A transistorized pulse code repeater

Two methods are available for the simultaneous transmission of several telephone conversations on the same radio beam or cable. These are frequency division multiplex (FDM) in which different conversations are assigned different portions of the electromagnetic spectrum, and pulse time modulation (PTM) in which different conversations are sampled at intervals and the samples interleaved in time for transmission. Among the methods of pulse time multiplex are pulse position modulation (PPM), pulse width modulation (PWM), pulse amplitude modulation (PAM), and pulse code modulation (PCM). This last method is perhaps the most elegant of all, and the most complicated in terms of the necessary terminal equipment. The advantages procured by this increased complexity are a relatively high immunity to noise in the transmission system and, of special importance in some applications, the possibility of encrypting the signals for greater privacy by means other than the usual ??scrambler.?? Under many circumstances, PCM systems will be less expensive and more reliable than other multiplex methods.