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Speed Reference

From Chace Audio

The running speed of a film camera or projector is determined by the electricity coming through the electrical outlet. Standard AC (alternating current) electricity in North America flows at 60Hz (or cycles) per second; film is run at 24 frames per second. If the AC is exactly 60Hz, then the film will be running at exactly 24 frames per second. Any variance in the electrical current will cause the film to speed up or slow down.

As long as sound and picture are contained on the same element, such as videotape or an optical film print, the fact that the electricity alternates and is rarely exactly 60 cycles per second isn’t important. The audio is tied to the picture so they both play back at the same speed regardless. But what happens when some elements have sound and others have picture that were recorded on different days in different cities with different machines? If their start marks are lined up, can they simply be expected to run at the correct speed? The answer is no. This practice is referred to as running something “wild” or without reference.

The earliest method used to provide a speed reference for an audio element on tape was to record a 60Hz tone – based on the AC at the time the recording was made – onto the element. The tone can then be “resolved” when the element is played back. This is accomplished by referencing the 60Hz AC that the playback machine is receiving through the electrical outlet to the 60Hz on the tape. The audiotape speed is increased or decreased to match the current coming through the outlet. If multiple audio elements are lined up via start marks and are all being played back at once, resolving each element in this fashion will ensure that they playback at the correct speed and do not drift out of sync with each other. This type of tone is called a “Pilot Tone.” Other versions exist including Rangertone™ sync, a short-lived but specialized tone-based resolving method requiring a special type of audiotape playback head to be properly resolved.

Of course, this is how it was done in North America. Europeans opted to go with 50Hz AC rather than 60Hz AC for mathematical reasons. Therefore, audiotape recorded in Europe was recorded with a 50Hz Pilot Tone.