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 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 Optical sound eventually superseded all other sound film technologies until the advent of digital sound became the standard in cinema projection booths.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.
1914-1921: Naval and military use Building on the principle first demonstrated by the Photophone of Alexander Graham Bell in 1880, optical sound was developed by several inventors with an interest in wireless communication through transmission of light, primarily for ship-to-ship use. The idea was that sound pulses could be converted into light pulses, beamed out from one ship and picked up by another, where the light pulses would then be reconverted into sound. A pioneer in this technology was American physicist Theodore Case. While studying at Yale, Case became interested in using modulated light as a means of transmitting and recording speech. In 1914, he opened the Case Research Lab to experiment with the photoelectric properties of various materials, leading to the development of the Thallofide (short for thallium oxysulfide) Cell, a light-sensitive vacuum tube. The Thallofide tube was originally used by the United States Navy in a top secret ship-to-ship infrared signaling system developed at Case's lab with his assistant Earl Sponable. Case and Sponable's system was first tested off the shores of New Jersey in 1917, and attending the test was Thomas Edison, contracted by the Navy to evaluate new technologies. The test was a success, and the U.S. Navy used the system during and after World War I. Contemporary with the work of Case and Sponable was Charles A. Hoxie's Pallophotophone (from Greek roots meaning "shaking light sound"), manufactured by General Electric (GE). Similar to the Case infrared system used by the Navy, the Pallophotophone was also intended for wireless communications at sea but was then adapted for recording speech. With GE's backing, Hoxie's invention was used in 1922–1923 to record then-Vice-president Calvin Coolidge and others for radio broadcasts. The early work by Case, Sponable and Hoxie was instrumental in the development of sound-on-film systems for motion pictures during the 1920s.
Film and radio
Most of the inventions which led to optical sound-on-film technology employed the use of an electric lamp, called an 'exciter', shining through a translucent waveform printed on the edge of a film strip. When the light shines through the film, it is read by a photo-sensitive material and fed through a processor, which converts the photovoltaic impulse into an electrical signal that is then amplified and converted into analog sound waves through a speaker. Three types of optical sound-on-film technology emerged in the 1920s: Phonofilm, Photophone and Movietone. A fourth major contender for the sound film market - Warner Brothers' Vitaphone sound-on-disc system which synchronized large-size (16") phonographic records with a film's projector was used on early talkies, such as their' 1927 hit The Jazz Singer (which was marketed as being "all singing" though the talking was sporadic, used in only several isolated sequences), utilized Vitaphone discs, but by 1931, optical sound-on-film would supplant the separate sound-on-disc technology.
1919-1926: Phonofilm
After the war, Theodore Case and Earl I. Sponable collaborated with fellow wireless communications pioneer Lee de Forest, inventor of the Audion tube, to apply their optical sound system to motion pictures. De Forest had been granted general patents for a sound-on-film process in 1919, though it was the Case Research Lab's inventions that made de Forest's systems workable. Case Lab first converted an old silent-film projector into a recording device in 1922, using the projector's light to expose a soundtrack onto film. The process (which de Forest called Phonofilm) recorded sound as parallel lines of variable shades of gray, photographically transcribing the electrical waveforms from a microphone, which were translated back into sound waves when the movie was projected. Case Lab fine-tuned the process with an invention called the 'Aeo-light' for use in sound cameras. During filming, audio signals modulated the Aeo-light to expose the film's audio directly inside the camera, streamlining Phonofilm's process for synchronizing a motion picture with its soundtrack. In 1924, Sponable focused on the design of these single-system cameras, in which both sound and picture were recorded on the same negative. He approached Bell & Howell to modify one of their cameras for his design, but the results were unsatisfactory. Later, the Wall Camera Corporation rebuilt the machine with improved results. De Forest also worked with early newsreel maker, Freeman Harrison Owens, who by 1921, had developed his own patented sound camera, and spent time in Berlin working with the Tri-Ergon corporation and researching the development of European sound film systems. There, he met Finnish inventor, Eric Tigerstedt ("Finland's Thomas Edison"), who improved Phonofilm's amplification system to be audible in a large theater. Phonofilm was used mainly to record stage performances, speeches, and musical acts in and around New York City, but Hollywood movie studios expressed little interest in the system. Since the Hollywood studios controlled the major theater chains, de Forest showed his films in independent theaters in a short-form series, akin to vaudeville, which included Max and Dave Fleischer's Song Car-Tunes. The Fleischers used the Phonofilm process for their animated shorts, which included the now-classic bouncing-ball gimmick. In 1924, Owens parted ways with de Forest, and Case followed suit in 1925, because of de Forest's taking sole credit for Phonofilm. In August 1926, Warner Brothers introduced their Vitaphone sound-on-disc system, developed by Western Electric, with the John Barrymore film Don Juan. One month later, the Phonofilm Company filed for bankruptcy. Case and Sponable went on to implement their optical sound-on-film innovations as the Movietone sound system, and the UK rights to Phonofilm were bought up by theater chain owner Isadore Schlesinger, who used the technology to release short films of British music hall performers through 1929.
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