Hello all, I greatly appreciate your comments and feedback! More notes:
1/ There were 33 patents granted to BTL overall, some by very well-known engineers like Homer Dudley, inventor of the VOCODER. Many were classified secret at the time and only declassified in 1976.
Claude Shannon's fundamental paper Communication Theory of Secrecy Systems was inspired by his work at BTL on the algorithms of SIGSALY and classified during the war. It was finally declassified in October 1949 and published in BSTJ.
http://pages.cs.wisc.edu/~rist/642-spring-2014/shannon-secrecy.pdfhttps://archive.org/details/bstj28-4-6562/ RE The transmission rate and BW: voice was analyzed into 12 parameters, 10 sub-band levels and 2 pitch parameters, also one bit for voiced/unvoiced. Each of the 12 analog parameters had 25 Hz bandwidth. 12 X 25 = 300 Hz, or 10X compression of voice information. Twelve quantizers converted the 12 vocoder parameters into six level digital signals. Bit rate: 50 Hz sample rate, 12 channels 2.5 bits (6 levels) per ch, thus total 30 bits at 50 Hz or 1500 BPS.
See for info on vocoder theory
https://en.wikipedia.org/wiki/Vocoder3/ The coding of the 12 quantized parameters employed MOD-6 addition. Each 6 level parameter was added to a 6 level band limited noise, and indeed recorded on 16" phonograph records.
4/ The final step before SW transmission was spread spectrum with SSB and FM, which expanded the bandwidth back to 3 kHz. SIGSALY had the first working spread spectrum, used not for secrecy but to reduce transoceanic SW radio fading by 20 dB.
5/ I know of no original SIGSALY recordings. But NSA and NCM have recordings of post war versions of SIGSALY using the same techniques. Very few if any are on the Internet.
See my interview and a short sound clip on PRI 99% invisible program
https://99percentinvisible.org/episode/vox-ex-machina/6/ To probe further, see the IEEE and NSA papers, and my AES, NAB and SMPTE papers on the Origins of Digital Media and Origins of Compression.
This link has a precise technical description, which should answer many of your questions:
https://www.cryptomuseum.com/crypto/usa/sigsaly/index.htm7/ My 3 minute demo of the quantizer in action (pardon the amateur video quality, it was my very first video, done with limited equipment)
https://youtu.be/4TLMnFyapjA8/ Connection from SIGSALY to a local phone eg at Cabinet War rooms in London, used secure copper circuits up to a few miles. The 4 wire SIGSALY secure circuits used noise and phantom circuits. They connected a SIGSALY terminal to a remote telephone e.g. Churchill War Rooms to SIGSALY miles away in the basement of Selfridges Department store. ( "phantom" is a circuit that uses common mode currents, for a second channel e.g. transmitted thru the center tap of a balanced line transformer).
https://en.wikipedia.org/wiki/Phantom_circuitIts origin was in the 1800s to add a circuit on a telegraph line.
Again thanks to all for your fine comments!
Jon Paul