Pilottone - Technology

Technology

The synchronization is obtained when a pulse cable is connected from Motion picture camera to an audio recorder such as those made by Nagra. A camera with a Sync motor sends a 60/50 Hz signal to the recorder, which is recorded as a sine wave pilot tone.

The recorder has a double recording head, similar to a two-track recorder. Each of the two cores of the recording head records both the audio signal, and the pilot tone. The audio signal to be recorded is applied identically to both cores of the recording head, and the pilot tone sine wave is applied in a push-pull arrangement (180 degrees out-of-phase).

Unlike the recording head, the playback head has a single core. The playback head gap covers both of the tracks created by the record head. The magnetic field changes across the width of the gap are effectively added in the playback head, so the playback head reproduces the audio signal, which is the same in both tracks. But the pilot tone cancels itself in the playback head: at a point on the tape where it has a certain intensity on the upper track, it has the opposite intensity on the lower track, and so always sums to zero.

On playback, the record head is used as a push-pull playback head in order to reproduce the pilot tone. All speed variations of the camera and tape can be detected as deviations from 60/50 Hz, and compared at the time of playback with in built quartz reference oscillator. For cinematic audio, speed variations are rectified (resolved) at the time of transfer to the perforated 35mm/16mm audio tape. At that time the mains power supply frequency is also taken as reference. (The selection of the 60/50 Hz equipment depends on the power supply in the country where filming is being conducted. North America has a supply of 60 Hz whereas Europe and some Asian countries have 50 Hz.)

Normal audio tape recorders have good regulation of tape speed, but not sufficiently precise to guarantee that a playback machine will exactly match the speed of the recorder over long periods 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 a quarter inch after 800 feet or more. Pilottone provides such a system.

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.