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The card reads:
"Empress PCM tandem exchange 1968.
After many years work, British engineers opted to use Pulse
Code Modulation (PCM), the principle behind all digital
electronics, in telephones exchanges. A trial PCM exchange,
the world's first, was installed at Empress telephone
exchange, west London, in 1968. It was successful amd led to
the development of the System X digital switching system
which became standard in Britain."
Photo: An Empress
PCM rack, Science Museum, London © Light Straw Oct 2014.
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