Adaptive differential PCM speech transmission

Because of the preponderance of long distance calls in the Defense Communication System, a significant tradeoff exists between the transmission cost of a digital speech channel and the terminal (or processing) cost required to translate speech into digital form. Preliminary cost estimates have indicated a significant overall system savings will occur even if relatively expensive terminals are employed provided the digital speech channel rate can be reduced from the conventional 64 kBps (PCM) to 10 kBps. This study is devoted to analyzing the potential of adaptive differential PCM techniques for reducing the bit rate to the 10 kBps range. It examines ADPCM in which only the quantizer is adapted, both theoretically and via simulation. In addition, ADPCM in which both the quantizer and the predictor operate adaptively is investigated, with emphasis on simple systems rather than on LPC class systems. (Modified author abstract)