Short ‘pip’ that sounds at a specified place in the leaders.
A ‘sync pulse’ is an inaudible timing reference recorded on the magnetic tape used in double-system recording. The source can be a generator in the camera cabled to the tape recorder, or an oscillating crystal in the recorder when the camera also has a crystal.
When the sound is transferred to magnetic film for editing, a resolver reads the reference and ensures that the tape runs at the same speed as during shooting. In this way the magnetic work print can be placed in sync with the images for which the original sound was recorded.