An adaptive-rate digital communication system for speech

Current digital voice communication systems allow only modest levels of protection of the coded speech and often do not follow the dynamic changes that occur in the transmission channel. We present a method that provides optimal voice quality and intelligibility for any given transmission channel condition. The approach is performed via adaptive-rate voice (ARV) coding using an adaptive-rate modem, channel coding, and a multimode sinusoidal transform coder. In general, the receiver utilizes channel state information to not only optimally demodulate and decode the currently corrupted symbols from the channel, but also to inform the transmitter, via a feedback channel, of the optimal strategy for voice/channel coding and modulation format. We compare several source-channel coding schemes at multiple transmission symbol rates and compare the performance to fixed aggregate-rate channel-controlled variable rate voice coding systems