Sound-on-film is a class of sound film processes where the sound accompanying a picture is recorded onto 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.
Analog sound-on-film recording
The most prevalent current method of recording analogue sound on a film print is by stereo variable-area (SVA) recording, a technique first used in the mid-1970s as Dolby Stereo. A two-channel audio signal is recorded as a pair of lines running parallel with the film's direction of travel through the projector's screen. The lines change area (grow broader or narrower) depending on the magnitude of the signal. The projector shines light from a small lamp, called an exciter, through a perpendicular slit onto the film. The image on the small slice of exposed track modulates the intensity of the light, which is collected by a photosensitive element: a photocell, a photodiode or CCD.
In the early years of the 21st century distributors changed to using cyan dye optical soundtracks on color stocks instead of applicated tracks, which use environmentally unfriendly chemicals to retain a silver (black-and-white) soundtrack. Because traditional incandescent exciter lamps produce copious amounts of infra-red light, and cyan tracks do not absorb infra-red light, this change has required theaters to replace the incandescent exciter lamp with a complementary colored red LED or laser. These LED or laser exciters are backwards-compatible with older tracks.
Earlier processes, used on 70 mm film prints and special presentations of 35 mm film prints, recorded sound magnetically on ferric oxide tracks bonded to the film print, outside the sprocket holes. 16 mm and Super 8 formats sometimes used a similar magnetic track on the camera film, bonded to one side of the film on which the sprocket holes had not been punched ("single perforated") for the purpose. Film of this form is no longer manufactured, but single-perforated film without the magnetic track (allowing an optical sound track) or, in the case of 16 mm, utilising the soundtrack area for a wider picture (Super 16 format) is readily available.