Movie Cameras - Sound Synchronization

Sound Synchronization

One of continuing problems in film is synchronizing a sound recording with the film. Most film cameras do not record sound internally; instead, the sound is captured separately by a precision audio device (see double-system recording). The exceptions to this are the single-system news film cameras, which had either an optical—or later—magnetic recording head inside the camera. For optical recording, the film only had a single perforation and the area where the other set of perforations would have been was exposed to a controlled bright light that would burn a waveform image that would later regulate the passage of light and playback the sound. For magnetic recording, that same area of the single perf 16 mm film that was prestriped with a magnetic stripe. A smaller balance stripe existed between the perforations and the edge to compensate the thickness of the recording stripe to keep the film wound evenly.

Double-system cameras are generally categorized as either "sync" or "non-sync." Sync cameras use crystal-controlled motors that ensure that film is advanced through the camera at a precise speed. In addition, they're designed to be quiet enough to not hamper sound recording of the scene being shot. Non-sync or "MOS" cameras do not offer these features; any attempt to match location sound to these cameras' footage will eventually result in "sync drift", and the noise they emit typically renders location sound recording useless.

To synchronize double-system footage, the clapper board which typically starts a take is used as a reference point for the editor to match the picture to the sound (provided the scene and take are also called out so that the editor knows which picture take goes with any given sound take). It also permits scene and take numbers and other essential information to be seen on the film itself. Aaton cameras have a system called AatonCode that can "jam sync" with a timecode-based audio recorder and prints a digital timecode directly on the edge of the film itself. However, the most commonly used system at the moment is unique identifier numbers exposed on the edge of the film by the film stock manufacturer (KeyKode is the name for Kodak's system). These are then logged (usually by a computer editing system, but sometimes by hand) and recorded along with audio timecode during editing. In the case of no better alternative, a handclap can work if done clearly and properly, but often a quick tap on the microphone (provided it is in frame for this gesture) is preferred.

One of the most common uses of non-sync cameras are the spring-wound cameras used in hazardous special effects, known as "crash cams". Scenes shot with these have to be kept short, or resynchronized manually with the sound. MOS cameras are also often used for second unit work or anything involving slow or fast-motion filming.

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