A Companded Coder for an Experimental PCM Terminal

An earlier paper 1 has described the framework of an experimental pulse code modulation (PCM) system for short-haul telephone trunks. The system consists of two 24-channel PCM terminals interconnected by two regenerative-repeatered lines. Within each terminal are compressor-encoder and decoder-expandor functional blocks that constitute a "companded coder system." The latter all-semiconductor system, which performs analog-to-digital and digital-to-analog conversions, is the subject of this paper. The compressor-encoder block, in the transmitting portion of each terminal, accepts 7.4-microsecond bipolar sample pulses from the multiplex circuits. During a particular 4.5 microseconds of each pulse duration, the amplitude of the sample pulse is accurately compressed in accordancc with a desired law, measured to the nearest half-quantum step out of 128, and converted to a pattern of seven ^-microsecond on-off pulses in accordance with 7-digit binary notation. A succession of the latter pulse patterns, constituting a megabit-per-second P C M signal, is finally transmitted to another terminal. 173