Code excitation linear predictive (CELP) encoder and decoder and

Details Code excitation linear predictive (CELP) encoder and decoder and Code excitation linear predictive (CELP) encoder and decoder and

1995-02-09

1998-03-10

MacDonald, Allen R.

395 231, 395 225, 395 226, 395 227, 395 275, G10L 302

Patent

active

057271224

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD OF THE INVENTION

This invention relates to an encoder and a decoder based on the code excitation linear predictive coding (CELP) system.


BACKGROUND OF THE INVENTION

Conventionally, as a highly efficient coding system for a speech signal including an audible signal in the field of digital transportable communication systems, code excitation linear predictive coding and a modification, thereof have been used. The modification is a vector sum excitation linear predictive coding system (VSELP). The coding apparatus which uses the code excitation linear predictive coding (CELP) is disclosed in, for example, N. S. Jayant and J. H. Chen, "Speech Coding with Time-varying Bit Allocation to Excitation and LPC Parameters", Proc. ICASSP, pp 65-68, 1989.
A fundamental construction of a coding system relative to the speech signal obtains vocal tract parameters representing vocal tract properties and excitation source parameters representing excitation source information. In a recent CELP system, an excited signal as an excitation source information is encoded by means of both adaptive excitation code vectors, which contribute to a stochastically stronger periodic excitation signal, and stochastic excitation code vectors which contribute to a stochastic less periodic random excitation signal. Then the coded excitation signals are stored in a codebook, and optimum adaptive excitation code vectors and stochastic excitation code vectors are located in each codebook so that a weighted error power sum between an input speech vector and a synthetic speech vector becomes minimum. Then, whatever it is of a forward-type coding system which obtains vocal tract parameters from an input speech vector or of a backward-type coding system which obtains vocal tract parameters from synthetic speech vectors, at least the excitation source parameters, that is, adaptive excitation code and stochastic excitation code information are transmitted.
By utilizing the code excitation linear predictive (CELP) system as described above, it is known that high quality regenerated speech signals are obtained at a coding rate of 6 kbit/s to 8 kbit/s.
However, some communication systems require lower coding rate, for example 4 kbit/s or less. In such a lower coding rate, regardless of whatever the forward type which transmits both vocal tract parameters and excitation source parameters or the backward type which transmits excitation source parameters is used, the number of coded bits which are assigned to the excitation source parameters is smaller and the number of adaptive excitation code vectors stored in the adaptive excitation codebook and the number of stochastic excitation code vectors stored in the stochastic excited codebook become smaller. Consequently, the quality of the regenerated speech signal inevitably degrades at the lower coding rate as described above.
Besides, the adaptive excited codebook is adaptively renewed by synthetic code vectors of the optimum adaptive excitation code vectors and stochastic excitation code vectors and, accordingly, it can be determined that the adaptive excitation code vectors are formed on the basis of the stochastic excitation code vectors. Therefore, the current CELP coding has a poor tracking capability for a voice signal having a nature of strong periodicity. Consequently, the generated speech signal lacks clearness.


SUMMARY OF THE INVENTION

The present invention is based upon the foregoing problems and an object of the present invention is to provide a code excitation linear predictive coding encoder and decoder which can provide a high quality regenerated speech signal even when pulse-like noise components are contained in the input speech vectors.
Another object of the present invention is to provide a code excitation linear predictive coding encoder and decoder which can provide a high-quality regenerated speech signal even when a lower coding rate is employed.
According to the present invention, there is provided a code excitation linear predictive coding apparatus which uses,

REFERENCES:
patent: 5142584 (1992-08-01), Ozawa
patent: 5293449 (1994-03-01), Tzeng
patent: 5371853 (1994-12-01), Kao et al.
Proc. ICASSP, pp. 65-68, 1989, "Speech Coding With Time-Varying Bit Allocation To Excitation and LPC Parameters", N. S. Jayant, et al.
Proc. ICASSP, pp. 1681-1684, 1986, "High-Quality At Low Bit Rates: Multi-Pulse and Stochastically Excited Linear Predictive Coders", B. S. Atal.
Sekine et al., "A CELP Coder Using Fully-Searched Pulse Excitation," All Hitachi, Ltd., SA-5-3, 1993 Convention of Electronic Information Communication Society.
Gerson et al., "Vector Sum Excited Linear Prediction (VSELP) Speech Coding at 8 KBPS" Proceedings ICASSP 90 IEEE Signal Processing Society Apr. 3-6, 1990 pp. 461-464.

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