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Pulse-coded modulation.

In pulse-coded modulation (PCM), the intelligence signal converts the carrier into a series of constant-amplitude pulses spaced in such a manner that the desired intelligence is contained in coded form. Continuous signals, such as voice messages, television pictures, and computer data, are commonly transformed into the Baudot Code or its variations, which are composed of patterns of 5 or 7 "on" and "off" pulses. PCM minimizes transmission losses and eliminates noise and interference problems because the receiving unit need only detect and identify simple pulse patterns. Moreover, the pulses, unlike continuous signals, can be regenerated electronically by repeater stations along the transmission route with virtually no distortion.

PCM, invented by H.A. Reeves of the United States in 1939, is employed by many communications companies and organizations, including Comsat and Intelsat, for telegraph, telephone, and television transmission. The technique has proved especially useful for the exchange of digital information between computer terminals.

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