What is meant by the term sound-on-film?

Sound-on-film is a class of sound film processes in which the sound that accompanies a picture is physically recorded onto a photographic film. It can either record an analog soundtrack for digital soundtrack and signal may be recorded optically, or magnetically.

The most prevalent method of recording analog sound on a film print is by stereo variable area (SVA) recording. The technique was first used in the mid-1970s.

During the 1990s, three different digital soundtrack systems for 35 mm cinema release prints were introduced- the Dolby digital, stored between the perforations on the sound side, Sony Dynamic Digital Sound or SDDS, that stored in two redundant strips along the outside edges, and DTS, in which sound data is stored on separate compact discs.

Since the three systems appear on different parts of the print, one movie can contain all of them, allowing broad distribution regardless of the sound system installed at individual theatres.