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 ). [1] The prototype was called the "Green Hornet" after the 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. [2]
At the time of its inception, long-distance telephone communications were broadcast using the "A-3" voice scrambler developed by Western Electric . The Germans had a listening station on the Dutch coast which could intercept and break A-3 traffic. [1]