In a multitrack tape recorder, the use of the record head to replay material from other tracks to be heard by the musicians while they simultaneously record a new track. This is essential for accurate synchronization as the extra few milliseconds afforded by its position will compensate for the inevitable delay if the signal was taken from the main replay head which is some millimeters downstream in the tape stream. The replay quality is less good when using the record head for a function for which it was not designed, thus it is important that the machine returns to monitoring from the main replay head during mixdown.
(2) On a recorder used for synchronization of sound with motion pictures or videotape, a separate head that records and plays the synchronizing tone or signal. See sel-sync(2).