Sound-recording techniques
The art of sound recording for motion pictures has developed dramatically. Most of the improvements fall into three areas: fidelity of recording; separation and then re-synchronization of sound to picture; and ability to manipulate sound during the post-production stage.
Until the early 1950s the normal recording medium was film. Sound waves were converted into light and recorded onto 35-mm film stock. Today the principal use of optical recording is to make a master optical negative for final exhibition prints after all editing and rerecording have been completed.
Magnetic recording offers better fidelity than optical sound, can be copied with less quality loss, and can be played back immediately without development. Magnetic tracks were first used by filmmakers in the late 1940s for recording music. The physical principles are the same as those of the standard tape recorder: the microphone output is fed to a magnet past which a tape coated with iron oxide runs at a constant speed. The changes in magnetic flux are recorded onto the tape as an invisible magnetic “picture” of the sound.
At first the sound was recorded onto 35-mm film that had a magnetic coating. Today sprocketed 35-mm magnetic tape is used during the editing stages. For onset recording, however, the film industry converted gradually to the same unperforated quarter-inch tape format widely used in broadcasting, the record industry, and even the home. Documentary and independent filmmakers were the first to develop and use the portable, more compact apparatus.
Improvements in magnetic recording have paralleled those in the recording industry and include the development of multiple-track recording and Dolby noise reduction.
Double-system recording
Although it is possible to reproduce sound, either optically or magnetically, in the same camera that is photographing a scene (a procedure known as single-system recording), there is greater flexibility if the sound track is recorded by a different person and on a separate unit. The main professional use for single-system recording is in filming news, where there is little time to strive for optimal sound or image quality. Motion-picture sound recording customarily uses a double system in which the sound track remains physically separate from the image until the very last stages of post-production.
Double-system shooting requires a means of re-matching corresponding sounds and images. The traditional solution is to mark the beginning of each take with a “clapper,” or “clap stick,” a set of wooden jaws about a foot long, snapped together in the picture field. The instant of clacking then is registered on both picture and sound tracks. Each new take number is identified visually by a number on the clapper board and aurally by voice. A newer version of the clapper is a digital slate that uses light-emitting diodes and an audio link to synchronize film and tape.
Precise synchronism must be maintained between camera and recorder so that sound can be kept perfectly matched to the visuals. (Lack of perfect synchronism is most conspicuous in close-up shots in which a speaker’s lips do not match his voice.) On some occasions several cameras shoot a scene simultaneously from different points of view while only one sound recording is made, or several sound records may be taken of a single shot. Thus, to maintain synchronism, all sound and picture versions of a particular scene must be recorded at the same speed; the camera and the recorder cannot fluctuate in speed. One way to achieve this is to drive all cameras and recorders from a common power supply. Alternatively, synchronization may be achieved through the automatic, continual transmission from cameras to recorders of a sync-pulse signal sent by cable or wireless radio. More convenient yet is crystal sync, whereby the speed of both cameras and recorders is controlled through the use of the oscillation of crystals installed in each piece of equipment. The most advanced system uses a time-code generator to emit numbers in “real-time” on both film and tape.
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