Telephone exchange regenerative repeater, c 1980s.
© Science Museum / Science & Society Picture Library
This device would have been used in a 30 channel PCM (pulse-code modulation) system on a telephone exchange. The regenerative repeater is part of a circuit that detects incoming pulses, and re-transmits stronger and more sharply defined output pulses. Digital speech signals travel as interleaved, coded samples. Every few kilometres, the signal is pased through a repeater circuit that examines it, decides whether each digit is a 0 or a 1 (in binary code) and generates a new signal accordingly. In this way, it helps to prevent any noise or distortion from accumulating, even over long paths.