What is SIGSALY and how does it work?

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Offline Pumblechook

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What is SIGSALY and how does it work?
« on: 07/11/2008 18:35:28 »
Anyone know how SIGSALY worked? I thought it was analogue - simple addition of noise and then subtraction at the receiving end but it looks as if it was 'digital'.
« Last Edit: 07/11/2008 19:50:50 by chris »



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What is SIGSALY and how does it work?
« Reply #1 on: 09/11/2008 11:25:02 »
wikkers describes thebasics of the system. It was pretty much the earliest use of PCM, so it was a digitally transmitted signal. It also involved the use of a vocoder (analogue) which was a very crude method of information rate reduction, involving analysing the sound spectrum of the speech into bands and then, rather than sending the original sound signal, sending the (6 level, digital, sampled) levels of energy in each band. The random element was a true random sequence of digits - recorded and distributed to all receiveing stations on disc - which was added in the coder and subtracted in the decoder (modulo 6 arithmetic, apparently).
It must have sounded truly awful.
The 'one time pad' is an excellent method of encryption but you need a very secure channel to send this key- i.e. transferring it by hand.
The random signal was generated in an analogue electronic device, then sampled and digitised into six levels.
The amount of digital circuitry had to be kept to a minimum - hence the use of an analogue random signal and the very simple digital codec with analogue at each end.
A nightmare to keep it running, involving a large bucket of replacement valves.

« Last Edit: 09/11/2008 11:27:36 by sophiecentaur »