In recent years there has been increasing interest in the use of digital methods to transmit speech signals over mobile communication networks. The advent of transistors has enabled the necessary equipment to be produced in a cheap and convenient form, combining the advantages of lightness, ease of operation, and low power consumption. The paper illustrates the use of transistors in this field by describing the more interesting design features of a 4-channel 5-digit pulse-code-modulation system employing signal compression and expansion. Consideration is also given to the transistor properties which limit the accuracy of such equipment, and it is shown that an 8-digit system is a practical proposition.