Channel and source considerations of a MELP derived vocoder operating at reduced bit rates

Abstract

Robust low bit rate speech coders are essential in commercial and military communication systems. They operate at fixed bit rates and those bit rates cannot 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 2400 b/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. A statistical tool called bootstrap is also used to determine the accuracy of these test results. Design parameters such as codec complexity and delay are also investigated.