Last month I was puzzled by an unconventional version of a familiar spy tech story. In a 1949 Passing Parade episode, John Nesbitt said:

Again in the late 30s, just before the world blew up again, our country was infested with enemy spies. And yet somehow we had invented a code machine for sending our own messages, which the immense intelligence forces of Hitler failed to figure out. That machine, still I believe a secret, is said to be a sort of thinking machine based upon the human brain.

I’ve been looking around since then, and I think the answer is SIGSALY, patented in 1941 by Bell Tel, using Bell’s recently developed vocoder technology. The patent was kept secret by the Patent Office, and finally declassified and published in 1976.

Previous speech scramblers simply modulated the speech audio with a 2500 cps tone, which made the speech hard to hear but not really incomprehensible. (The voice of Daleks was done with this type of modulation.)

SIGSALY was part analog and part digital. It sampled the incoming speech 50 times a second and did an analog spectral analysis with a bank of 9 filters. The outputs from the filters were then turned into a sequence of digital pulses, more or less like Morse. Each 20 millisecond interval included a count from each of the 9 frequencies. The pulses were turned into an audio pattern using frequency-shift keying.

This sequence would be easy enough to decode if the enemy knew it was a spectrum, but the sequence was then mixed with a pre-recorded sequence of noise before it was sent over phone or radio channels. The noise was the real trick. Every message had a new noise-mix. The SIGSALY crew had created a huge stack of records, each with a different noise pattern. Each record was played exactly once and then destroyed.

The receiving end had copies of the same stack of records, and played them in the same order to demodulate the incoming signal. The pulse counts were then fed into the inverse of the input filter: a set of 9 sine oscillators. A larger pulse count in one frequency segment mixed in a larger proportion of that frequency.

In other words, SIGSALY was an audio equivalent of the classic one-time pad text code.

Several SIGSALY setups were built. One was in the Pentagon serving the US military and the White House. Another was in the basement of Selfridge’s department store in London, serving Churchill. Others were in semi-trailers or ships, but the unit was definitely not portable, and couldn’t be airdropped into a combat zone.

The setup looks more like a radio station than a computer.

Especially the turntables for the one-time audio pads! [Note the supersecret milspec needle-cleaning equipment.]

From the government story:

While the Germans monitored the system, they never succeeded in breaking it. The details of SIGSALY’s technical features remained classified until 1976. The best proof perhaps is a telling 1943 statement by the once successful Deutsche Reichspost on the future possibilities of intercept of high-level Allied communications, “there is not much to be gotten from them now.”

This sounds a lot like Nesbitt’s narrative.

Even better, the spectral analysis system actually did work like a human brain, which wasn’t known in 1941. The hair cells of the cochlea analyze the spectrum of incoming sound and send their relative spectral strengths to the brainstem AS PULSE CODES.

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Footnote: The SIGSALY story is yet another disproof of the STUPIDEST STATEMENT IN THE WORLD. What’s the STUPIDEST STATEMENT?

“Secret conspiracies are impossible because large groups can’t keep secrets.”

The parallel Enigma project already disproved the STUPID. Enigma involved about 10000 people in Britain and America. Only the top leaders were professional spies trained and committed and paid for secrecy. All the rest were civilians from academia or business who were called up for the duration and returned to normal life in 1945. ALL of the civilians kept the secret perfectly, until one of the leaders decided to declassify and publish in 1973. The SIGSALY project was smaller, about 1000 civilians, mainly from Bell. They returned to normal life and kept the secret, until one of the leaders declassified in 1976. Huge groups of ordinary non-professional people kept a secret for 30 years with no pay or encouragement. They didn’t even hear about the declassification through official channels, so most of them just stayed silent.