Optimum codec companding for high-speed PCM data transmission in telephone networks

Codecs used in telephone networks in the United States have a -law companding characteristic solely designed for the purpose of transmitting voice signals [1, 2]. These -law codecs are not well suited for the latest generation of high-speed digital voiceband modems requiring PCM data transmission at the quantization levels of the codec [3, 4, 5, 6, 7, 8, 9]. In this paper, we have shown that using a linear codec in the telephone network is optimal for PCM transmission (in terms of a minimum symbol error rate) for the range of SNRs observed over most modern telephone lines. We demonstrate that with a linear codec we get a dramatic improvement in the performance of these PCM modems without any increase in the complexity of the modem. In practice, it would be necessary and relatively simple to implement an additional feature in the -law codec to detect a voiceband modem during the initial phase of training and switch to a linear companding characteristic.