SIGSALY – Secure Speech System Used in WWII by the Allies

In order to guarantee uninterrupted operation, two turntables were used, allowing the next disc to be ‘cued up’ whilst the current one was running. Towards the end of each KEY record, a control tone or pilot tone is recorded (with the existing key tones) to mark the start of the other turntable. The simplified block diagram above shows how the rising and falling edges of the control tones are used to start and stop the motors of the two turntables, and to select which record is used as the source of the key stream.

Source: www.cryptomuseum.com