocks of digital samples of speech signal by a linear-prediction inverse filter, whose coefficients are chosen out of a codebook of quantized filter coefficient vectors, obtaining a residual signal subdivided into vectors. The weighted mean-square error made in quantizing said vectors with quantized residual vectors contained in a codebook and forming exitation waveforms is computed. The coding signal for each block of samples consists of the coefficient vector index chosen for the inverse filter as well as of the indices of the vectors of the exitation waveforms which have generated minimum weigthed mean-square error. During the decoding phase, a synthesis filter having the same coefficients as chosen for the inverse filter is exited by quantized-residual vectors chosen during the coding phase. In "International Conference on Acoustics Speech and Signal Processing", vol. 1, 14-17 May 1991, Toronto, pages 589-592, J. Haagen et al., "A 2.4 kbps high-quality speech coder" an algorithm for coding speech at 2.4 kbps is presented. The coder is fundamentally a base-band coder where short-term correlation is predicted by LPC-analysis. Coding of the short-term parameters is performed by vector quantization. An open-loop long-term predictor is applied to the lowpass filtered short-term residual to reduce the quasi-periodic pitch structure before the signal is down-sampled. A method for coding the down-sampled residual based on voiced/unvoiced classification is described, wherein for unvoive frames a simple white ganssien codebook is applied, and for voiced frames a pulse codebook is used. In "International Conference on Acoustics Speech and Signal Processing", vol. 3, 7-11 April 1986, Tokyo, pages 1693-1696, J.H. Chen et.al., "Vector Adaptive Predictive Coding of Speech at 9.6 kb/s" a speech coder is shown which significantly enhances adaptive predictive coding (APC) by using vector quantization. The coder gives very good speech quality at 9.6 kb/s and reasonably good quality at 4.8 kb/s. Redundancy is first removed by a long-delay predictor and then by a short-delay predictor; the prediction residual is than quantized by a gain-adaptive vector quantizer. in the receiver, decoded residual vectors are used to excite a synthesis filter to obtain the coded speech. Further, "International Journal of Electronics, vol. 67, no. 2, August 1989, London, pages 173-178, K.K. Paliwel "Reduced-complexity stochastically-excited coder for the low-bit-rate coding of speech" relates to a two-stage method for reducing the computational complexity of a stochastically-excited linear predictive coder. This method can be used in conjunction with the Trancoso-Atal transform-domain filtering method. SUMMARY OF THE INVENTION It is the object of this invention to provide a vocoder device for encoding and decoding speech signals by which the complexity of the calculations of the spectral and voice source parameters is reduced and the procedure for the determining the parameters is stabilize, such that a high quality synthesized speech is produced. This object is accomplished in accordance with the principle of this invention by a vocoder device comprising the features of claim 1. Preferred embodiment of the vocoder device according to the invention are defined in the subclaims. The vocoder device for encoding and decoding speech signals according to the present invention comprises: an encoder unit for encoding an input speech signal including: (a) a first spectral code-book storing a plurality of spectral code words each corresponding to a set of spectral parameters and identified by a spectral code word identification number; (b) a first voice source code-book storing a plurality of voice source code words each representing a voice source waveform over a pitch period and identified by a voice source code word identification number; (c) voice source generator means for generating voice source waveforms for each pitch period on the basis of the voice source code words; (d) speech synthesizer means for producing synthesized speech waveforms for respective combinations of the spectral code words and the voice source code words in response to the spectral code words and the voice source waveforms; (e) optimal code word selector means for selecting a combination of a spectral code word and a voice source code word corresponding to a synthesized speech waveform having a smallest distance to the input speech signal, the optimal code word selector means outputting the spectral code word identification number and the voice source code word identification number corresponding to the spectral code word and the voice source code word, respectively, of the combination selected by the optimal code word selector means; and a decoder unit for reproducing a synthesized speech from each combination of the spectral code word and the voice source code word encoding the input speech signal, the decoder unit including: (f) a second spectral code-book identical to the first spectral code-book; (g) a second voice source code-book identical to the first voice source code-book; (h) spectral inverse quantizer means for selecting from the second spectral code-book a spectral code word corresponding to the spectral code word identification number; (i) voice source inverse quantizer means for selecting from the voice source code-book a voice source code word corresponding to the voice source code word identification number; (j) voice source generator means for generating a voice source waveform for each pitch period on the basis of the voice source code word selected by the voice source inverse quantizer; and (k) speech synthesizer means for producing a synthesized speech waveform on the basis of the spectral code word selected by the spectral inverse quantizer means and the voice source waveform generated by the voice source generator means. Preferably, the spectrum analyzer means extracts a set of the spectral parameters for each analysis frame of predetermined time length longer than the pitch period; and the encoder unit further includes voice source position detector means for detecting a start point of the voice source waveform for each pitch period and outputting the start point as a voice source position; the voice source generator means generating the voice source waveforms in synchronism with the voice source position output from the voice source position detector means for each pitch period; the optimal code word selector means selecting a combination of the spectral code word and the voice source code word which minimizes the distance between the voice source position detector and the input speech signal over a length of time including pitch periods extended over a current frame and a preceding and a succeeding frame; and the decoder unit further includes: spectral interpolator means for outputting interpolated spectral parameters interpolating for each pitch period the spectral parameters of the spectral code words of current and preceding frames; voice source interpolator means for outputting interpolated voice source parameters interpolating for each pitch period the voice source parameters of the voice source code words of current and preceding frames; wherein the voice source generator generates the voice source waveform for each pitch period on the basis of the interpolated voice source parameters, and the speech synthesizer means producing the synthesized speech waveform for each pitch period on the basis of the interpolated spectral parameters and the voice source waveform output from the voice source generator. Further, according to this invention, a method is provided for generating a voice source waveform g(n) for each pitch period on the basis of predetermined parameters: A, B, C, L 1 , L 2 , and pitch period T: g(n) = An - Bn 2 (for 0 ≦ n ≦ L 1 ) g(n) = C(n - L 2 ) 2 (for L 1 < n ≦ L 2 ) g(n) = 0 (for L 2 < n ≦ T) where n represents time. Furthermore, it is preferred that the encoder unit further includes: (l) pitch period extractor means for determining a pitch period length of the input speech signal; (m) order determiner means for determining an order in accordance with the pitch period length; and (n) first converter means for converting the spectral code words into corresponding spectral parameters, the spectral code words each consisting of a set spectral envelope parameters corresponding to a set of the spectral parameters; and the decoder unit further includes: (o) second converter means for converting the spectral code word retrieved by the spectral inverse quantizer means from the second spectral code-book into a set of corresponding spectral parameters of an order equal to the order determined by the order determiner of the encoder unit. BRIEF DESCRIPTION OF THE DRAWINGS The features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The structure and method of operation of this invention itself, however, will be best understood from the following detailed description, taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram showing the structure of the encoder unit of a vocoder device according to this invention; Fig. 2 is a block diagram showing the structure of the decoder unit of a vocoder device according to this invention; Fig. 3 shows the waveforms of the input and the synthesized speech to illustrate a method of operation of the optimal code word selector of Fig. 1; Fig. 4 shows the waveform of synthesized speech to illustrate the method of interpolation within the decoder unit according to this invention; Fig. 5 shows the voice source waveform model used in the vocoder device according to this invention; Fig. 6a is a block diagram showing the structure of the encoder unit of another vocoder device according this invention; Fig. 6b is a block diagram showing the structure of the decoder unit coupled with the encoder unit of Fig. 6a; Fig. 7a is a block diagram showing the structure of the encoder unit of still another vocoder device according to this invention; Fig. 7b is a block diagram showing the structure of the decoder unit coupled with the encoder unit of fig. 7a; Fig. 8a is a block diagram showing the structure of a speech analyzer unit of a conventional vocoder; Fig. 8b is a block diagram showing the structure of a speech synthesizer unit of a conventional vocoder; and Fig. 9 shows the voice source waveform model (the glottal flow derivative) used in the conventional device of Figs. 8a and 8b. In the drawings, like reference numerals represent like or corresponding parts or portions. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the accompanying drawings, the preferred embodiments of this invention are described. Fig. 1 is a block diagram showing the structure of the encoder unit of a vocoder device according to this invention. Based on the well-known LPC (linear predictive analysis) method, the AR analyzer 4 analyses the input speech signal 1 to obtain the AR parameters 5. The AR parameters 5 thus obtained represent a good approximation of the set of the AR parameters a i 's minimizing the error of the equation (1) above. The AR code-book 7 stores a plurality of AR code words each consisting of a set of the AR parameters and an identification number thereof. An AR preliminary selector 6 selects from the AR code-book 7 a finite number L of AR code words which are closest (i.e., at smallest distance) to the AR parameters 5 output from the AR analyzer 4. The distance between two AR code words, or two sets of the AR parameters, may be measured by the sum of the squares of the differences of the corresponding a i 's. The AR preliminary selector 6 outputs the selected code words as preliminarily selected code words 8, preliminarily selected code words representing sets of AR parameters which are relatively close to the set of the AR parameters determined by the AR analyzer 4. To each one of the preliminarily selected code words 8 output from the AR preliminary selector 6 is attached an identification number thereof within the AR code-book 7. The analysis of the input speech signal 1 is effected for each frame (time interval), the length of which is greater than that of a pitch period of the input speech signal 1. A voice source position detector 2 detects, for example, the peak position of the LPC residual signal of the input speech signal 1 for each pitch period and outputs it as the voice source position 3. A voice source code-book 10 stores a plurality of voice source code words each consisting of a set of voice source parameters and an identification number thereof. A voice source preliminary selector 9 selects from the voice source code-book 10 a finite number M of voice source code words which are close (i.e., at smallest distances) to the voice source code word that was selected in the preceding frame. The measure of closeness or the distance between two voice source code words may be a weighted squared distance therebetween, which is the weighted sum of the squares of the differences of the corresponding voice source parameters of the two code words. The voice source preliminary selector 9 outputs the selected voice source code words together with the identification numbers thereof as the preliminarily selected code words 11. Each of the preliminarily selected code words 11 represents a set of voice source parameters corresponding to a voice source waveform over a pitch period. In response to the preliminarily selected code words 11 output from the voice source preliminary selector 9 and the voice source position 3 output from the voice source position detector 2, a voice source generator 12 produces a plurality of voice source waveforms 13 in synchronism with the voice source position 3. In response to the input speech signal 1, the voice source position 3, the preliminarily selected code words 8, and the voice source waveforms 13, an MA calculator 14 calculates a set of MA parameters 15 which gives a good approximation of the MA parameters b j 's minimizing the error of the equation (1) above. The MA code-book 17 stores a plurality of AR code words each consisting of a set of the MA parameters and an identification number thereof. An MA preliminary selector 16 selects from the MA code-book 17 a finite number N of MA code words which are closest (i.e., at smallest distances) to the MA parameters 15 determined by the MA calculator 14. The closeness or distance between two sets of the MA parameters may be measured by a squared distance therebetween, which is the sum of the squares of the differences of the corresponding b j 's. The MA preliminary selector 16 outputs the selected code words as preliminarily selected MA code words 18. The preliminarily selected code words represent sets of MA parameters which are relatively close to the set of the MA parameters calculated by the MA calculator 14. On the basis of the preliminarily selected code words 8, the voice source waveforms 13 and the preliminarily selected MA code words 18, a speech synthesizer 19 produces synthesized speech waveforms 20. As described above, the preliminarily selected code words 8 and the preliminarily selected MA code words 18 includes L and N code words, respectively, and the voice source waveforms 13 includes M voice source waveforms. Thus, the speech synthesizer 19 produces a plurality (equal to L times M times N) of synthesized speech waveforms 20, all in synchronism with the voice source position 3 supplied from the voice source position detector 2. The difference between the input speech signal 1 and each one of the synthesized speech waveforms 20 is calculated by a subtractor 21a and is supplied to an optimal code word selector 21 together with the code word identification numbers corresponding to the AR, the MA, and the voice source code words on the basis of which the synthesized waveform is produced. The differences between the input speech signal 1 and the plurality of the synthesized speech waveforms 20 may be supplied to the optimal code word selector 21 in parallel. The optimal code word selector 21 selects the combination of the AR code word, the MA code word, and the voice source code word which minimizes the difference or the error thereof from the input speech signal 1, and outputs the AR code word identification number 22, the MA code word identification number 23, and the voice source code word identification number 24 corresponding to the AR, the MA, and the voice source code words of the selected combination. The combination of the AR code word identification number 22, the MA code word identification number 23, and the voice source code word identification number 24 output from the optimal code word selector 21 encodes the input speech signal 1 in the current frame. The voice source code word identification number 24 is fed back to the voice source preliminary selector 9 to be used in the selection of the voice source code word in the next frame. Fig. 3 shows the waveforms of the input and the synthesized speech to illustrate a method of operation of the optimal code word selector of Fig. 1. First, the optimal code word selector 21 determines the combination of the AR code word, the MA code word, and the voice source code word which minimizes the distance E1 between the input speech signal 1 (solid line) and the synthesized speech (dotted line) over a distance evaluation interval a which includes several pitch periods before and after the current frame. If the distance E1 is less than a predetermined threshold level E0, then the combination giving the distance E1 is selected and output. On the other hand, if the distance E1 exceeds the threshold E0, a new distance evaluation interval b (b < a) consisting of several pitch periods within which the input speech signal 1 is at a greater power level is selected, and the combination of the AR code word, the MA code word, and the voice source code word which minimizes the distance between the input speech signal 1 (solid line) and the synthesized speech (dotted line) over the new distance evaluation interval b is selected and output. By the way, the entries of the AR code-book 7, the voice source code-book 10, and the MA code-book 17 consist of the AR parameters, voice source parameters, and the MA parameters, respectively, which are determined beforehand from a multitude of input speech waveform examples (which are collected for the purpose of preparing the AR code-book 7, the voice source code-book 10, and the MA code-book 17) by means of the "analysis by synthesis" method for respective parameters. For example, the sets of the AR parameters a i 's, the MA parameters b j 's, and the voice source parameters corresponding to the waveform g(n) which give stable solutions of the equation (1) above for each input speech waveform are determined by means of the "analysis by synthesis" method, and then are subjected to a clustering process on the basis of the LBG algorithm to obtain respective code word entries of the AR code-book 7, the voice source code-book 10, and the MA code-book 17, respectively. Fig. 2 is a block diagram showing the structure of the decoder unit of a vocoder device according to this invention. The decoder unit decodes the combination of the AR code word identification number 22, the MA code word identification number 23, and the voice source code word identification number 24 supplied from the encoder unit and produces the synthesized speech 43 corresponding to the input speech signal 1. Upon receiving the AR code word identification number 22, an AR inverse quantizer 25 retrieves the AR code word 27 corresponding to the AR code word identification number 22 from the AR code-book 26, which has identical organization as the AR code-book 7. Further, upon receiving the MA code word identification number 23, an MA inverse quantizer 30 retrieves the MA code word 32 corresponding to the MA code word identification number 23 from the MA code-book 31, which has identical organization as the MA code-book 17. Furthermore, upon receiving the voice source code word identification number 24, a voice source inverse quantizer 35 retrieves the voice source code word 37 corresponding to the voice source code word identification number 24 from the voice source code-book 36, which has identical organization as the voice source code-book 10. Fig. 4 shows the waveform of synthesized speech to illustrate the method of interpolation within the decoder unit according to this invention. Each frame includes complete or fractional parts of the pitch periods. For example, the current frame includes a complete pitch period Y and fractions of pitch periods X and Z. On the other hand, the preceding frame includes complete pitch periods V and W and a fraction of the pitch period X. The speech is synthesized for each of the pitch periods V, W, X, Y, and Z. As described above, however, the combination of the AR, the MA, and the voice source code words which encode the speech waveform is selected for each one of the frame by the optimal code word selector 21 of the encoder unit. Thus, the AR, the MA, and the voice source parameters must be interpolated for those pitch periods (e.g., the pitch period X in Fig. 4) which are divided among two frames. Thus, in response to the AR code word 27, an AR interpolator 28 outputs a set of interpolated AR parameters 29 for each pitch period. The interpolated AR parameters 29 is a linear interpolation of the AR parameters of the preceding and current frame for the fractional pitch periods (e.g., the pitch period X in the current frame) divided among the two frames. However, for the pitch period Y, for example, which is completely included within the current frame, the interpolated AR parameters 29 may be identical with the parameters of the AR code word 27 of the current frame. Similarly, an MA interpolator 33 outputs a set of interpolated MA parameters 34 for each pitch period. The interpolated MA parameters 34 is a linear interpolation of the MA parameters of the preceding and current frame for the fractional pitch periods divided among the two frames. For the pitch period which is completely included within the current frame, the interpolated MA parameters 34 may be identical with the parameters of the MA code word 32 of the current frame. Further, a voice source interpolator 38 outputs a set of interpolated voice source parameters 39 for each pitch period. The interpolated voice source parameters 39 is a linear interpolation of the voice source parameters of the preceding and current frame for the fractional pitch periods divided among the two frames. For the pitch period which is completely included within the current frame, the interpolated voice source parameters 39 may be the parameters of the voice source code word 37 of the current frame. On the basis of the interpolated voice source parameters 39, a voice source generator 40 generates a voice source waveform 41 for each pitch period. Further, on the basis of the interpolated AR parameters 29, the interpolated MA parameters 34, and the voice source waveform 41, a speech synthesizer 42 generates a synthesized speech 43. As described above, according to this invention, the AR parameters, the MA parameters, and the voice source parameters are interpolated for those pitch periods which are divided among the frames, such that in effect the speech is synthesized in synchronism with the frames that generally includes a plurality of pitch periods. Thus, a low and fixed bit rate encoding of speech can be realized. Fig. 5 shows the voice source waveform model used in the vocoder device according to this invention. The voice source waveform may be generated by the voice source generator 12 of Fig. 1 and the voice source generator 40 of Fig. 2 on the basis of the voice source parameters. The voice source waveform g(n), defined as the glottal flow derivative, is plotted against time shown along the abscissa and the amplitude (the time derivative of the glottal flow) shown along the ordinate. The interval a represents the time interval from the glottal opening to the minimal point of the voice source waveform. The interval b represents the time interval within the pitch period T after the interval a . The interval c represents the time interval from the minimal point to the subsequent zero-crossing point. The interval d represents the time interval from the glottal opening to the first subsequent zero-crossing point. Then, the voice source waveform g(n) is expressed by means of five voice source parameters: the pitch period T, amplitude AM, the ratio OQ of the interval a to the pitch period T, the ratio OP of the interval d to the interval a , and the ratio CT of the interval c to the interval b . Namely, the voice source waveform g(n) as used by the embodiment of Figs. 1 and 2 is defined by: g(n) = An - Bn 2 (0 ≦ n ≦ T · OQ) g(n) = C(n-L) 2 (T · OQ < n ≦ L) g(n) = 0 (L < ≦ T) where A = AM T·OQ·(T·OQ-1)/OP B = A OQ·T·OP C = - AM (1-OQ)·T·CT L = T · (1 - OQ) · CT + T · OQ In the case of the above embodiment, a combination of the AR code word, the MA code word, and the voice source code word is selected for each frame. It is possible, however, to select plural combinations of code words for each frame. Further, although the AR and the MA parameters are used as the spectral parameters in the above embodiment, the AR parameters alone may be used as spectral parameters. Furthermore, in the case of the above embodiment, the synthesized speech is produced from the spectral parameters and the voice source parameters. However, it is possible to generate the synthesized speech while interpolating the spectral parameters and the voice source parameters and calculating the distance between the synthesized speech and the input speech signal. Still further, in the case where the distance between the synthesized speech and the input speech signal is determined to be above an allowable limit by the optimal code word selector 21, the parameters for the current frame may be calculated by interpolation of the spectral parameters and the voice source parameters for the frames preceding and subsequent to the current frame. Still further, in the case of the above embodiment, the voice source code word includes the pitch period T and the amplitude AM. The voice source code-book may be prepared with code word entries which are obtained by clustering the voice source parameters excluding the pitch period T and the amplitude AM. Then the pitch period and the amplitude may be encoded and decoded separately. Fig. 6a is a block diagram showing the structure of the encoder unit of another vocoder device according to this invention, which is discussed in an article by the present inventors: Seza et al., "Study of Speech Analysis/Synthesis System Using Glottal Voice Source Waveform Model," Lecture Notes of 1991 Fall Convention of Acoustics Association of Japan, I, 1-6-10, pp. 209 - 210, 1991. The encoder of Fig. 6a is similar to that of Fig. 1. However, the encoder unit includes pitch period extractor 51 for detecting the pitch period of the input speech signal 1 and outputs a pitch period length 52 of the input speech signal 1. The voice source code-book 10 of Fig. 6a (corresponding to the combination of the voice source code-book 10 and the voice source preliminary selector 9 of Fig. 1) stores a plurality of voice source code words, and outputs the voice source code words 11a together with their identification numbers. The MA code-book 17 (corresponding to the combination of the MA calculator 14, the MA preliminary selector 16 and the MA code-book 17 of Fig. 1) stores as the MA code words sets of MA parameters converted into spectral envelope parameters, and outputs these MA code words 18a together with the identification numbers thereof. The voice source generator 12 generates the voice source waveforms 13 in response to the pitch period length 52 and the voice source code words 11a. The speech synthesizer 19 produces synthesized speech waveforms 20 on the basis of the AR code words 8a, the MA code words 18a, and the voice source waveforms 13. Otherwise, the structure and method of operation of the encoder of Fig. 6a are similar to those of the encoder of Fig. 1. Fig. 6b is a block diagram showing the structure of the decoder unit coupled with the encoder unit of Fig. 6a, which is similar in structure and method of operation to the decoder of Fig. 2. However, the decoder unit of Fig. 6b lacks the AR interpolator 28, the MA interpolator 33, and the voice source interpolator 38 of Fig. 2. Further, the voice source generator 40 generates the voice source waveform 41 in response to the pitch period length 52 and the voice source code word 37 output from the voice source inverse quantizer 35. The speech synthesizer 42 produces the synthesized speech 43 on the basis of the AR code word 27 output from the AR inverse quantizer 25, the voice source waveform 41 output from the voice source generator 40, and the MA code word 32 output from the MA inverse quantizer 30. It is noted that the AR interpolator 28, the MA interpolator 33, and the voice source interpolator 38 of Fig. 2 may also be included in the decoder of Fig. 6b. As described above, according to this invention, the input speech signal is encoded using voice source waveforms for each pitch period. Under this circumstance, the MA parameters serve to compensate for the inaccuracy of the voice source waveforms, especially when the pitch period becomes longer, such that the higher order MA parameters become necessary for accurate reproduction of the input speech signal. Thus, for the purpose of accurate and efficient encoding of the input speech signal, the order of the MA parameters should be varied depending on the length of the pitch period of the input speech signal. It is thus preferred that the degree or order q of the MA (the number of the MA parameters b j 's excluding b 0 in the equation (1) above) is rendered variable. Fig. 7a is a block diagram showing the structure of the encoder unit of still another vocoder device according to this invention, by which the order of the MA parameters is varied in accordance with the pitch period of the input speech signal. Generally, the encoder of Fig. 7a is similar to that of Fig. 6a. However, the encoder unit of Fig. 7a further includes an order determiner 53 and an MA converter 55. The pitch period extractor 51 determines the pitch period of the input speech signal 1 and outputs the pitch period length 52 corresponding thereto. In response to the pitch period length 52 output from the pitch period extractor 51, the order determiner 53 determines the order 54 (the number q of the MA parameters b j excluding b 0 ) in accordance with the length of the pitch period of the input speech signal 1. For example, the order determiner 53 determines the order 54 as an integer closest to 1/4 of the pitch period length 52. The MA code-book 17 stores MA code words and the identification numbers corresponding thereto. The MA code words each consist, for example, of a set of cepstrum coefficients representing a spectral envelope. The MA code-book 17 outputs the MA code words 18a to the MA converter 55 together with the identification numbers thereof. The MA converter 55 converts the MA code words 18a into corresponding sets of MA parameters 18b of order q determined by the order determiner 53. The MA converter 55 effects the conversion using the equations: where Cn is the cepstrum parameter of the n'th order and b n is the n'th order MA coefficient (linear predictive analysis (LPC) coefficient). The sets of the MA parameters 18b thus obtained by the MA converter 55 are output to the speech synthesizer 19 together with the identification numbers thereof. Otherwise, the encoder of Fig. 7a is similar to that of Fig. 6b. Fig. 7b is a block diagram showing the structure of the decoder unit coupled with the encoder unit of Fig. 7a, which is similar in structure and method of operation to the decoder of Fig. 6b. However, the decoder of Fig. 7b includes an order determiner 60 which determines the order q of the MA parameters equal to the integer closest to the 1/4 of the pitch period length 52 output from the pitch period extractor 51 of the encoder unit. The order determiner 60 outputs the order q 61 to the MA converter 62. The MA code-book 31 is identical in organization to the MA code-book 17 and stores the same MA code words consisting of cepstrum coefficients. The MA inverse quantizer 30 retrieves the MA code word corresponding to the MA code word identification number 23 output from the optimal code word selector 21 and outputs it as the MA code word 32a. In response to the order q 61, the MA converter 62 converts the MA code word 32a into the corresponding MA parameters of order q, using the equation (3) above. The MA converter 62 outputs the converted MA parameters 32b to the speech synthesizer 42. Otherwise the decoder of Fig. 7b is similar to that of Fig. 6b. As described above, the order q of the MA parameters is varied in accordance with the input speech signal 1. Thus, the distance or error between the input speech signal 1 and the synthesized speech 43 is minimized without sacrificing the efficiency, and the quality of the synthesized speech can thereby be improved. In the embodiment of Fig. 7b, the decoder unit includes the order determiner 60 for determining the order of MA parameters in accordance with the pitch period length 52 received from the encoder unit. However, the optimal code word selector 21 of the encoder unit of Fig. 7a may select and output the order of MA parameters minimizing the error or distortion of the synthesized speech with respect to the input speech signal, and the order selected by the optimal code word selector 21 is supplied to the MA converter 62. Then the order determiner 60 of the decoder of Fig. 7b can be dispensed with. Further, it is noted that the LSP and the PARCOR parameters may be used as the spectral envelope parameters of the MA code words. Furthermore, the order p of the AR parameters may also be rendered variable in a similar manner. Then, the LSP, the PARCOR, and the LPC cepstrum parameters may be used as the spectral envelope parameters of the AR code words. It is also noted that the AR preliminary selector 6, the voice source preliminary selector 9, and the MA parameters 15 of the embodiment of Fig. 1 may also be included in the embodiments of Figs. 6a and 7a for optimizing the efficiency and accuracy of the speech reproduction.

Claims: A vocoder device for encoding and decoding speech signals, comprising: an encoder unit for encoding an input speech signal including: (a) a first spectral code-book (7, 17) storing a plurality of spectral code words each corresponding to a set of spectral parameters and identified by a spectral code word identification number; (b) a first voice source code-book (10) storing a plurality of voice source code words each representing a voice source waveform over a pitch period and identified by a voice source code word identification number; (c) voice source generator means (12) for generating voice source waveforms for each pitch period on the basis of said voice source code words; (d) speech synthesizer means (19) for producing synthesized speech waveforms for respective combinations of preliminarily selected spectral code words and preliminarily selected voice source code words in response to said preliminarily selected spectral code words and the voice source waveforms corresponding to said preliminarily selected voice source code words; (e) optimal code word selector means (21) for selecting a combination of a spectral code word and a voice source code word corresponding to a synthesized speech waveform having a smallest distance to said input speech signal, said optimal code word selector means (21) outputting said spectral code word identification number and said voice source code word identification number corresponding to said spectral code word and said voice source code word, respectively, of said combination selected by said optimal code word selector means (21), and a decoder unit for reproducing a synthesized speech from each combination of said spectral code word and said voice source code word encoding said input speech signal, said decoder unit including: (f) a second spectral code-book (26, 31) identical to said first spectral code-book (7, 17); (g) a second voice source code-book (36) identical to said first voice source code-book (10); (h) spectral inverse quantizer means (25, 30) for selecting from said second spectral code-book (26, 31) a spectral code word corresponding to said spectral code word identification number; (i) voice source inverse quantizer means (35) for selecting from said voice source code-book (36) a voice source code word corresponding to said voice source code word identification number; (j) voice source generator means (40) for generating a voice source waveform for each pitch period on the basis of said voice source code word selected by said voice source inverse quantizer (35); and (k) speech synthesizer means (42) for producing a synthesized speech waveform on the basis of said spectral code word selected by said spectral inverse quantizer means (25, 30) and said voice source waveform generated by said voice source generator means (40). A vocoder device according to claim 1, characterized in that said encoder unit encodes an input speech signal for each analysis time frame equal to or longer than a pitch period of said input speech signal and includes: spectrum analyzer means (4) for analyzing said input speech signal and successively extracting therefrom a set of spectral parameters corresponding to a current spectrum of said input speech signal; spectral preliminary selector means (6, 16) for selecting from said spectral code-book (7, 17) a finite number of spectral code words representing sets of spectral parameters having smallest distances to said set of spectral parameters extracted by said spectrum analyzer means (5); a voice source preliminary selector means (9) for selecting a finite number of voice source code words having smallest distances to a voice source code word selected in a immediately preceeding analysis time frame; said optimal code word selector means (21) comparing said synthesized speech waveforms with said input speech signal. A vocoder device according to claim 2, characterized in that said optimal code word selector means (21) outputs a combination of a spectral code word identification number and a voice source code word identification number encoding said input speech signal, wherein said decoder unit reproduces a synthesized speech from each combination of said code word identification number and said voice source code word identification number. A vocoder device according to one of claims 1 to 3, characterized in that said spectrum analyzer means (4) extracts a set of said spectral parameters for each analysis time frame longer than said pitch period; and said encoder unit further includes voice source position detector means (2) for detecting a start point of said voice source waveform for each pitch period and outputting said start point as a voice source position; said voice source generator means (12) generating said voice source waveforms in synchronism with said voice source position output from said voice source position detector means (2) for each pitch period; said optimal code word selector means (21a, 21) selecting a combination of said spectral code word and said voice source code word which minimizes said distance between said voice source position and said input speech signal over a length of time including pitch periods extended over a current frame and a preceding and a succeeding frame; and said decoder unit further includes: spectral interpolator means (28, 33) for outputting interpolated spectral parameters interpolating for each pitch period said spectral parameters of said spectral code words of current and preceding frames; voice source interpolator means (38) for outputting interpolated voice source parameters interpolating for each pitch period said voice source parameters of said voice source code words of current and preceding frames; wherein said voice source generator (40) generates said voice source waveform for each pitch period on the basis of said interpolated voice source parameters, and said speech synthesizer means (42) producing said synthesized speech waveform for each pitch period on the basis of said interpolated spectral parameters and said voice source waveform output from said voice source generator (40). A vocoder device according to one of claims 1 to 4, characterized in that said encoder unit further includes: pitch period extractor means (51) for determining a pitch period length of said input speech signal; order determiner means (53) for determining an order in accordance with said pitch period length; and first converter means (55) for converting said spectral code words into corresponding spectral parameters, said spectral code words each consisting of a set spectral envelope parameters corresponding to a set of said spectral parameters; and said decoder unit further includes: second converter means (62) for converting said spectral code word retrieved by said spectral inverse quantizer means (30) from said second spectral code-book (30) into a set of corresponding spectral parameters of an order equal to said order determined by said order determiner of said encoder unit. A vocoder device according to claim 5, characterized in that said first spectral code-book comprises: a first auto-regressive (AR) code-book (7) storing a plurality of AR code words each corresponding to a set of AR parameters and identified by an AR code word identification number; and a first moving-average (MA) code-book (17) storing a plurality of MA code words each representing a set of spectral envelope parameters corresponding to MA parameters and identified by a MA code word identification number; said first converter means (55) converting said MA code words into corresponding MA parameters of said order determined by said order determiner means (53); and said second spectral code-book comprises: a second AR code-book (26) identical to said first AR code-book (7); a second MA code-book (31) identical to said first MA code-book (17); said spectral inverse quantizer means comprises: AR inverse quantizer means (25) for selecting from said second AR code-book (26) an AR code word corresponding to said AR code word identification number; MA inverse quantizer means (30) for selecting from said second MA code-book (31) a MA code word corresponding to said MA code word identification number; and said second converter means (62) converting said MA code word, retrieved by said MA inverse quantizer means (30) from said MA code-book (31), into a set of corresponding MA parameters of an order equal to said order determined by said order determiner (53) of said encoder unit.

Inventors: NAKAJIMA KUNIO, SEZA KATSUSHI, TASAKI HIROHISA, NAKAJIMA, KUNIO, SEZA, KATSUSHI, TASAKI, HIROHISA, Nakajima, Kunio, c/o Mitsubishi Denki K. K., Seza, Katsushi, c/o Mitsubishi Denki K. K., TASAKI, HIROHISA, C/O MITSUBISHI DENKI K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: G10L5
IPC codes: G10L19