Double System Sound

A method of producing sound motion pictures where the sound track is recorded on a magnetic tape recorder that is synchronized with the movement of the film in the camera. The synchronization is done by recording a special tachometer signal on one track of the tape. This signal is generated by the camera and is at 60 Hz when running at 24 f.p.s (the standard frame rate for film). When the tape of the sound track is played back, the sync signal is sensed by a device called a resolver, which adjusts the speed of the tape player to match the speed of the movie projector.