Effects of Channel Errors on the Signal-to-Noise Performance of Speech-Encoding Systems

The suitability of digital coding and transmitting speech signals for mobile telephone systems is a question of current interest. In UHF systems, Rayleigh fading causes the carrier-to-interference ratio and the carrier-to-noise ratio to be low in frequent intervals. This leads to high bit-error probabilities in the transmission of the coded signal; the errors occur in bursts. This paper compares the effects of channel transmission errors on the objective signal-to-noise ratio (s/n) for different encoding schemes. Most of these schemes use an adaptive quantizer with time-varjdng step sizes or, equivalently, a time-varying gain control of an amplifier in front of a quantizer with fixed step sizes. The side information about the step size (or about the amplifier gain) is derived from stored samples of the input signal and has to be transmitted together with the message block of coded samples (adaptive quantization with forward estimation = AQF). These AQF schemes have an excellent idle channel performance, even in the presence of channel errors, if the side informa1615