Analysis-synthesis telephony, with special reference to the vocoder

The requirements of a commercial speech-telephone system are considered from first principles and shown to lead naturally to the idea of analysis-synthesis telephony when bandwidth is the main criterion of cost, i.e. in practice, for long distances. The more thoroughly the analysis-synthesis principle is exploited the greater is the resulting potential band economy; the price paid is a restriction in versatility, but not any inherent loss in quality of the reproduced speech. The vocoder, first developed in America before the recent war, is an example of an analysis-synthesis system. Details are given of British Post Office experience in the design and performance of vocoders. Using a high-quality microphone, an articulation efficiency approximately the same as the highest reached on an ordinary commercial circuit has been reliably achieved, though the rather ¿inhuman¿ quality of the synthesized speech may be a disadvantage in some circumstances. A brief review of transmission systems which might be suitable for the vocoder is also given.