soundonfilm

Sound on film refers to any method of recording and reproducing audible sound using the same strip of film carrying the picture, so that audio playback is synchronized with the motion image. The most common form is optical sound, in which the audio waveform is converted into a light modulation read by a projector's sensor. There are two principal optical formats: variable-density, where the soundtrack's opacity varies with the sound, and variable-area, where the width of a transparent track is modulated. In both cases the light signal is converted back into an electrical signal and then into sound.

Historically, sound-on-film emerged in the 1920s as an alternative to sound-on-disc systems. Early experiments by inventors such as Lee de Forest with Phonofilm and other European efforts demonstrated the concept of printed soundtracks on film. By the late 1920s and into the 1930s, optical sound-on-film became the dominant cinema soundtrack technology, enabling tighter synchronization and paving the way for sound films worldwide. Magnetic sound-on-film was introduced later as a higher-fidelity option on some formats, using a magnetic stripe or coating alongside the image track, particularly on 16mm and some 35mm prints.

Today, most modern cinema sound uses digital formats, but optical sound-on-film remains a historical and archival technology, and some contemporary prints retain an optical soundtrack for compatibility with older playback equipment. The term "sound on film" is thus a broad descriptor for any system that stores audio directly on the film to accompany the moving images.