d modulo 6 with elabo- rately generated speech samples, to provide the complete security of a one-time-pad crypto-system, at a data rate of 1500 bps. Copies of each key were pressed on phonograph discs by Muzak, and the best studio-quality The Historical Origins of Spread-Spectrum Communications turntables for playing these records were incorporated into the thirty racks of equipment making up a Sigsaly radio station. The weight of the million dollar station, i.e., the operational equipment, a 30 kw power source, air con- ditioning, a complete set of spare parts including duplicates of the required 1100+ vacuum tubes, and other ancillary equipment, has been informally estimated at a staggering 80 tons. The official government designation for this station was the RC-220-T1 Terminal. The use of pseudorandom keys in combination with speech bandwidth compression and re-expansion into a pulse-code modulation (PCM) format to utilize the full bandwidth of a telephone channel could be viewed as a SR spectrum-spreading system. The intended application, admirably achieved, was unbreakable enciphered telephony; the system "contained no anti-jam- ming provisions, and for the most part, did not require any" [55]. Surprisingly, synchronization of the turntables was not as difficult as expected, because of the use of excellent frequency standards. Both Claude Shannon of Bell Laboratories and Alan Turing of the Government Code and Cypher School at Bletchley Park, England, inde- pendently verified the security of the X-System [3], [225]. Robert C. Mathes [57] and Ralph K. Potter [58] applied for patents on X-System apparatus in 1941, the disclosures being immediately placed under secrecy order. Thirty more patents, with X-System applications, followed [225, p. 297]. BTL continued its work on key-stream generation and in the mid-1940s filed for patents on all-electronic key generators which combined several short keys of relatively prime lengths to produce key streams possessing long periods [59], [60]. Such schemes also had been 