Speech coding using sparse vector codebook and cyclic shift tech

Details Speech coding using sparse vector codebook and cyclic shift tech Speech coding using sparse vector codebook and cyclic shift tech

1989-05-09

1991-02-05

Harkcom, Gary V.

Electrical audio signal processing systems and devices

One-way audio signal program distribution

Public address system

G10L 702

Patent

active

049912143

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION

A common technique for speech coding is the so-called LPC coding in which at a coder, an input speech signal is divided into time intervals and each interval is analysed to determine the parameters of a synthesis filter whose response is representative of the frequency spectrum of the signal during that interval. The parameters are transmitted to a decoder where theiy periodically update the parameters of a synthesis filter which, when fed with a suitable excitation signal, produces a synthetic speech output which approximates the original input.
Clearly the coder has also to transmit to the decoder information as to the nature of the excitation which is to be employed. A number of options have been proposed for achieving this, falling into two main categories, viz.
(i) Residual excited linear predictive coding (RELP) where the input signal is passed through a filter which is the inverse of the synthesis filter to produce a residual signal which can be quantised and sent (possibly after filtering) to be used as the excitiation, or may be analysed, e.g. to obtain voicing and pithc parameters for transmission to an excitation generator in the decoder.
(ii) Analysis by synthesis methods in which an excitation is derived such that, when passed through the synthesis filter, the difference between the output obtained and the input speech is minimised. In this category there are two distinct approaches: One is multipulse excitation (MP-LPC) in which a time frame corresponding to a number of speech samples contains a, somewhat smaller, limited number of excitation pulses whose amplitudes and positions are coded. The other approach is stochastic coding or coded excited linear prediction (CELP). The coder and decoder each have a stored list of standard frames of excitations. For each frame of speech, that one of the codebook entries which, when passed through the synthesis filter, produces synthetic speech closet to the actual speech is identified and a codeword assigned to it is sent to the decoder which can then retrieve the same entry from its stored list. Such codebooks may compiled using random sequence generation; however another variant is the so-called `sparse vector ` codebook in which a frame contains only a small number of pulses (e.g. 4 or 5 pulses out of 32 possible positions with a frame). A CELP coder may typically have a 1024-entry codebook.
The present invention is defined in the appended claims.
Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:


BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1(a-c) illustrate three typical members of a set of cyclically related excitations to be used in the invention;
FIG. 1(d) shows a single excitation representing the excitations shown in FIGS. 1(a-c);
FIG. 2 is a block diagram of one form of speech coder according to the invention; and
FIG. 3 is a block diagram of a suitable decoder.


DESCRIPTION OF THE PREFERRED EMBODIMENTS

It will be appreciated from the introduction that multipulse coders and sparse vector CELP coders have in common the features that the exciation employed is in both cases a frame containing a number of pulses significantly smaller than the number of allowable positions within the frame.
The coder now to be described is similar to CELP in that it employs a sparse vector codebook which is, however much smaller than that conventionally used; perhaps 32 or 64 entries. Each entry represents one excitation from which can be derived other members of a set of excitations which differ from the one excitation --and from each other--only by a cyclic shift. Three such members of the set are shown in FIGS. 1a, 1b and 1c for a 32 position frame with five pulses, where it is seen that 1b can be formed from 1a by cyclically shifting the entry to the left, and likewise 1c from 1a. The amount of shift is indicated in the figure by a double-headed arrow. Cyclic shifting means that pulses shifted out of the left-hand end wrap around and reenter

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ICASSP 86 (IEEE IECEJ-ASJ International Conference on Acoustics, Speech and Signal Processing, 7-11 Apr. 1986, Tokyo, JP), vol. 1, IEEE (New York, U.S.) L. A. Hernandez-Gomez et al.: "On the Behaviour of Reduced Complexity Code-Excited Linear Prediction (CELP)", pp. 469-472.
ICASSP 87 (International Conference on Acoustics Speech and Signal Processing, 4-6 Apr. 1987, (Dallas, U.S.), vol. 3, IEEE (New York, U.S.) D. Lin: "Speech Coding Using Efficient Pseudo-Stochastic Block Codes", pp. 1354-1357.

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