A flexible bit rate switching method for low bit rate vocoders

Robust low bit rate speech coders are essential in commercial and military communication systems. They operate at fixed bit rates and bit rates cannot be altered without major modifications in the vocoder design. In this paper we introduce a novel approach to vocoders by coding the time-scale modified input speech signal. The proposed method offers any bit rate from 2400 to 720 bits/ s without modifying the vocoder structure. Simulation results, which mainly concentrate on intelligibility, talker recognisability and voice quality versus codec complexity and delay, are also presented. The proposed scaled speech coder delivers communication quality speech at half the bit rate of the new US Federal Standard, mixed excitation linear prediction vocoder.