Science Fair Project Encyclopedia
Alec Reeves (1902-1971) invented pulse-code modulation PCM in 1937 when he worked for the International Telephone and Telegraph Company. This is a very early invention in the history of electronics, since it is only a few years after Edwin Armstrong invented wideband FM, a method of high-quality radio broadcasting. Reeves, instead of following tradition by sending an electrical current being proportional with the sound level, proposed that the electrical sound signal be sampled and digitised at regular intervals. Then the analogue value of each sample would be rounded to the nearest integer value, which, in turn, is represented by a binary number and transmitted as binary on-off pulses. In principle, the binary, two-level, signalling was a return to the simple, robust technique used by the telegraph. Noise immunity and fidelity benefit tremendously because the sound signal is no longer stored as a delicate analogue signal but as a much more robust sequence of binary numbers. Because PCM is a method of representing an analogue signal in digital form, it is particularly well adapted to work directly with digital data-processing equipment.
In his 1937 patent, Reeves formulated the major advantages of digital PCM transmission, namely
- Quality depends on conversion steps ONLY
- Quality independent of transmission media
- Compatibility with different media and traffic (video, audio, and data)
- Low cost
- New features can easily be embedded.
These are gigantic and also visionary conclusions. Reeves showed his enormous engineering foresight, as there are two essential assumptions that are implicit in the above characteristics. Firstly, each quantized sample can be transmitted with arbitrarily small probability of error. It was absolutely not clear in 1937 that this could be accomplished in theory; let alone that he, or others, knew about practical methods for achieving error-free transmission. There was no research, not even by an obscure Russian mathematician, on the topic of error correcting codes. It would take another ten years and a world war, before research on error free digital communication would take off. Secondly, he assumed that conversion from the analog to the digital domain, and vice versa, could be done, either in theory or practice, with arbitrary small accuracy by use of sufficiently frequent sampling, and by quantizing each sample with a sufficiently large number of levels. Early theoretical work by mathematicians had been published, but we may well assume that Reeves was unaware of that literature.
A notable disadvantage of PCM is the required high (analogue) bandwidth of the transmission or storage system. And though Reeves' extraordinary patent of 1937 showed how this might be done in theory, the valve-based technology of the time was not up to the job. Pulse Code Modulation could not be implement economically until the invention of the transistor decades later. But economy was not always a priority. PCM was first used by the armed forces for the scrambled radio system on which Churchill and Roosevelt talked in total secrecy for much of World War 2, SIGSALY.
He was awarded over 100 patents, as well as a CBE.
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