Double-system recording

Double-system recording is a form of sound recording used in motion picture production whereby the sound for a scene is recorded on a machine that is separate from the camera or picture-recording apparatus.

Double-system recording is the standard procedure on motion pictures that are originally photographed on film. Recording sound-on-film directly at the time of photography has several technical limitations, and no professional motion picture camera supports this option, so all production sound is recorded on a separate recorder. This procedure requires that both camera and sound recorder share a very accurate time reference, and that the speed of the camera and sound recorders be carefully governed. Originally this was done with an electro-mechanical interlock between the camera and recorder, necessitating a physical link, a cable, between recorder and camera. As quartz-based timers came into common use, film cameras and sound recorders adopted these, and these were accurate enough to remove the need for an interlock cable.