Method for vector quantizing speech signals

Details Method for vector quantizing speech signals Method for vector quantizing speech signals

1998-05-18

2001-01-16

Zele, Krista (Department: 2748)

Data processing: speech signal processing, linguistics, language

Speech signal processing

For storage or transmission

Reexamination Certificate

active

06175817

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a method for coding of signal scanning values, making use of vector quantization and, more particularly, to a method of coding speech signals by vector quantization.
A CELP speech coding method is known from “Speech Communication” 8 (1989), pp. 363 to 369, wherein the coder parameters are optimized together. In comparison with sequential optimization, it is possible to considerably reduce the length of the excitation codebook.
A digital speech coder is known from WO 91/01545, wherein excitation vectors entered in a codebook are accessed for selecting an excitation vector which best represents the original speech scanning value. Two excitation vectors from two respective codebooks are employed for describing a scanned speech value in the speech coder in accordance with WO 91/01545. First, a first excitation vector is selected there independently of pitch information. The second excitation vector is selected in a corresponding manner. During orthogonalization of the second excitation vector from the second codebook, the resulting vector as well as the first selected excitation vector from the first codebook are taken into consideration. This selection process is then repeated with an orthogonalized excitation signal from the second codebook in order to finally identify those excitation vectors which best match the original speech scanning value.
SUMMARY OF THE INVENTION
It is the object of the instant invention to increase dependability in the selection of the optimized scanning value without too greatly increasing the processing effort and expense.
According to the invention, the method for vector quantizing of speech signals includes:
a) entering fixed excitation vectors of an LPC filter for speech prediction in a first codebook;
b) entering excitation vectors of a pitch synthesis filter in a second codebook;
c) modifying the excitation vectors in the second codebook (CB
2
) according to each speech sample sub-frame;
d) establishing a predetermined error criterion for selection of excitation vectors from the second codebook;
e) selecting at least two excitation vectors from the second codebook to obtain in optimum prediction value according to the predetermined error criterion;
f) linking the at least two excitation vectors selected in step e) with a number of excitation vectors from the first codebook to form a set of linked vectors; and
g) selecting a resulting linked vector having a minimal variation from the speech signal according to a predetermined variation parameter.
There are several preferred embodiments of the method according to the invention. The predetermined variation parameter may be the same as the predetermined error criterion or different from it.
In a particularly preferred embodiment the method also includes thinning out the fixed excitation vectors in the first codebook. This thinning can occur by suppressing vector components taken from sum bits of two frame sections into which the speech signal is divided. The thinning out of the first codebook, in some embodiments, occurs to the extent that processing efforts are approximately as great as processing efforts would be with no thinning out and with only one selected excitation vector from the second codebook.
Advantageously the error or deviation of each excitation vector in the first codebook with respect to the speech signal can be determined considering the at least two pitch predictors selected from the second codebook.
The invention is based on the following realizations: If, in contrast to the known methods (as described in the prior art references, “Speech Communication” 8 (1989), pp. 363 to 369 or WO 91/01545), more than one vector with a minimal error from the adaptive (second) codebook is employed for linking with all vectors of the first (fixed) codebook, the processing effort (calculation effort) will increase, but the dependability in the optimization of the scanning value with the least error is increased. This increase of dependability means an increase in the speech quality when processing speech scanned scanning samples. Since, when taking into consideration more than one vector from the adaptive codebook, the processing effort increases less greatly than linearly, it is possible with a moderate reduction of the fixed codebook, for example by codebook thinning (frame thinning) in accordance with the U.S. patent application Ser. No. 08/530,204, filed Sep. 25, 1995, entitled “Method for Preparing Data, in particular Encoded Speech Signal Parameters”, by the inventors of the present invention to keep the processing effort approximately constant, wherein the original codebook length without thinning is made the comparison basis. It is possible to obtain considerably better speech quality by means of the steps of the invention along with approximately the same processing effort as in conventional methods.


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“Improving performance of Code Excited LPC-Coders by Joint Optimization”, Muller, Speech Communication, Jun. 15, 1989.
“Improvements to the analysis by synthesis loop in CELP code”, Radio Receivers and Associated Systems, Woodard et al., Sep. 1995.
“Pitch Sharpening for Perceptually improved CELP, and the spa”, ICASSP '91, Taniguchi et al, Jul. 1991.

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