band amplitude signals. 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 record was then duplicated, with the records being distributed to SIGSALY systems on both ends of a conversation. 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 records were played on turntables, but since the timing – the clock synchronization – between the two SIGSALY terminals had to be precise, the turntables were by no means just ordinary record-players. 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. Since each record only provided 12 minutes of key, each SIGSALY had two turntables, with a second record "queued up" while the first was "playing". Usage The SIGSALY terminal was massive, consisting of 40 racks of equipment. It weighed over 50 tons, and used about 30 kW of power, necessitating an air-conditioned room to hold it. Too big and cumbersome for general use, it was only used for the highest level of voice communications. A dozen SIGSALY terminal installations were eventually set up all over the world. 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 second was installed below street level in the basement of Selfridges department store on Oxford Street, London, close to the US Embassy on Grosvenor Square. 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. One was installed in a ship and followed General Douglas MacArthur during his South Pacific campaigns. In total during WW2, the system supported about 3,000 high-level telephone conferences. The installation and maintenance of all SIGSALY machines was undertaken by the