SIGSALY

It pioneered a number of digital communications concepts, including the first transmission of speech using pulse-code modulation.

[2] At the time of its inception, long-distance telephone communications used the "A-3" voice scrambler developed by Western Electric.

[2] The voice encoding used the fact that speech varies fairly slowly as the components of the throat move.

The pitch signal, which required greater sensitivity, was encoded by a pair of six-level values (one coarse, and one fine), giving thirty-six levels in all.

An inversion of the vocoder process was employed, which included: The noise values used for the encryption key were originally produced by large mercury-vapor rectifying vacuum tubes and stored on a phonograph record.

The records served as the SIGSALY one-time pad, and distribution was very strictly controlled (although if one had been seized, it would have been of little importance, since only one pair of each was ever produced).

For testing and setup purposes, a pseudo-random number generating system made out of relays, known as the "threshing machine", was used.

The rotation rate of the turntables was carefully controlled, and the records were started at highly specific times, based on precision time-of-day clock standards.

The first was installed in the Pentagon building rather than the White House, which had an extension line, as the US President Franklin Roosevelt knew of the British Prime Minister Winston Churchill's insistence that he be able to call at any time of the day or night.

The first conference took place on 15 July 1943, and it was used by both General Dwight D. Eisenhower as the commander of SHAEF, and Churchill, before extensions were installed to the Embassy, 10 Downing Street and the Cabinet War Rooms.