A Bit Rate Reduction Technique for Standard MELP Vocoder for a Possible Wireless Communications System’s Performance Enhancement

H. Gokhan Ilk, S. Tugac, A. Akbulut, and Z. Telatar (Turkey)

Keywords

speech coding, simulation andmodelling, MELP, time-scale modification, ChannelCharacterisation and Simulation

Abstract

Robust low bit rate speech coders are essential in commercial and military communication systems. They operate at fix bit rates and those bit rates can not be altered without major modifications in the vocoder design. In this paper we introduce a Scaled Speech Coder, which operates on time-scale modified input speech. The proposed method offers any bit rate from 2,400 bits/s to downwards without modifying the principle vocoder structure, which is the Mixed Excitation Linear Prediction (MELP) vocoder. We consider the application of transmitting MELP-encoded speech over noisy communication channels after time scale compression is applied. Computer simulation results, both source and channel, are presented in terms of objective speech quality metrics and informal subjective listening tests. Simulation results lead to a possible wireless communications system, whose performance might be enhanced by using the spared bits offered by the procedure.

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