1939: Pulse Code Modulation

Discoveries occasionally occur long before the current technology is ready to accept and support them. This is certainly the case with British physicist Alec Harley Reeves, who proposed Pulse Code Modulation (PCM), one of the cornerstone principles of digital audio, more than four decades before the first commercial digital audio recorder.
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Alec Harley Reeves

Discoveries occasionally occur long before the current technology is ready to accept and support them. This is certainly the case with British physicist Alec Harley Reeves, who proposed Pulse Code Modulation (PCM), one of the cornerstone principles of digital audio, more than four decades before the first commercial digital audio recorder.

In his 1939 patent for a Signaling System, Reeves describes a “system for transmitting complex waveforms—for example, speech—wherein the waveform is scanned at the transmitter at predetermined instants” and the “amplitude range of the waveform to be transmitted is divided into a finite number of predetermined amplitude values according to the fidelity required.” He also proposed a tube-based PCM circuit, but this was in 1939—well before the introduction of transistors, ICs or microprocessors—and Reeves’ PCM proposal remained mostly unused until the 1950s, when it was employed in low-fidelity applications such as telephone systems.