Techniques for improving the robustness of an adaptive predictive coder in the presence of channel errors

A study was performed to evaluate various techniques for improving the robustness of an Adaptive Predictive Coder operating in the presence of channel errors. The communications environment consisted of a packetized, symbol oriented system in which channel errors were manifested as both 5-bit symbol errors in the received bit stream and the occasional loss of an entire packet of data. To reduce the effects of symbol errors forward error correction was applied to a selected subset of the transmitted parameter bits. At the receiver, smoothing strategies were employed to replace those parameters in which errors could be detected but not corrected and to cope with situations in which entire packets were lost. The introduction of these techniques to the basic APC vocoder added 900-1100 bps to the bit rate and resulted in significantly improved robustness in channel errors.