"Optical sound is a means of storing sound recordings on transparent film. Originally developed for military purposes, the technology first saw widespread use in the 1920s ...
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as a sound-on-film format for motion pictures. Optical sound eventually superseded all other sound film technologies until the advent of digital sound became the standard in cinema projection booths. Optical sound has also been used for multitrack recording and for creating effects in some musical synthesizers." - (en.wikipedia.org 29.11.2023)
