by Edmund M. DiGiulio ASC Associate Member Cinema Products Corporation
When recording sound simultaneously with filming, it is necessary to provide some means of guaranteeing that the soundtrack will be in perfect synchronism with the film, hi single-system filming, where the sound is recorded directly on the film in the camera, on either a magnetic strip or optical sound track, this is automatically accomplished. In double-system filming, however, speed variations of camera and recorder, as well as the elasticity of the magnetic recording tape, require 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 is the most common method in use today. The sync pulse is typically a sine wave of 50 to 60 Hz with an RMS amplitude of approximately 1 volt. Back in the lab, a "resolver" transfers the sound track onto oxide-coated sprocketed film stock using the sync track as a reference so that the transferred sound track will correspond, frame for frame, with the camera negative. Until the introduction of crystal sync systems, this sync pulse was derived from the camera by another means.
If, for example, the camera was being 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. (See Fig. 1.)
An alternate method, used most commonly on soundstages but also on location, was for the camera to be driven by a synchronous motor operating from AC mains, or on location from an AC generator. In this case the recorder used the mains or alternator as a sync pulse source (Hg. 2).
In crystal drive systems, a crystal oscillator of extremely high accuracy at (or in) the recorder provides the sync pulse. The camera is in turn driven by a specially designed DC motor and control circuit which is capable of operating in exact synchronism with a self-contained crys tal oscillator of comparable accuracy (Fig. 3). The crystal-controlled motor operation is analogous to that of a sync motor operating in synchronism with AC mains. In the case of AC synchronous operation, both camera and recorder are tied to the AC source as a common reference. In the case of crystal operation both camera and recorder reference to self-contained crystal oscillators which are so accurate that the effect is the same as if they had been tied together.
Since the reference is absolute, any number of cameras can be operated simultaneously, in perfect synchronism, with a single recorder. The basic advantage to the crystal drive system, however, is that it eliminates the need for power cables and any umbilical connection between the camera and recorder. Most crystal motors commonly in use today employ some means of indicating when the motor is running out of synchronism. This is usually a beep tone or a blinking light. This is a reliable indicator of good synchronous operation and is a corollary benefit.
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