Sound-on-film

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Simulated 35 mm film with soundtracks - The outermost strips (on either side) contain the SDDS soundtrack as an image of a digital signal. In from them are the perforations used to drive the film through the projector, along with the Dolby Digital soundtrack between them. On the left side only the two tracks of analog soundtrack can be seen, often encoded using Dolby SR to simulate four tracks. Just in from the analog track is the timecode used to synchronize a DTS soundtrack. Finally, in the center, is the image, in this case compressed horizontally by anamorphic optics.

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 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 magnitude 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, which allowed the two-channel format to record a center and surround channel, and companding noise reduction, which allows a constant signal-to-noise ratio to be delivered over a wide dynamic range.

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, introduced by William Fox in 1927,
Fantasound, where sound and picture were recorded on separate strips of film,
Phonofilm, patented by Lee De Forest in 1919,
RCA Photophone, introduced in 1928,

Contrast with: sound-on-disc

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