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Pulse Code Modulation (PCM) is a method of recording sound as digital data. The amplitude of the audio signal is sampled at a regular sampling rate and quantized with a fixed bit depth.

This technology was created by Alec Reeves in 1937.

A Page History On PCM

Taken from here: http://web.archive.org/web/20030406235542/http://homepage.ntlworld.com/quantium/ahr/pcm.htm

In the 1930s the necessity to convert analogue data to digital format for computer use was unknown - there were no computers capable of using it. However the idea was seen by Alec Reeves as a means of communicating with perfect fidelity - no errors could arise in a system based on ones and noughts. In 1938 he filed a patent in France that introduced the concept of digital communication which he called Pulse Code Modulation. It is now commonplace for computers, radio, television, CDs, recording and so on.

Although the idea had military application for coded transmissions, it was totally impractical for commercial use as it required racks of valves to implement. By the time transistors and integrated circuits made it practicable, the patents had run out.

When Alec Reeves invented pulse code modulation in 1937 a prediction that the GPO would issue a commemorative postage stamp on the subject in 1969 would probably have struck him as unlikely.

The significance of the invention, although clear enough to Reeves, was not widely appreciated at the time. Furthermore, it was to be many years before the advent of the transistor would make it a practical proposition. Even in 1969, well over a quarter of a century later, it is only just beginning to come into widespread use in the UK telecommunications network.

Reeves and many others in telecommunications became convinced that PCM and associated digital systems eventually will be the principal methods of transmitting not only speech, but all forms of information from one point to another by wire, radio, or optical links. In most electronics laboratories in the 1960s onwards prediction of the future was being taken perfectly seriously and what might have been described as "science fiction thinking" a year or two previously was then considered to be legitimate scientific activity. Alec Reeves has given a good deal of thought to the future of telecommunications when asked to give lectures in the late 1960s and came up with some fascinating prospects.

For his work on PCM he won The Ballantine Medal of the Franklin Institute (1965) The City of Columbus Gold Medal (1966) and PCM was commemorated on a shilling (5p) postage stamp (1969)