Talk:SIGSALY

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Contents 1 Stimulus 2 Duplicate article 3 Spread spectrum 4 Bit rate 5 Accurate to call it PCM?

[edit] Stimulus

James H. Ellis' non-secret encryption for which SIGSALY or something simliar seems to have been the stimulus

Do we have any evidence that Ellis was motivated by SIGSALY? — Matt 12:17, 22 Jun 2004 (UTC)

[edit] Duplicate article

Blast! I've just spent a day writing an article on the "X System", as my reference book (see 'Further reading' I just added) calls it, only to discover that we already have an article on this topic, just one which wasn't listed under all its names.

Which raises the question of what we ought to call the article. Both the Bell book, and the Turing bio by Hodges, which also covers it in some detail, call it the "X System" (or some typographical variant thereof, such as "X-system"). Do we want to leave it here (which is, I gather, the name the NSA museum has it under), or use the one in all the references? Noel (talk) 19:25, 6 Mar 2005 (UTC)

Sorry to hear about your reduplication of effort, although you've appear to have merged in a lot good stuff into this article regardless — nice work! Regarding the name: I don't have strong feelings either way, but a few minutes with Google suggests that SIGSALY is the more common name on the Web. — Matt Crypto 01:09, 8 Mar 2005 (UTC)

[edit] Spread spectrum

I don't see how this system was a spread-spectrum system. To me, SS implies that the signal is spread out across a wide frequency range so as to be imprevious to narrow-band jamming. It's true that the transmission technique used by this system was relatively good against atmospheric distortion (natural "jamming", as it were), but I would describe it more as a "frequency modulation" system, rather than SS. They used FM because an amplitude modulated signal wouldn't give the required S/N (the vocoded signals needed to be reproduced +- 10% in amplitude for the unvocoded speech to be intelligible), and there was too much fading on long hops to meet this target if they used AM. After all, I don't think anyone would describe ordinary FM commercial radio as SS! Noel (talk) 23:02, 6 Mar 2005 (UTC)

I'm out of my depth technically, but perhaps we could directly attribute the claim to Bennett's IEEE article? That is, something like "Bennett (1983) suggests that SIGSALY can be thought of as being one of the very first successful applications of spread spectrum technology".— Matt Crypto 01:09, 8 Mar 2005 (UTC)

A spread spectrum system uses a larger range of frequencies than is necessary to transmit the signal. Different modulation schemes do this to different degrees. FM uses more bandwidth (Δf_FM ≈ 2(f_Modulation + Δf_max), where Δf_max is the maximum frequency shift) to achieve the higher resistance than AM (Δf_AM ≈ 2f_Modulation). In this sense, FM is a spread-spectrum method.

SIGSALY transmits a form of noise (because the one-time-pad is noise) and probably uses the entire frequency bandwidth of the transmission medium to transmit that "noise". Even for the transmission of a payload signal consisting of just a single tone, the entire bandwidth is used. In this sense, SIGSALY uses a spread-spectrum technique. It is comparable to CDMA, which also uses "noise" for modulation.

It would be interesting to see if it would work to transmit two (or several) SIGSALY payloads over the same transmission medium (at the same time), encrypted using two (or several) independent SIGSALY systems each using a different "key record". My guess is that only one noise bit is used for each payload bit. In this case it would not work. --RainerBlome 20:30, 24 October 2005 (UTC)

[edit] Bit rate

According to Diffie and Landau, Privacy on the Line, SIGSALY's vocoder ran at 2400 bps. It must have sounded terrible. Phr (talk) 09:40, 24 July 2006 (UTC)

[edit] Accurate to call it PCM?

The article claims that SIGSALY transmitted speech using PCM, but don't think that is a fair claim. SIGSALY transmitted the quantized parameters of an entirely analog vocoder rather than digitizing the the speech waveform directly as in a true PCM system. The vocoder parameters were not even transmitted via PCM: instead a subranging quantizer was used and its output was sent directly rather than in PCM form (see US patent 3,912,868). Although the output of a subranging quantizer can be losslessly converted to PCM, the distinction there is important because SIGSALY was in production use before Alec Reeves's patent on PCM was issued in the US. As far as I can tell, SIGSALY never adopted true PCM principles, which is not shocking since it mostly predated that information theoretic basis which allows us to understand the fundamental mathematical equivalence of multi-level and binary encoding. --Gmaxwell 19:35, 19 March 2007 (UTC)