Even before 1920, silent films had become popular and were being shown in many cities around the world. Inventors, including heavyweights such as Thomas Edison, had spent thirty years trying to perfect a way to link the phonograph and motion pictures. Following his success with his Audion vacuum tube invention, Lee De Forest also experimented with sound systems for motion pictures, but he took a unique approach.
Instead of trying to match up sounds recorded on a phonograph disk with a motion picture projector, De Forest wanted to put the sound right on the film, next to the image. He did not do this with a groove like the phonograph, but used a completely different kind of recording called optical recording. His optical recorder converted sound into a pulsing beam of light, and then photographed the light to preserve a record of it on a narrow strip along the edge of the film. It is reproduced by a sensitive light detector, which converts the image into an electrical signal that can be amplified and heard through a loudspeaker.
About the same time, however, several companies including Fox Movietone, Western Electric, Tonfilm (in Germany), and Tri-Ergon (also in Germany) were preparing to offer similar sound-on-film systems, and this was the type of technology that remained standard for motion pictures until the late 1940s.