Sound-on-film

Sound-on-film refers to a class of sound film processes where the sound accompanying picture is physically recorded onto photographic film, usually, but not always, the same film strip of film carrying the picture. Sound-on-film processes can either record an analogue sound track or digital sound track, and may record the signal either optically or magnetically.

Analogue sound-on-Film recording

The most prevalent modern method of recording sound on a film print is by stereo variable-area (SVA) recording, encoding a two-channel audio signal as a pair of lines running parallel with the film's direction of travel through the projector. The lines change area (grow broader or narrower) depending on the energy of the signal. The projector shines a light or LED, 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 photodiode or CCD.

Commonly, the audio signal recorded onto an SVA track is encoded through a phase matrix and companding noise reduction, allowing the two-channel format to record, a center and surround channel, and significantly increasing the recording's signal-to-noise ratio.

Earlier processes, used on 70mm film prints and special presentations of 35mm film prints, recorded sound magnetically on ferric oxide tracks bonded to the film print, outside the sprocket holes.

Sound-on-film formats

Almost all modern motion picture sound formats are sound-on film formats, including:

• Dolby Stereo and Dolby SR,
• Ultra-Stereo,
• Dolby Digital,
• Sony Dynamic Digital Sound

Older, now obsolete formats include:

• Fox Movietone,
• Fantasound, where sound and picture were recorded on separate strips of film,

Contrast with: sound-on-disc