The Synthesis of Speech: Part 2 SIGSALY

This edition of the Music of Radio continues to explore developments around  electronically generated speech. Homer Dudley, an engineer and acoustics researcher who worked for Bell Telephone Laboratories (BTL), made significant contributions to this field beginning with his invention of the Vocoder and Voder. The development of these two instruments was detailed in last month’s column. Now I will turn my attention to how the Vocoder was employed in encrypting the transmissions of high ranking officials during WWII for the SIGSALY program. SIGSALY, by-the-way, is simply a cover name for the system and is not an acronym.

In 1931 BTL had developed the A-3 scrambler that was used by Roosevelt and Churchill, but the security of this device was eventually compromised by German’s at a radio post in South Holland who had been intercepting the Prime Ministers telephone calls. The A-3 had worked with the Trans-Atlantic Telephone by splitting speech up into different bands, but it wasn’t difficult to reassemble as the Germans proved in 1941, making the situation surrounding communications security to become intolerable to the Allies.

In 1942 the Army contracted BTL to assist with the communication problem and create “indestructible speech” or speech that could withstand attempts at code breaking. From this effort the revolutionary 12-channel SIGSALY system was born. To create SIGSALY workers sifted through over 80 patents in the general area of voice security. None of these fit the needs of the allies, but Homer Dudley’s Vocoder did and formed the basis of the system.  For SIGSALY a twelve-channel Vocoder system was used. Ten of the channels measured the power of the voice signal in a portion of voice frequency spectrum (generally 250-3000 Hz). Two channels were devoted to “pitch” information and whether or not unvoiced (hiss) energy was present. The Vocoder enciphered the speech as it went out over phone or radio. In order to be deciphered at each end of the conversation an audio crypto-key was needed. This came in the form of vinyl records.

From the standpoint of music history it is interesting to note, as Dave Tompkins did in his book How to Wreck A Nice Beach: The Vocoder fromWWII to Hip-hop, that the SIGSALY system employed two-turntables alongside the microphone/telephone. The classified name for this vinyl part of the operation was SIGGRUV.  The turntables were used to solve the problem of needing a cryptographic key. They played vinyl records produced by the Muzak Corporation, a company famous for the creation of elevator music. The sounds on these records weren’t aimed at soothing weekend shoppers or people sitting in waiting rooms.  Muzak had been contracted into pressing vinyl that contained random white noise, like channel 3 on an old television set. The noise was created by the output of very large mercury-rectifier tubes that were four inches in diameter, and over a foot high. These generated wide band thermal noise that was sampled every twenty milliseconds. The samples were then quantized into six levels of equal probability. The level information was converted into channels of a Frequency Shift Keyed audio tone signal recorded onto a vinyl master. From the master only three copies of a key segment were made. If these platters had been commercial entertainment masters thousands would have been pressed from its blueprint. If any SIGGRUV vinyl still exists, and for security reasons they shouldn’t have, those grooves are critically rare.

It had to be insured that no pattern could be detected so the records had to be random noise. If the equipment had somehow been duplicated by the Axis powers, the communications would still be uncompromised as the they required the crypto key of the matching vinyl, required at each terminal. This made the transportation of these records, via armored truck, the most secure since Edison invented the Phonograph. Just as the masters were destroyed after making three keys, each vinyl key was only ever to be played once, as operators were instructed to burn after playing. The official instruction read, “The used project record should be cut-up and placed in an oven and reduced to a plastic biscuit of ‘Vinylite'”. As another precaution against the grooves falling into enemy hands the turntables themselves had a self-destruct mechanism built into them that could be activated in case a terminal was compromised. Thinking of all this sheds new light on the idea of a DJ-Battle.

sigsaly-terminalKeeping the turntables at two different terminals across the globe synchronized was another  technical hurdle that BTL overcame. If a needle jumped or the system went out of synch only garbled speech was heard. At the agreed upon time, say 1200 GMT, operators listened for the click of the phonograph being cued to the first groove. The turntables were started by releasing a clutch for the synchronous motor that kept the turntable running at a precise speed. Fine adjustments were made using 50-Hertz phase shifters (Helmholtz coils) to account for delays in transmission time. The operators would listen for full quieting of the audio as synchronization was established. Oscilliscopes and HF receivers were also used to keep systems locked to international time.

A complete SIGSALY system contained about forty racks of heavy equipment composed of vacuum tubes, relays, synchronous motors, turntables, and custom made electromechanical equipment. In the pre-transistor era all of this gear required a heavy load of power so cooling systems were also required to keep it all from getting fried. The average weight of a set up was about 55 tons.

The system passed Alan Turing’s inspection (if not his test) as he had been briefly involved with the project on the British side. On July 15, 1943 the inaugural connection was established between the Pentagon and a room in the basement below Selfridges Department Store in London. Eventually a total of twelve SIGSALY encipherment terminals were established, including some in Paris, Algiers, Manila, Guam, Australia and one on a barge that ended up in the Tokyo Bay. In the year 1945 alone the system trafficked millions of words between the Allies.

To keep all of this operational a special division of the Army Signal Corp was set up, the 805th Signal Service Company. Training commenced in a school set up by BTL and members were sent to various locations. Their tasks required security clearances and a firm grasp on cutting edge technology which they were tasked to operate and maintain. For every eight hours of operation the SIGSALY systems required 16 hours of maintenance.

In putting the system together eight remarkable engineering “firsts” were achieved. A review conducted by The Institute of Electronic and Electrical Engineers in 1983 lists them  as follows:

1. The first realization of enciphered telephony
2.The first quantized speech transmission
3.The first transmission of speech by Pulse Code Modulation (PCM)
4.The first use of companded PCM
5.The first examples of multilevel Frequency Shift Keying (FSK)
6.The first useful realization of speech bandwidth compression
7.The first use of FSK – FDM (Frequency Shift Keying-Frequency Division Multiplex) as a viable transmission method over a fading medium
8.The first use of a multilevel “eye pattern” to adjust the sampling intervals (a new, and important, instrumentation technique)

To do all these things required precision and refinement in new technology. SIGSALY has left the world with a rich inheritance that spans developments in cryptology, digital communications, and even left its mark on music.


How to Wreck A Nice Beach: The Vocoder from WWII to Hip-hop: The Machine Seaks by Dave Tompkins, Melville House, 2010

SIGSALY: The Start of the Digital Revolution by J.V. Boone and R.R. Peterson, retrieved at:

[This article originally appeared in the January 2017 issue of the Q-Fiver.]

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