Rebuilding a Piece of the First Digital Voice Scrambler

This 1943 analog-to-digital converter helped make an unbreakable code

In the years before World War II, German intelligence could decode band-scrambled U.S. radiotelephone conferences. After Pearl Harbor, an unbreakable speech scrambler was developed with top priority, and by 1943, it was deployed. Known as SIGSALY, the device pioneered many advances critical to modern digital media technologies, including spread-spectrum communications and the first use of pulse-code modulation (PCM) to transmit speech.

SIGSALY was top secret, so even today information about the details of its construction are hard to come by. I’ve spent 20 years researching the history of digital technology and digital media, especially SIGSALY. I searched IEEE and U.S. National Security Agency (NSA) journals, and Bell Telephone Laboratories patents. Finally, I found Lieut. Donald Mehl, a WWII SIGSALY technician, who gave me invaluable assistance. In 2015, I realized that it might be possible to re-create a key element of SIGSALY—the quantizer—using vintage parts.

SIGSALY was unbreakable because, unlike earlier analog systems, it scrambled voices by using a one-time random digital encryption key. Before a digital key can be applied, a speaker’s voice must first be converted from analog to digital, thus the quantizer.

Modern analog-to-digital conversion makes it easy to capture the entire audio spectrum. With the technology available at the time, the creators of SIGSALY used 12 speech parameters that best encoded speech. SIGSALY used vocoders to analyze incoming speech and classify them into 10 frequency sub-bands; a pitch parameter; and a bit that indicated whether the speech sound was voiced or unvoiced.

SIGSALY had 72 identical quantizers to digitize the 12 vocoder parameters and the encryption key (stored as a vinyl recording of noise). The digitized voice and key were enciphered by modulo six addition. Each quantizer employed five VT-109/2051 thyratrons. Thyratrons are a type of electronic tube, but they differ from conventional ones in that they don’t respond in a linear way: They are off until the grid voltage exceeds a trigger; then an arc forms and they “latch up” and conduct until the anode current falls to zero, resetting them. The five thyratrons formed a flash converter, in which an incoming signal is compared to a reference voltage at a number of tap points in the circuit. In SIGSALY, the analog input voltage is fed into a five-tap logarithmic resistor divider ladder, whose taps drive the thyratron grids. As the input voltage increases, the five thyratrons thus trigger in sequence (producing not binary but a logarithmic “thermometer code”). These five outputs were sampled every 20 milliseconds to form a six-level quantized output signal (it’s a six-level output, not five, because zero is one level). [READ MORE]