In 1919, American inventor Lee De Forest was awarded several patents that would lead to the first optical sound-on-film technology with commercial application. In De Forest's system, the sound track was photographically recorded on to the side of the strip of motion picture film to create a composite, or "married", print. If proper synchronization of sound and picture was achieved in recording, it could be absolutely counted on in playback. Over the next four years, he improved his system with the help of equipment and patents licensed from another American inventor in the field, Theodore Case.

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