Sound-on-film

Sound-on-film is a motion-picture sound system that records audio as a photographic track on the film strip. The track is converted back into sound during projection.

Optical sound: Audio encoded as variations in a photographic or optical track and recovered by reading changes in transmitted or reflected light. Sound-on-film is the motion-picture use of this broader optical recording method.
Lee de Forest: An American inventor whose Audion vacuum tube helped amplify electrical signals and who developed the Phonofilm sound system. His Phonofilm demonstrated synchronized optical sound on film in the 1920s.
35 mm film: A motion-picture film gauge with a film strip 35 millimeters wide, used widely in theatrical cinema. Its standard frame and soundtrack layouts made it the dominant carrier for optical cinema sound.
Sound-on-disc: A motion-picture sound system that records audio on a separate disc synchronized with the film during playback. It contrasts with sound-on-film because its audio and picture occupy separate media.
Variable-area recording: An optical sound-recording method in which the width of a transparent or opaque track varies with the audio signal. This track shape is one common way sound-on-film encodes changes in audio.
Phonofilm: A sound-on-film system developed by Lee de Forest that recorded audio as a photographic track alongside motion-picture images. It was an early commercial demonstration of the system’s central approach.
Cinema sound system: The equipment and methods used to record, reproduce, and deliver sound in movie theaters. Sound-on-film is one recording and playback format within the broader cinema sound system.
Magnetic soundtrack: A motion-picture audio track recorded as magnetic patterns on film or a separate magnetic medium. It replaced photographic encoding in some productions and offered different recording characteristics.
Variable-density recording: An optical sound-recording method in which a track’s transparency varies with the audio signal. This alternative encodes sound through changes in film-track darkness rather than width.
The Jazz Singer: A 1927 American feature film combining synchronized sound sequences with silent-film intertitles. Its success helped establish demand for synchronized sound in feature films.

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