I've seen multivibrators as frequency dividers in the repair work I've done. The clever thing here would be in the initialization, and synchronization at the receive end. I'd imagine that wheel has to be spun up to some fraction of 4000 rotations/second and uses photodiodes or perhaps just contact, to generate the starting pulses.
This reminds me: are there even any good scans of the Portsmouth Papers that Keynes bought? Or is this stuff still just buried in various museum archives, seen only by certified friends of the museum's director?
It seems it already is owned by a private party, which is essentially the same as a personal collection.
If it was a real A to D with digital encryption, it would have needed far more tubes. The digital side would have had to be all electronic. It wasn't. Look at the picture. That's an Enigma mechanism with a very few tubes out back.
Something like that would have sync problems. Not clear what they used as a time base for the demo device. Maybe for demo purposes they just used the power line as a clock.
Before SIGSALY, there was a pre-war Western Electric system, the A-3 [1], used for transatlantic phone calls. That had only five channels, advanced the key only every 15-20 seconds, and only had six channel-swapping patterns. The A-3 was a very low grade encryption system, and was broken by the Germans.
SIGSALY was the A-3 concept scaled up. More channels, faster key changes, additional analog processing. It filtered analog voice into ten channels, and swapped the channels around based on a one-time key stored on phonograph records. This seems to have taken way too much hardware. But it had redundancy, each direction was separate, and there was a pseudorandom key generator for testing without using up one-time keying material, all of which added bulk.
Audio speed analog to digital converters were invented in 1937 by Alec Reeves.[2] But they were not used until after WWII. A usable voice-speed A to D seems to have been beyond the capabilities of WWII electronics. Reeves had the basic concept - a counter, a ramp generator, and a comparator. All of those were very hard to do in 1937.
Reeves is an obscure figure. He invented pulse-code modulation as well as the A to D converter. He spent WWII working on aircraft navigation systems rather than encryption. If he'd been involved in encryption, high-security WWII voice encryption probably would have been all-digital. After the war, he was involved in the beginnings of fiber optic links, so he did quite a bit.
[1] http://chris-intel-corner.blogspot.com/2012/02/intercepted-c...
The 7-tube "combiner" is clearly analog. The key generator seems to be three multivibrators phase-locked with relatively prime ratios. That's cute. It's a pseudo-congruential random number generator in the analog domain.
This exploits the effect that oscillators that are weakly linked will tend to come into sync. Not necessarily at 1:1; you can sync at various ratios, with difficulty. Must have been a pain to get to work reliably. I'd like to see the schematics.
With phase-locked loops and counters this would be easy; that's how modern radio tuners work. Not easy to make reliable with 1940s technology. Getting things to sync up was a huge headache until the 1970s.
[1] https://www.hmgcc.gov.uk/news/turing-s-rebuilt-delilah-machi...
That seems curious -- surely the military would have been interested in a finished portable voice encryption system, even after the war?
However, in 1952 Turing was convicted of being gay and that meant he lost his security clearance. His name became anathema, and a lot of his projects were immediately buried or forgotten.
What is your opinion on visiting Bletchley Park? Is it worth it? Or is it more oriented to kids and whatnot?
Both museums do have some kid-friendly activities, but their focuses are very different. You might wish to visit one, or both, and you should figure that out before going as they aren't even necessarily open at the same time.
TNMOC is about Computers generally, but has some exhibits about Enigma including a Bombe reconstruction and then of course Colossus - to break the Lorenz cipher, only exists due to the war and would have been at Bletchley. If you don't much care about Computers that's not too interesting, maybe worth a half hour if you've time.
The main Bletchley Park museum is about Codebreaking and particularly Ultra, the secret project to break German codes, most famously Enigma but also Lorenz and others, at Bletchley Park in WW2. It has some exhibits about spycraft, and a lot more about the practical undertaking of this codebreaking. Who are these people, what are they all doing here, what was their life like? If you care about the people you will want to visit this museum, but it has relatively little about the technical nuances of what was done.
Another thing about the place is that modern archeology doesn't favour restoration of everything. Limited amounts of structural change happen. So a lot of the site looks like dismal ww2 badly poured concrete, not well looked after. Since GCHQ occupied the space for some time, some of it is probably post war, and historically relevant to different stories.
Also when I was last there, The National Museum Of Computing had free parking, but Bletchley Park museum charged for their car park, even though it is right next door. You could park at the computing museum car park and visit either or both.
Alan Turing – The Delilah Project - https://news.ycombinator.com/item?id=38862147 - Jan 2024 (7 comments)
> Bell Labs’ pioneering SIGSALY speech-encryption system was constructed in New York City, under a U.S. Army contract, during 1942 and 1943. It was gigantic, weighing over 50 thousand kilograms and filling a room. Turing was familiar with SIGSALY and wanted to miniaturize speech encryption.
Well, yes; given that he _helped design it_ (https://en.wikipedia.org/wiki/SIGSALY#Development) he probably would be familiar with it.