Sound-on-film

Sound-on-film is a class of sound film processes where the sound accompanying a picture is recorded on photographic film, usually, but not always, the same strip of film carrying the picture. Sound-on-film processes can either record an analog sound track or digital sound track, and may record the signal either optically or magnetically. Earlier technologies were sound-on-disc, meaning the film's soundtrack would be on a separate phonograph record.[1]

Left: Movietone track with variable density. Right: Variable area track.
Film picture with complementary Variable Density audio track.
Edge of a 35mm film print showing the soundtracks. The outermost strip (left of picture) contains the SDDS track as an image of a digital signal; the next contains the perforations used to drive the film through the projector, with the Dolby Digital track (grey areas) with the Dolby Double-D logo, between them. The two tracks of the analog soundtrack on the next strip are bilateral variable-area, where amplitude is represented as a waveform. These are generally encoded using Dolby Stereo matrixing to simulate four tracks. Finally, to the far right, the timecode used to synchronize with a DTS soundtrack CD-ROM is visible.