Data compression of speech signals using time code modulation

The authors previously (1988, 1990) proposed a uniform amplitude coder as an alternative to traditional time domain coders. The encoder of time code modulation (TCM) transmits the quantized intervals corresponding to the time at which the input signal passes through uniform spaced amplitude levels (of a predetermined step size). The decoder reconstructs the signal by using linear interpolation in the amplitude domain. Allowing for a variable rate of transmission, TCM systems can effectively be used with packet switched networks. The sampling frequency and step size are shown to be the two dominant factors that affect TCM performance; the higher the sampling frequency and the smaller the step size the higher the SNR that can be achieved by TCM, but that also the higher the transmission rate. In order to determine the optimum of each factor, a comparison is conducted on the sampling theorems between time and amplitude domains. Several parallels between Nyquist-based PCM and TCM systems are described. Simulation results are presented