Normal audio recorders have a good system for regulating their speed with a tachometer, but such a system cannot guarantee that a playback machine will exactly match the speed of the recorder over long spaces of time. Such a system would need to record exactly how much tape passes the head in such an amount of time, and would have to be accurate to the quarter-inch after 800 feet or more. Pilottone provides such a system.
The pilottone signal itself is very simple. On a given track of analogue tape medium, a pilottone-recording audio tape recorder (ATR) records a 60 Hz sine wave (Europeans record a 50 Hz sine wave.) This sine wave is generated in the recorder by a highly accurate quartz oscillator. This pilottone is recorded alongside the audio being recorded. This is useful because the pilottone is in effect an absolute record of how fast the tape was moving at the time the audio was recorded.
When the tape is played back on a pilottone-reading tape player, it needs to only resolve the pilottone signal on the tape. The player has a quartz oscillator of its own, and when the operator hits play, the player tries to match the sine wave of the recorded pilottone with the pilottone being generated by its own quartz crystal. When they match up, the player knows that the tape is moving across its play head exactly as fast as it was across the record head when it was originally recorded.
Pilottone provides a near-perfect system for speed resolution, but was quickly made obsolete by the advent of SMPTE timecode. Additionally, digital audio recorders intrinsically resolve, without a need for a separate clock signal.