In the early years of the 20th Century there were efforts to associate sound with motion pictures, usually using gramophone records. Few of these have survived. The permanent introduction of sound into the cinema came at the end of the 1920s. At that time the sound was provided by two different methods, then known as ‘sound-on-disc’ and ‘sound-on-film’. The sound-on-disc used a gramophone type of record, which was 12 inches or 16 inches in diameter, and rotated at 33 1/3 revolutions per minute but the sound started near the center and continued toward the outer edge.
Sound-on-film used a photographically recorded track placed on the film in the space formerly occupied by the extreme left-hand part of the picture. These ‘photographic’ tracks have since become more commonly known as ‘optical’ tracks. Since about 1951 ‘magnetic’ sound tracks, in several different formats, have come into wide use. All photographic or optical sound tracks fall into one or two categories, i.e. variable area or variable density. (Basic Film Handling)
Brown, H., 1985, Basic Film Handling, in FIAF Technical Manual First Edition, FIAF Preservation Commission, Federation Internationale des Archives du Film, Brussels, Belgium