Excitation codebook search method in a speech coding system

Abstract

A method for searching an excitation (or fixed) codebook in a speech coding system. In a speech coding system including a synthesis filter for synthesizing a speech signal, a fixed codebook searcher according to the present invention segments a speech signal frame into a plurality of subframes to generate an excitation signal to be used in a synthesis filter, segments again each of the subframes into a plurality of subgroups, and searches the respective subframes each comprised of a plurality of pulse position / amplitude combinations for pulses. The fixed codebook searcher searches the respective subgroups for a predetermine number of pulses having non-zero amplitude, and generates the searched pulses as an initial vector. Next, the fixed codebook searcher selects a pulse combination including at least one pulse among the pulses of the initial vector, and then substitutes pulses of the selected pulse combination for pulses in other positions in the subgroups. The selection and the substitution are repeatedly performed on all the pulses of the initial vector.

Description

PRIORITY [0001] This application is a continuation of U.S. application Ser. No. 10 / 155,272, filed May 23, 2002, and claims priority to an application entitled “Excitation Codebook Search Method in a Speech Coding System” filed in the Korean Industrial Property Office on May 23, 2001 and assigned Serial No. 2001-28451, the contents of which are incorporated herein by reference.BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates generally to a speech coding system, and in particular, to a method for searching an excitation codebook. [0004] 2. Description of the Related Art [0005] There are several types of vocoders, which compress speech signals. A vocoder typically used in a current mobile communication system is a CELP (Code Excited Linear Predictive coding) vocoder based on a liner prediction technique. The CELP vocoder is divided into a linear prediction filter for managing a linear prediction operation and a section for generating an ...

AI Technical Summary

Benefits of technology

[0015] It is another object of the present invention to provide a codebook search method with improved coding performance in a speech coding system.

[0016] To achieve the above and other objects, the present invention provides a new codebook search method. The codebook search method first searches for positions and amplitudes of a desired number of initial pulses, and then repeatedly exchanges the positions of or the positions and amplitudes of a predetermined number of pulses, thereby updating positions of new pulses. A cost function value calculated by the new codebook search method shows better results compared with the cost function value calculated by the conventional ACELP technique, resulting in an improvement in speech quality of a vocoder.

Problems solved by technology

The CELP technique using the physical codebook requires a large amount of memory and takes a great deal of time to search the codebook.

Compared with the ACELP technique, an LD-CELP (Low-Delay Code Excited Linear Predictive coding) technique using a stochastic codebook is susceptible to the channel error, since even a single-bit error of a codebook index affects the overall excitation signal.

However, whether this assumption is reasonable is uncertain.

This method, however, increases calculations, so it is not possible to securely search for a code vector having a higher cost function value than the previously searched code vector, although the codebook is searched in various ways.

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