1943 SIGSALY terminal, photo from National Cryptologic Museum
2003 SIGSALY terminal recreated at National Cryptologic Museum
SIGGRUV black vinyl disc on gold master exhibit at National Cryptologic Museum

SIGSALY began operation in World War II as a digital encrypted voice telephone system connecting London and the Pentagon. It had been developed by Bell Labs, using early work since 1936 that had produced the vocoder for the 1939 World's Fair in New York. Bell Labs had developed the A-3 telephone scrambler for Roosevelt and Churchill in 1939 using older non-digital technology, but it was not secure and by 1941 was broken by Germany. A Bell Labs contract with the Army in 1942 developed the revolutionary digital SIGSALY 12-channel system by 1943. The official inaugural transatlantic encrypted telephone call took place July 15, 1943. Ten channels were used to sample the voice frequency spectrum from 250 Hz to 3000 Hz and two channels sampled voice pitch and unvoiced background hiss. The digital sampling was done by circuits using the model 2051 Thyratron vacuum tube, requiring 384 tubes in each terminal. Each SIGSALY terminal used 40 racks of equipment weighing 55 tons and filled a large room. This equipment included radio transmitters and receivers and large phonograph turntables. The voice was keyed to two 16-inch vinyl phonograph records that contained a Frequency Shift keying (FSK) audio tone. The records were played on large precise turntables in synch with the voice transmission. The system was called SIGGRUV when vinyl records were used in 1943, and was later called SIGJINGS when acetate-coated aluminum records were used. Terminal rooms were established in the Pentagon, London, Paris, North Africa, Hawaii, Guam, Manila, and Australia during the war, and in Berlin, Frankfurt, Tokyo after the war. Twelve terminal rooms were deployed until the system was removed from service in 1946. In London, most of the equipment was placed in the basement of an annex to Selfridge's Department Store and the telephone terminal was located a mile away in the Admiralty Building War Rooms near 10 Downing Street. Each SIGSALY installation was maintained by the 805th Signal Service Company and members of the Women's Army Corps. The patents and encryption system remained secret until 1976. SIGSALY is considered to represent the first digital quantization of speech and the first transmission of speech by PCM.



1999-2007 by Steven E. Schoenherr. All rights reserved.

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