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# >>The         Start of the Digital Revolution

by J. V. Boone and R. R. Peterson

****Contents:

IntroductionBackground and OverviewDevelopment and DeploymentConclusionsNotesAppendix AAppendix B

**Introduction**

 Today, digital technology is the backbone of our entire information                     industry. As a part of this, the transformation of audio               information into digital signals is now a routine process which               is incorporated                     into our telephone, television, and music equipment (both               recorded and live). Digital communication, measurement and data               techniques                     are quite commonplace. This fairly recent situation was               enabled by many things including the invention of the transistor               in                     1947 and the later evolution of semiconductor microelectronics                     techniques. However, the pioneering work for many of these                     capabilities was performed early in World War II in a successful                     effort to provide secure voice communications for high-level                     government officials. This brochure presents a brief overview                     of this revolutionary development.1

Contents

**Background and Overview**

 Before the full involvement of the United States in WWII, the               United States and the United Kingdom were using transatlantic               high-frequency radio for voice communications between senior leaders.               The analog voice privacy system in use, called the "A-3," provided               reasonable protection against the casual eavesdropper, but it               was vulnerable to anyone with sophisticated unscrambling capability.               This system continued to be used during the early part of the               war, and government officials were warned that they could be overheard.               In fact, it was later discovered that a German station in the               Netherlands was breaking out the conversations in real time. This               situation was intolerable, but neither the U.S. nor the U.K. had               a ready solution.

Fortunately, the technical groundwork for a solution was already               in place. About 1936, Bell Telephone Laboratories (BTL) started               exploring a technique to transform voice signals into digital               data which could then be reconstructed (or synthesized) into intelligible               voice. It was called a "vocoder," short for voice coder.               An early demonstration of the voice synthesizer portion of the               vocoder was even a part of the 1939 World's Fair in New York.               The approaching war stimulated the investigation of true voice               security. The BTL staff soon discovered that there were about               eighty patents issued on the general topic, but analysis indicated               that all of the methods were really unsatisfactory from a national               security viewpoint. New technology was required. Spurred on by               added interest from the U.K. and some early research results,               the vocoder was selected as the basis of a new high-tech voice               security system. BTL proceeded on its own to develop this much-needed               capability and was soon able to demonstrate it to the satisfaction               of the Army. A U. S. Army contract was awarded in 1942 for the               production of the first two systems. This system eventually came               to be called SIGSALY2 and was first deployed in 1943.

               This conference, on 15 July 1943, marked the official start of                  SIGSALY service. The encrypted transmissions were between the                  Pentagon and the London systems. Lt. Gen. J. T. McNarney, deputy                  chief of staff, USA, is at the head of the table on the right.                  Dr. O. E. Buckley, president of Bell Telephone Laboratories,                  is at the left end of the table. Lt. Gen. Brehon Somervell,                  USA, commanding general, Army Services Forces, is using the                  phone while others listen on headsets. The London end of the                  conference was attended by Lt. Gen. J. L. Devers, USA, commanding                  general, European Theater of Operations, and Maj. Gen. I. H.                  Edwards, USA, chief of staff, European Theater of Operations,                  among others. (Photo from the National Archives)

The system was formally put into use on 15 July 1943 in a conference               between the Pentagon and London. A photograph of that event is               shown in figure 1. As a part of this conference, the president               of BTL addressed the participants. His comments, which predicted "far-reaching               effects," are contained in Appendix A. Twelve terminals were               eventually deployed. Before they were removed from service in               1946, they were to support over 3,000 official secret conferences.               The mere existence of the capability to conduct secure voice communications               during this period remained secret until 1976.

Contents

**Development and Deployment**

The BTL development group worked under the direction of A. B.               Clark (who later led the Research and Development activities of               the new National Security Agency from 1954 to 1955) and developed               a vocoder-based system which emphasized the preservation of voice               quality. They chose a twelve-channel system. Ten of the channels               each measured the power of the voice signal in a portion of the               total voice frequency spectrum (generally 250-3000 Hz), and two               channels were devoted to "pitch" information of the               speech as well as whether or not unvoiced (hiss) energy was present.               This work was essentially completed in 1942, and patents were               filed. Most of the patents would be kept secret until 1976! BTL               had invented the fundamentals of digital, encrypted voice,3 and               they had also invented the means to transmit it.4

A 1983 review of this remarkable system for the Institute of Electrical               and Electronic Engineers (IEEE)5 attributes no fewer than eight "firsts" to               SIGSALY. They are as follows:
        - The first realization of enciphered telephony
        - The first quantized speech transmission
        - The first transmission of speech by Pulse Code Modulation (PCM)
        - The first use of companded PCM
        - The first examples of multilevel Frequency Shift Keying (FSK)
        - The first useful realization of speech bandwidth compression
        - The first use of FSK - FDM (Frequency Shift Keying-Frequency               Division Multiplex) as a viable transmission method over a fading               medium
        - The first use of a multilevel "eye pattern" to adjust               the sampling intervals (a new, and important, instrumentation               technique)

The IEEE article also points out that the system can be thought               of as being one of the very first successful applications of spread               spectrum technology.

An overview of the general scheme for digitizing the voice information               and encrypting it is shown in Appendix B. This was a completely               new approach to the problem and was based on the extensive communications               research then being conducted at BTL.6

It is a rare thing indeed to produce a new system with so many               unique features. These were not simply "improvements";               they were fundamentally new and absolutely necessary for the system               to work. The concepts were proven in the lab, but before the system               was ready for final development and deployment, there were several               important system features which needed refinement.

Key generation was a major problem. The basic requirements for               the key (one essential part of the total encryption system) was               that it should be completely random, and must not repeat, but               could still be replicated at both the sending and receiving ends               of the system.

This was accomplished for SIGSALY by using the output of large               (four-inch diameter, fourteen-inch high) mercury-vapor rectifier               vacuum tubes to generate wideband thermal noise. This noise power               was sampled every twenty milliseconds and the samples then quantized               into six levels of equal probability. The level information was               converted into channels of a frequency-shift-keyed (FSK) audio               tone signal which could then be recorded on the hard vinyl phonograph               records of the time.7

The FSK signal was recorded on sixteen-inch diameter wax platters               which were then transformed into "masters." The masters,               which in commercial use would have been used to make thousands               of records each, were used to produce only three records of a               particular key generation segment. Key distribution, always a               problem, was accomplished by means of transporting and distributing               the phonograph records. These records were taken to Arlington               Hall Station in Virginia, and the masters were destroyed. Once               the systems were deployed, the key-pair was distributed by courier               from Arlington Hall to the sending and receiving stations.8 Each               recording provided only twelve minutes of key plus several other               functional signals which were necessary for seamless key output.

Later advances in recording technology permitted the simultaneous               direct recording of the key on two acetate disks backed by aluminum.               This technique reduced drastically the time required to make each               record and also reduced cost. Since the keys were changed for               every conference and on a regular schedule, there were a very               large number of recordings made and distributed under strict controls.               The key recordings were destroyed after use.

The system required that the key be used in twenty millisecond               segments. Therefore, it was necessary for each record to be kept               in synchronism within a few milliseconds for fairly long periods               of time (one hour or so). This was accomplished by the use of               very precisely driven turntables. The turntables themselves were               remarkable machines. Each was driven by a large (about thirty-pound)               synchronous electric motor with hundreds of poles. The motor was               kept in constant operation, and the power for it was derived directly               from dividing down the terminal's frequency standard. The frequency               standard was a 100 kHz crystal oscillator. The accuracy of the               standard had to be maintained within about one part in ten million               so that the system would stay in synchronism for long periods               of time. The system frequency standard could be corrected by comparing               it to an available national frequency standard (which was WWV               in the U.S.).

Since one record held only about twelve minutes of key, it was               necessary to have two transmit and two receive turntable subsystems               at each terminal. In this way, a transition could be made from               one key-pair to another. However, there was another problem: how               did the system get started in synchronism?

There were no synchronizing signals passed from one terminal to               another. Each terminal was a standalone which depended on its               own internal clock referenced to a national time standard. Systems               were started at prearranged times. For example, Washington and               London might agree to start at precisely 1200 GMT. Prior to starting,               the phonograph pickup was indexed to the first record groove.               This process consisted of listening to the pickup output click               as it was slid along the edge of the record in order to determine               when it fell into the first groove. This was easier with the original               hard records than with the softer acetate records.

Once the pickup position was established and the time came to               start the session, a clever mechanical device was used to obtain               a simultaneous start at both ends of the link. The turntables               were attached to the shaft of the synchronous motor by means of               a clutch and spring arrangement. At the startup time, the clutch               was automatically energized and a pinball type plunger was activated               which provided the initial starting motion to the turntable by               releasing a spring which then pulled the turntable from its starting               position to the same speed and in lock with the rotation of the               synchronous motor. All of this enabled the motor to keep its synchronized               position. At the end of the process, each turntable would be running               at a definite and precise rate, and the key-pair would be synchronized               to the accuracy required for operation.

Of course, fine adjustments in relative timing were required.               These were made by the use of 50-Hertz phase shifters (Helmholz               coils) in the basic power/timing circuitry which drove the turntables.               A separate phase shifter could be adjusted by the operator to               control the timing within the receiving system to account for               the transmission time, which was on the order of 16 milliseconds               for a transatlantic circuit. Operators initially established the               synchronization by carefully adjusting the phase of the synchronous               motor and listening to the quieting in the audio output when the               key-pairs were in synchronism. Operators often monitored the quality               of the conversations and adjusted the system in a similar manner.               The entire synchronization process was complex, but it worked.               The turntable-based key-record system was called SIGGRUV when               the vinyl records were used and SIGJINGS when the acetate records               were used.

There was also a mechanical alternative to the recorded key. This               was called Alternate Key, or AK. The AK subsystem consisted of               a large number of stepping switches, relays and other devices.               It started the key derivation process with a rotor device normally               associated with teletype encryption systems. It was a very complex               and relatively unreliable system which required constant maintenance               attention. There was also an interesting difference in system               operating characteristics between using the recorded key and the               AK subsystem.

When the system was using a recorded key and lost synchronization,               there was almost always an abrupt and total loss of system capability.               When the AK system began to deteriorate, it usually did so in               small increments, which resulted in a sound like a horse galloping.               In just a few seconds, a small "gallop" developed into               a full gallop as the first small error (caused, for example, by               a faulty relay contact) was multiplied and propagated throughout               the system. AK was used by the system operators mainly for daily               maintenance purposes. This subsystem was called SIGBUSE.

A very simplified overview of the total system is shown in the               figure below, representing a one-way transmission. Return transmissions               used               the same key setup.

               Each complete SIGSALY terminal consisted of about forty racks                  of equipment and was very heavy. A portion of a complete system                  is shown in the photograph in figure 3. Installations were                  customized to their surroundings, which resulted in a variety                  of actual physical configurations. Each system contained a                  large assortment of vacuum tubes,9 relays, synchronous motors,                  turntables, and other unique electromechanical equipment. Because                  of the technology of the time, it also used large amounts of                  power. Special cooling systems were required to dissipate the                  heat.

Each installation was unique, but this photograph conveys the               complexity and size of the system. The phonograph turntables which               played the cryptographic key are on the right of the picture.               The oscilloscope in the center of the rear rack of equipment had               many uses, but, along with an HF radio receiver, was a major tool               in the process of assuring that the local system time standard               matched the international time signals. The approximate weight               of the forty racks of equipment and other materials in a terminal               was 55 tons!

Terminals were eventually established in Washington, D.C., London,               Paris, North Africa, Hawaii, Guam, Manila and Australia, among               others. In each case, there were installation challenges. Systems               were also deployed after the conclusion of the war to other locations,               including Berlin, Frankfurt, and Tokyo.

In London, the bulk of the SIGSALY equipment was housed in the               basement of an annex to Selfridge's Department Store while the               actual instrument used by Churchill and his staff was about a               mile away in the War Rooms under the Admiralty Building and near               the prime minister's residence at 10 Downing Street. The Washington,               D.C., end of the system was installed in the recently completed               Pentagon in the summer of 1943. The original installation schedule               called for a system to be in the White House itself, but the Pentagon               location was chosen to permit the system to be more easily used               by senior members of the military (and possibly to give President               Roosevelt better control over his own schedule since he would               not be interrupted at all hours by callers).

It was very clear from the start that this complex equipment was               going to require a specially trained operations and maintenance               staff. The Army Signal Corps responded by establishing the 805th               Signal Service Company. Special training was provided in a BTL               school, and members of the 805th were sent to all SIGSALY locations.               The 805th was a very unusual company, to say the least. It consisted               of eighty-one officers and two hundred and seventy-five enlisted.               The officers were mostly lieutenants and captains, and the enlisted               men were technical and master sergeants. This special company               had the highest average grade of any company in World War II.               A picture of part of the unit assigned to operate the equipment               in the I. G. Farben building in Frankfurt, Germany, is in figure               4. Although technically not assigned to the 805th, members of               the Women's Army Corps (WACs) performed a wide variety of duties               related to SIGSALY including creating stenographic records of               many of the important conversations.10

The members of the 805th had a challenging task. In addition to               the requirements for special security, they had to deal with the               complexities of the technology. Western Electric Company specifications               were used, and immaculate daily records were kept. The equipment               was normally operated for about eight hours a day, and the remaining               sixteen were used for maintenance. The large number of vacuum               tubes required constant checking. In some cases they were removed               and checked so often that the sockets started to go bad. Maintenance               schedules were adapted with experience to avoid such problems,               but maintenance remained a challenging job. Power supplies were               critical elements in the system and were adjusted using a standard               cell and galvanometer system to a probable accuracy of one-tenth               of a volt in one hundred and fifty volts. Tens of power supplies               were adjusted daily. Ninety-six stepper circuits also needed daily               adjustment.

            At first, BTL provided members of the 805th their training in               the system (called the RC-220-T-1) at the BTL School for War Training               in New York City. Classes started in January 1943 and continued               until the Army took over the training in July 1944. This photograph               shows only a few of the members of the 805th and was taken circa               1945 in the I. G. Farben building in Frankfurt, Germany. L to               R in the front row are TSgt. Jim Atkinson, Lt. Massey (the 1st               C.O. of this unit), Lt. Golliday (the 2nd C. O.), Lt. Mc White,               Lt. Chope, and MSgt. Charlie Haas. L to R in the rear row are:               Staff Sgt. Fred Buck, TSgt O'Neil, Staff Sgt. Jim Biggerstaff,               Staff Sgt. Ed Dessert, Staff Sgt. Frank Zylius and "Willy" Wilchenski.            (Photo courtesy of Fred Buck)

Despite all the complexity, the efforts of the 805th, coupled               with the fundamentally sound design, made the SIGSALY system operationally               effective. There was surprisingly little operational downtime.

Contents

**Conclusions**

 The ability to use truly secure voice communications at high               organizational levels was a great advantage to the Allies in the               conduct of the                  war and in the critical activities which followed it. Not only                  was SIGSALY a highly successful secure voice system, but it               provided a springboard into the digital communications world.               It encouraged                  the BTL staff and their government counterparts to think more                  about communications in digital terms rather than in the traditional                  analog processes.

The rapid development and deployment activities also offer a challenge               for present-day members of the cryptologic community. We are all               indebted to the development teams in both government and industry               as well as to the members of the 805th Signal Service Company               and all of the others associated with this remarkable system.

Contents

***Notes***

1. A much more detailed description of the system,               as well as background on its development, deployment and operation,               is available               in a document entitled The Green Hornet...America's Unbreakable               Code for Secret Telephony. This study was authored by Donald E.               Mehl and was privately published in 1999. Copies are available               for study in the library of the National Cryptologic Museum. It               contains many diagrams, photographs, references and stories concerning               this system. This work is the source of much of the information               regarding the deployment of the system as well as some of the               technical details which are presented in this short paper. 

2. SIGSALY and similar names given to portions of the system are               simply "cover" names, not acronyms. 

3. Even today, the legalistic and somewhat arcane language used               in patent applications is difficult to comprehend. Yet it is interesting               to read the patent applications of the late 1930s and early 1940s               on this topic. The BTL inventors were establishing a completely               new way to transmit voice signals, and they were also inventing               the nomenclature at the same time. For example, a portion of the               United States Patent 3,967,067, "Secret Telephony," awarded               to Ralph K. Potter in 1976 (filed in September 1941) reads as               follows (under the heading "what is claimed is"):
> *In secret telephony, means to derive from the speech to be sent               a plurality of speech-defining signals each indicative of the               energy variations with time of a different frequency region of               the speech, means to translate each such signal above a certain               amplitude into a mark and each such signal below said certain               amplitude into a space, an individual key for each signal, consisting               of marks and spaces of random occurrence, and means to combine               each key with the marks and spaces obtained from an individual               one of said signals, to render transmission of the latter marks               and spaces secret.*

 Potter was describing a method of filtering the input speech               signal into a set of individual channels, digitizing each of those               channels,                  combining each of the digitized outputs with a digital key               to produce a signal which was unreadable without the key. Digital,                  encrypted voice! 

4. Once the encrypted signal was produced, they still needed the               means to transmit it in a reliable manner. The BTL staff solved               the problems by inventing multilevel frequency shift keying. And               again, the patent application is descriptive. A portion of United               States Patent 3,991,273, "Speech Component Coded Multiplex               Carrier Wave Transmission," awarded to Robert C. Mathes in               1976 (filed in October 1943) reads as follows (under the "what               is claimed is" heading):
> *...a group of frequency modulators of the same type and same average               frequency, means to impress said signals of stepped wave form               on the respective frequency modulators to produce a plurality               of frequency modulated waves of the same average frequency, and               means to translate the last waves into frequency modulated waves               accurately positioned at different frequency levels comprising               a separate amplitude modulator for shifting the frequency of each               such frequency modulated wave, and means to supply to said amplitude               modulators carrier waves of accurately spaced frequency comprising               a source of base frequency waves of highly constant frequency               and a harmonic generator for fixing the frequencies of said supplied               carrier waves.*

 They had shown the practicality of the techniques for digitizing                  the voice, encrypting and decrypting it, and had also invented               a practical way to transmit the signals over long distances.

5. William R. Bennett, Fellow, IEEE, "Secret Telephony as               a Historical Example of Spread-Spectrum Communications," IEEE               Transactions on Communications, Vol. COM-31, No. 1, January 1983,               99. 

6. As with almost any large-scale development, many individuals               made important contributions. Fortunately, the BTL team was highly               skilled and had access to many experts. For example, historian               David Kahn notes in his article in the IEEE Spectrum of September               1984, "Cryptology and the Origins of Spread Spectrum," that               Harry Nyquist and Claude Shannon made important contributions.               He also relates that the British cryptographer Alan Turing was               briefly involved in the development and approved the security               aspects of the system for the British. 

7. This type of system is generally described as a part of another               United States Patent, "Secret Telephony" 3,985,958,               which was awarded to Homer W. Dudley in 1976 (filed in 1941).               

8. The third, "spare," record was retained at Arlington               Hall to guard against the possibility of the destruction of a               record during shipment. This practice was eventually terminated.               

9. For example, the digitization was accomplished by circuits               which featured the use of model 2051 Thyratron vacuum tubes. There               were 384 of these tubes used in a single terminal. 

10. Many of the WACs were stationed in the Pentagon, but others               served in overseas locations. There were also civilian women associated               with the systems. 

Contents

**Appendix A**

*Remarks by Dr. O. E. Buckley, President, Bell Telephone Laboratories                  at the Formal Opening of SIGSALY Service, 15 July 1943*

We are assembled today in Washington and London to open a new               service, secret telephony. It is an event of importance in the               conduct of the war that others here can appraise better than I.               As a technical achievement, I should like to point out that it               must be counted among the major advances in the art of telephony.               Not only does it represent the achievement of a goal long sought               - complete secrecy in radiotelephone transmission - but it represents               the first practical application of new methods of telephone transmission               that promise to have far-reaching effects.

To achieve the result represented by this system, there have been               done several very remarkable things. Speech has been converted               into low frequency signals that are not speech but contain a specification               or description of it. Those signals have been coded by a system               that defies decoding by any but the intended recipient. The coded               signals have been transmitted over a radio circuit in such a way               that an interceptor cannot even distinguish the presence or absence               of the signals. At the receiving end, the signals have been decoded               and restored and then used to regenerate speech nearly enough               like that which gave them birth that it may be clearly understood.

To do these things called for a degree of precision and a refinement               of techniques that scarcely seemed possible when the researches               that led to this result were undertaken. That speech transmitted               in this manner sounds somewhat unnatural and that voices are not               always recognizable should not be surprising. The remarkable thing               is that it can be done at all.

All of the elements of this system were developed by the Bell               Telephone Laboratories in the interests of advancing the art of               telephony. Early in the course of the development, the system               was discussed with representatives of the Signal Corps, who at               once recognized its possible military value and encouraged our               efforts. When it had reached the point where its principles could               be demonstrated, the Signal Corps took prompt steps for its procurement.               In the present embodiment of the system, manufactured by the Western               Electric Company, representatives of the Signal Corps have worked               closely with us and are prepared to install and operate it.

We of Bell Telephone Laboratories and the Western Electric Company               are proud of this achievement in which the efforts of a large               number of our people were involved. We hope that it will be a               help in the prosecution of the war, and we are indebted to the               Signal Corps for the opportunity to put into practical use this               new system of telephony.

A secret War Department letter from the Adjutant General's Office               dated July 9, 1943, published the "Operating Procedure for               Overseas Telephone System" and announced that service between               Washington and London on the SIGSALY system would be available               by July 15, 1943. SIGSALY was "on the air."1

***Note***

1. This material was obtained from the work of Donald E. Mehl,               The Green Hornet, previously referenced. Back

Contents

**Appendix B**

*Overview of the Encryption Process*

The general concept of the encryption process for a single vocoder               channel is presented in figure A-1. In each of the lines of this               figure, the vertical axis segment represents levels of quantization,               and the segments of the horizontal lines represent time in the               20-msec increments used in the SIGSALY system.1

Line (a) in the figure represents the six-level quantization of the output of the vocoder channel. Although difficult to show in this diagram, the individual signal quantization steps do not represent equal power changes in the actual input signal. The steps were logarithmetic and a part of the "companding" process. This process had been shown to produce improved intelligibility in the reconstructed voice output.2

The random key is illustrated by line (b) in the figure. This               is the information produced from the recordings played on the               turntable subsystem. The key was combined with the quantized signal               information in a process which was then called "reentry." The               reentry combination process is mathematically described as "adding               the two quantized values mod 6."

Line (c) represents the encrypted data stream which would then               be transmitted by the FSK-FDM radio equipment. The essentials               of the receiving and decryption process are the reverse of the               encryption process. First, the transmitted multichannel signals               are demodulated. Then each channel is decrypted by subtracting               the key mod 6 from the signal. It is obvious that the 20-msec               segments of the quantized signal information and the key information               must be synchronized in exactly the same manner at both the transmitting               and receiving ends of the system. It remains to recombine the               vocoder channel outputs, incorporate the pitch information and               then reconstruct the audio signal and transform it into intelligible               speech. This entire new process was extremely difficult to implement               in the technology of the 1940s.

***Notes***

1. This figure is adapted from one in an article               on the SIGSALY techniques contained in M.D. Fagen, ed., A History               of Engineering               and Science in the Bell System: National Service in War and Peace               (1925-1975), Murray Hill, NJ: Bell Lab, 296-317. 

2. This topic is discussed in the IEEE article               by William R. Bennett, previously referenced. 

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