Film Sound & Crystal Sync Through The Ages
Information from Tobin Cinema Systems
It might seem logical to just record the audio optically on the same film as the picture. This was done for 35mm, 16mm and Fuji Single-8 newsreels and home movies; however the picture film is too grainy, unsharp and low in contrast for good photographic sound quality. Magnetic sound was used in 8mm, super-8 and 16mm but magnetically pre-striped camera films have now all been discontinued.
Double-System Perforated Magnetic Film:
Using two synchronous motors (big brothers of the motor in your electric clock), gear drive, and perforated magnetic film, the sound can be easily recorded in sync with the picture film, running precisely the same number of sprocket holes per second. A mag film recorder, however, is huge and heavy and it and the camera require a generator truck or long extension cord for location work! (A clapper board is used in filming, to enable syncing up each picture and sound take during editing, for this and for subsequent methods.)
Double-System Pilot Recording:
The sound is more conveniently recorded using a battery-powered portable tape recorder, either 1/4" or cassette. The tape has no sprocket holes, so sync is maintained by recording a pilot frequency in their stead. This is generated by the camera, and is a precise and meaningful representation of its running speed. In the U.S. exactly 2-1/2 cycles of pilot are generated for each film frame that is exposed, in the form of a continuous sine wave. The pilot signal is fed through a sync cable to the tape recorder, where it is recorded at the same time as the audio, but on a separate track. The finished tape is taken to a sound service where it is resolved (i.e., transferred in sync) to perforated magnetic film. Voila! You now have audio on magnetic film that is equivalent to your having lugged the mag film recorder on location, only without the lugging. The resolving is done in either of two basic ways: either the reproduced pilot signal is fed to a powerful amplifier driving the motor in the mag film recorder (the brute force method); or else the playback speed of the tape deck is automatically adjusted so the pilot frequency and phase are made to match the power line (mains) frequency that is being fed to the mag film recorder (the self-resolving method). Either way, 2-1/2 cycles of pilot tone is made to correspond to one frame of magnetic sound film, the same ratio as in the picture film.
Pilot recording works fine, but what if the camera is in a helicopter and there's no feasible way to connect a sync cable to the audio recorder on the ground? What if the plugs don't fit, or the cable breaks, or people and equipment are getting tangled in it? (This happened all the time.) What if you want to shoot with several cameras at once, and want to keep the sound in sync with all of them? The answer is crystal sync, where a camera's running speed is locked to the digitally divided oscillations of a quartz crystal, with an accuracy of a few parts per million. The function of the pilot cable was to feed a representation of the camera's speed to the recorder. Since with crystal control that speed is precisely known, the cable can be replaced by a similarly accurate crystal generator mounted in the audio recorder. This eliminates any connection between camera and recorder, but permits them to stay in sync with each other. A pilot signal is recorded as above, but it comes from the built-in crystal instead of the camera. The resulting tape is resolved to mag film just the same as a pilot tape. You still need a clapper board for a start mark.
What About Those Weird Speeds And Frequencies?
The traditional numbers are 24 FPS with 60 Hz pilot, and 25 FPS with 50 Hz pilot. For transfer to U.S. NTSC video only without making film prints, some people prefer using 29.970 or 30 FPS for eliminating “judder” (an irritating uneven strobing because of the 2-3 pulldown used to transfer 24 FPS film.) Filming at 29.970 FPS will also give a stationary shutter bar on any NTSC video monitors with a CRT (picture tube) that are in the scene. For the best results when filming from a video or computer monitor, use a camera with a 180° shutter opening and our now discontinued “Milliframe Controller” or “Videoframe Controller” to move the shutter bar to where you want it, and keep it there. (Special techniques are required with cameras lacking a mirror shutter.) Computer and video monitors with the latest LCD and plasma screens no longer need the film camera to run at an odd speed. Also, 29.97 and 30 FPS speeds have fallen out of favor lately as they make the action too smooth, reducing the desired “film look.”
©1998-2013 C. H. Tobin and used by permission