Wikipedia
SIGSALY
In cryptography, 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 popular radio show The Green Hornet, because it sounded like a buzzing hornet — resembling the show's theme tune — to anyone trying to eavesdrop on the conversation.