Movietone sound system

The Movietone sound system is an optical sound-on-film method of recording sound for motion pictures that guarantees synchronization between sound and picture. It achieves this by recording the sound as a variable-density optical track on the same strip of film that records the pictures. Although sound films today use variable-area tracks, any modern motion picture theater (excluding those that have transitioned to digital cinema) can play a Movietone film without modification to the projector (though if the projector has been fitted with red LED or laser photocells, the reproduction quality from a variable density track will be significantly impaired). Movietone was one of four motion picture sound systems under development in the U.S. during the 1920s, the others being DeForest Phonofilm, Warner Brothers' Vitaphone, and RCA Photophone, though Phonofilm was primarily an early version of Movietone.

As a student Theodore Case became interested in the use of modulated light to carry speech, and in 1916 set up a laboratory to study the photo-electric properties of materials. He developed the Thalofide cell, a sensitive photocell, which was used as part of an infra-red communication system by the U.S. Navy during, and for some years after, WW1.

In 1922 Case and his assistant Earl I. Sponable turned their attention to "talking pictures". In that same year Case was approached by Lee de Forest who had been attempting since 1919 to create an optical soundtrack for motion picture film in a system he called Phonofilm. De Forest was having little success and turned to Case for help, so from 1922 until 1925 Case and de Forest collaborated in the development of the Phonofilm system. Amongst other Case inventions Case contributed the Thalofide photocell and also the Aeo-light, a light source that could be readily modulated by audio signals and was used to expose the soundtrack in sound cameras.

In 1925, however, Case ended his relationship with de Forest due in the main to de Forest's tendency to claim the entire credit for the Phonofilm system for himself despite the fact that most of the critical inventions had come from Case. Documents supporting this, including a signed letter by De Forest that states that Phonofilms are only possible because of the inventions of Case Research Lab, are located at the Case Research Lab Museum in Auburn, New York. In 1925, therefore, Case and Sponable continued the development of their own system, now called Movietone.

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