Pilot tone is one system which has been historically used to keep the camera in sync with an external sound recording device. In the days of SMTPE timecode and digital recording, it has become less common for pilot tone to be used.
The pilot tone system was devised in the days when film sound was recorded on a battery-powered tape recorder, such as the Nagra. Tape has no sprocket holes, so sync is maintained by recording a pilot frequency in their stead. This is generated by the camera, and is a precise and meaningful representation of its running speed. In the US, exactly 2-1/2 cycles of pilot are generated for each film frame that is exposed, in the form of a continuous sine wave. The pilot signal is fed through a sync cable to the tape recorder, where it is recorded at the same time as the audio, but on a separate track. The finished tape is taken to a sound service where it is resolved (i.e. transferred in sync) to perforated magnetic film. Voila! You now have audio on magnetic film that is equivalent to your having lugged the magnetic film recorder on location, only without the lugging. The resolving is done in either of two basic ways: either the reproduced pilot signal is fed to a powerful amplifier driving the motor in the magnetic film recorder (the brute force method); or else the playback speed of the tape deck is automatically adjusted so the pilot frequency and phase are made to match the power line (mains) frequency that is being fed to the magnetic film recorder (the self-resolving method). Either way, 2-1/2 cycles of pilot tone is made to correspond to one frame of magnetic sound film, the same ratio as in the picture film.
