The Effects of Digital Errors on PCM Transmission of Compandored Speech

Abstract

The recent interest in the use of pulse code modulation (PCM) for the transmission of speech has made it desirable to determine the effects of digital line errors on certain codes. Improved performance for low-level talkers has been generally obtained by the use of instantaneous compressors and expandors (compandors) in order to avoid using a large number of digits in the PCM coder-decoder (codec). A comparison of the effects of digital errors for a logarithmically compandored system transmitting speech is made on the basis of mean square distortion power and the distribution of error magnitude. Results are obtained for the binary, Gray (reflected binary), and folded binary codes. The results indicate that the binary and Gray codes have a “click” as well as a “noise” component of distortion while the folded binary code produces only noise at low-talker levels. “Clicks” have been defined as errors which have amplitudes greater than half the full range amplitude; the remaining errors are considered “noise”. For the folded binary code, the most significant digit gives polarity information; the remaining digits represent the signal magnitude in binary code.