This paper describes a pulse-code modulator, using junction transistors, designed to provide a 5-digit transmission of a signal having a total bandwidth of 800 c/s. The system described uses the pulse-count method as being the most straightforward way of producing a digital code from an a.c. waveform. A monitor unit is included to decode any required number of digits of the output signal so that system-evaluation tests can be carried out.Temperature tests are also described, the results of which show that the equipment will operate satisfactorily up to a temperature of at least 65° C.