Speech coding at very low bit-rates for mobile communication

In this paper we discuss the various basic speech coding techniques such as waveform coding, parametric coding and the quantization schemes. We also review the enhanced waveform interpolative coding technique in detail. The EWI coding technique for low bit rates with several enhancements like analysis by synthesis (AbS) optimization of slowly evolving waveform (SEW), rapidly evolving waveform (REW) parametrization, REW quantization, etc. proves to be very efficient for mobile communications. Also discussed are enhanced post-filtering and a novel pitch search technique for speech enhancement. The subjective test results have indicated that the quality of the 2.8 Kb/s EWI exceeds that of the G.723.1 at 5.3 Kb/s. Based on the results, we conclude that speech coding low bit-rates especially the EWI coder has enormous vistas in future 4G mobile systems, Internet telephony, LEO systems, etc.