"If it can be written, or thought, it can be filmed..." STANLEY KUBRIC
From Encyclopedia PRO
Synchronous Sounds are those sounds which are synchronized or matched with what is viewed. For example: if the film portrays a character playing the piano, the sounds of the piano are projected.
Both Dialogue and Sound Effects may be recorded during shooting, but sound effects are usually added later. Synchronous Sound is recorded on either magnetic film or tape. On magnetic film perfect synchronization between the picture and the sound can be achieved only if the film and the tape are moving at the same speed. Otherwise, dialogue will not correspond with the lip movements of the actors, and sound effects will not match the action in the film. Synchronous motors on both camera and recorder, as well as the same sort of perforations on both the magnetic film and the camera film, assure synchronization, or sync.
If, however, Magnetic Tape is used, it is impossible to run both the tape and the camera at exactly the same speed because the tape both shrinks and expands and is unperforated. Instead, a pulse-sync signal is generated by the camera and recorded on the tape along with the dialogue. This signal is used later in the laboratory to allow for proper resynchronization when the tape-recorded sound is transferred to magnetic perforated film for editing and rerecording purposes.
Recording is handled by a Sound Crew. While shooting is in progress, a sound engineer, called a mixer, watches the action and regulates the volume and the balance of voices by means of an electronic console. Other members of the sound crew adjust microphones to follow the action and to keep them out of the camera's range.
In recording dialogue, the sound crew must ensure that the sound of each voice is consistent throughout the film, the direction from which voices come corresponds with the movement of the actors, and the sound perspective—the distance from which sound is heard—matches the visual perspective.