Exploring the Role of SIGSALY in the 2017 Film 'Darkest Hour'

We see a lot of films feature radio technology as a silent yet pivotal character, so we thought it might be fun to explore (and explain) what some of these films got right, or maybe not quite right, with the portrayal of tech. So today, we’re talking about SIGSALY and its use in the 2017 film Darkest Hour. 

There is a scene in which Winston Churchill speaks to President Franklin Roosevelt using a secure phone, linked to Washington D.C. via high-frequency shortwave radio.

Decoding SIGSALY: From 'Green Hornet' to Digital Communication

While the essence of the messaging is correct, the actual communication at the time was by hand-written correspondence, not a phone call.  The secure phone shown actually did come into existence, but not until later in the war.

Since voice communication via radio primarily used AM modulation at the time, it was not secure.  Anyone could listen in, so important messages were usually transcribed, encoded, and sent via Morse Code.  Something better was needed, so the government turned to AT&T for the solution, which came in the form of SIGSALY.

SIGSALY (also known as the X System, Project X, Ciphony I, and the Green Hornet) was a secure speech system used in World War II for the highest-level Allied communications. It pioneered a number of digital communications concepts, including the first transmission of speech using pulse-code modulation.

The name SIGSALY was not an acronym, but a cover name that resembled an acronym—the SIG part was common in Army Signal Corps names (e.g., SIGABA). The prototype was called the "Green Hornet" after the radio show… because it sounded like a buzzing hornet, resembling the show's theme tune, to anyone trying to eavesdrop on the conversation.

Wikipedia

Electronic computers, integrated circuits, and even transistors were still years in the future when SIGSALY was developed, yet Bell Labs managed to build a highly secure (even by today’s standards) encrypted digital voice transmission system. It was incredibly cumbersome (50 tons of gear), power hungry (30 kilowatts) and terribly expensive ($1 million dollars per terminal).

SIGSALY's Impact on Modern Communication Technology

However, consider this: SIGSALY used a primitive vocoder, a vital part of the system that takes the voice and turns it into digital bits in an efficient manner.  

These days, derivatives of that early technology can be found in every cell phone, virtually all landline phone systems, and just about every computer out there.  SIGSALY also used a very basic form of spread-spectrum transmission, also used in cellphones, satellite communications, wireless LANs, and a host of other radio-based communications systems.  

Much of the communication technology we take for granted today found its start in systems such as SIGSALY, such as vocoders, digital transmission which used a form of MIMO (multiple-input and multiple-output), encryption, and precise time and frequency synchronization, among other inventions.  Radio Design Group has done many projects using frequency sync as well as similar digital transmission methods to SIGSALY.

Delving Deeper: Exploring the Fascinating History of SIGSALY

There is a wealth of information on the Internet about SIGSALY, and a quick search will yield a variety of technical documents, should you be interested in the history of this amazing system. You can read more about the technology that made this innovation possible on our blog, too. If you go to visit Churchill’s underground bunker today, you will get to see the little room reserved for the Prime Minister to make those secure phone calls to Washington.

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