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.
Full article:
https://spectrum.ieee.org/geek-life/hands-on/rebuilding-a-piece-of-the-first-digital-voice-scrambler