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Phonofilm sound-on-film process

In 1921, de Forest ended most of his radio research in order to concentrate on developing an optical sound-on-film process called Phonofilm. In 1919 he filed the first patent for the new system, which improved upon earlier work by Finnish inventor Eric Tigerstedt and the German partnership Tri-Ergon. Phonofilm recorded the electrical waveforms produced by a microphone photographically onto film, using parallel lines of variable shades of gray, an approach known as "variable density", in contrast to "variable area" systems used by processes such as RCA Photophone. When the movie film was projected, the recorded information was converted back into sound, in synchronization with the picture.

From October 1921 to September 1922, de Forest lived in Berlin, Germany, meeting the Tri-Ergon developers (German inventors Josef Engl (1893–1942), Hans Vogt (1890–1979), and Joseph Massolle (1889–1957)) and investigating other European sound film systems. In April 1922 he announced that he would soon have a workable sound-on-film system. On March 12, 1923, he demonstrated Phonofilm to the press; this was followed on April 12, 1923, by a private demonstration to electrical engineers at the Engineering Society Building's Auditorium at 33 West 39th Street in New York City.

In November 1922, de Forest established the De Forest Phonofilm Company, located at 314 East 48th Street in New York City. But none of the Hollywood movie studios expressed interest in his invention, and because at this time these studios controlled all the major theater chains, this meant de Forest was limited to showing his experimental films in independent theaters (The Phonofilm Company would file for bankruptcy in September 1926.).

After recording stage performances (such as in vaudeville), speeches, and musical acts, on April 15, 1923, de Forest premiered 18 Phonofilm short films at the independent Rivoli Theater in New York City. Starting in May 1924, Max and Dave Fleischer used the Phonofilm process for their Song Car-Tune series of cartoons—featuring the " Follow the Bouncing Ball " gimmick. However, de Forest's choice of primarily filming short vaudeville acts, instead of full-length features, limited the appeal of Phonofilm to Hollywood studios.

De Forest also worked with Freeman Harrison Owens and Theodore Case, using their work to perfect the Phonofilm system. However, de Forest had a falling out with both men. Due to de Forest's continuing misuse of Theodore Case's inventions and failure to publicly acknowledge Case's contributions, the Case Research Laboratory proceeded to build its own camera. That camera was used by Case and his colleague Earl Sponable to record Calvin Coolidge on August 11, 1924, which was one of the films shown by de Forest and claimed by him to be the product of his inventions.

Believing that de Forest was more concerned with his own fame and recognition than he was with actually creating a workable system of sound film, and because of his continuing attempts to downplay the contributions of the Case Research Laboratory in the creation of Phonofilm, Case severed his ties with de Forest in the fall of 1925. Case successfully negotiated an agreement to use his patents with studio head William Fox, owner of Fox Film Corporation, who marketed the innovation as Fox Movietone. Warner Brothers introduced a competing method for sound film, the Vitaphone sound-on-disc process developed by Western Electric, with the August 6, 1926, release of the John Barrymore film Don Juan.

In 1927 and 1928, Hollywood expanded its use of sound-on-film systems, including Fox Movietone and RCA Photophone. Meanwhile, theater chain owner Isadore Schlesinger purchased the UK rights to Phonofilm and released short films of British music hall performers from September 1926 to May 1929. Almost 200 Phonofilm shorts were made, and many are preserved in the collections of the Library of Congress and the British Film Institute.