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Pulse Code Modulation

Foundation of digital telecommunications South East,

Empress PCM tandem exchange, 1968. Courtesy of BT Heritage.

Pulse-code modulation (PCM) was invented by Alec Reeves in 1937 while developing radio telephones for transatlantic calls. He wanted to make higher-frequency radios that could carry several calls at the same time, but these conversations interfered with each other.

Reeves realised that converting these waves – analogue representations of speech – into a digital form might avoid the troublesome interference. In this way, PCM was born. He designed circuits to measure the strength of each speaker's voice 8000 times a second and assign that signal strength to one of 32 levels. Each level was then represented by a sequence of five binary digits and all that was required was a receiver that could tell the binary 1s from the 0s to turn the stream of pulses back into interference-free speech.

However, there was no way to make this conversion of analogue to digital to analogue again economic using the valve technology of his day. His employer, at the Paris laboratories of International Telephone and Telegraph Corporation, ITT, patented pulse-code modulation, but made no royalties before the patent expired in the 1950s.
Reeves went on to work at Britain's vital scientific research establishments, the Royal Aircraft Establishment and the Telecommunications Research Establishment, before he was put in charge of exploratory research at the Standard Telecommunications Laboratories in Harlow, Essex. In that role, he launched a group to study laser communications, and supported work by Charles Kao that led to the fibre-optic network that today carries pulse-code modulated light signals across the planet. Today PCM is standard for digital audio in computers and various formats, from Blu-ray to DVD, and Reeves is recognized as the master architect of the information age.

Science Museum


Engineering, Computing,
South East
Key Individuals
Alec Reeves,