Synchronous sound

Sync sound (synchronized sound recording) refers to sound recorded at the time of the filming of movies. It has been widely used in movies since the birth of sound movies.

Even in the silent film era, films were shown with sounds, often with musical accompaniment by a pianist or an orchestra keeping time with the screen action. The first synchronization was a turning recording device marked with a white spot. As the white spot rotated, the cameraman hand cranked the camera to keep it in sync with the recording. The method was then repeated for playback, but with the projectionist hand cranking the film projector. "Single-system" sound recorded sound optically to part of the original camera film, or magnetically to a stripe of magnetic coating along the film edge.

"Double-system" sound used independent cameras and sound recorders. The first sync sound standard used recorders and cameras both powered by AC (alternating current) motors - essentially clock motors.

Later the 50 Hz or 60 Hz sine wave, called a Pilottone, was recorded on a second parallel track of an audio recorder.

In double-system film, speed variations of camera and recorder, as well as the elasticity of the magnetic recording tape, requires some positive means of keying the dialogue to its appropriate film frame. The inclusion on the sound recorder of a second, parallel, sync or "Pilotone" track, has been the most common method in use until today. In video recording, synchronism is electronically generated and generally called dual-system sound

If location, a camera is driven by a DC motor, with some sort of governor control to hold it fairly accurate at 24 fps, a sync pulse generator geared to the movement or motor shaft could be employed to provide the sync pulse output. A cable conducts the sync pulse from camera to sound recorder. The sync pulse is typically a sine wave of 50 or 60 Hz with an RMS amplitude of approximately 1 volt.

This double-system audio recording could then be transferred or "resolved" to sprocketed magnetic film, with sprocket holes that match one to one with the original camera film. These two sprocketed media could be run through a "Moviola" or flat-bed editing table such as the Steenbeck for synchronous sound editing.

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