During the period from 1956 to 1966 the UK Government's Joint Speech Research Unit was conducting research into channel vocoders, culminating in a laboratory-built design suitable for evaluation by potential users over digital transmission networks at 2400 bit/s. The success of the basic vocoder design was such that it has since been engineered in various forms for widespread operational use, using different technologies as they have evolved. In view of the JSRU vocoder's continued competitiveness with other narrow-band speech coding techniques, such as linear predictive coding, this paper has been written to give a full specification of all the essential features of the vocoder configuration, with comments on the reasons for the design decisions and reference to supporting research where appropriate. The two most important factors contributing to this vocoder's successful performance are the use of narrow-band single-resonant circuits for the synthesis filters and the use of differential coding between channels in the digitisation process.