Yesterday I was reading about SIGSALY, the WW2 encryption system used by the allies and I'm kind of staggered they were doing digital, encrypted speech transmission this early? Used vocoders just a couple of years after they were invented, and a one-time pad that was stored on an identical pair of vinyl records? Wild.
There's a simulation on this page of what SIGSALY probably sounded like. It seems barely understandable, and they required 40 racks of equipment on each end just to get this level of quality.
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