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[[File:35mm film audio macro.jpg|right|thumb|250px|Edge of a 35-mm film print showing four types of soundtrack. The stereo optical sound strip is located on the right, with waveforms for left and right channels.
To the far left is the SDDS digital track (blue area to the left of the sprocket holes), then the Dolby Digital (grey area between the sprocket holes labeled with the Dolby "Double-D" logo in the middle), and to the right of the analog optical sound is the DTS time code (the dashed line to the far right.)]]
Optical sound is a means of storing on transparent film. Originally developed for military purposes, the technology first saw widespread use in the 1920s as a sound-on-film format for . Optical sound eventually superseded all other sound film technologies until the advent of digital sound became the standard in cinema projection booths.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.
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 language 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.
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") 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.
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 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 Fleischer 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. Case Lab Museum website 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.
By the mid-1920s, GE adapted Hoxie's invention for motion picture sound playback, subsequently marketed as a commercial product by then-GE subsidiary RCA as the 'RCA Photophone'. The first demonstrations of the Photophone, were given in 1926, and in 1927 a sound version (music plus sound effects only) of the silent film Wings, toured to a dozen specially equipped theaters.History of Sound Motion Pictures, E.W. Kellog, Journal of the SMPTE Vol 64 June 1955
In 1926, Fox hired Sponible, bought Case's patents (they had already acquired Freeman Owens' and Tri-Ergon's),See Freeman Harrison Owens (1890–1979), op. cit. A number of sources erroneously state that Owens's and/or the Tri-Ergon patents were essential to the creation of the Fox-Case Movietone system. and mass-produced Case's Aeo-light for use in all Movietone News cameras from 1928 to 1939. These cameras recorded all Fox feature films during this period, beginning with F. W. Murnau's (1927). As the first professionally produced feature with an optical soundtrack, it included mostly music and sound effects, with a very few unsynchronized words.
After 1931, Fox's feature film production moved to a two-machine system which Western Electric had developed from the RCA Photophone, with the advent of a light-valve invented by Edward C. Wente. In this system, one camera shot the frames, and a second lens-less "sound camera" served as an optical recorder, which was mechanically interlocked with the picture. Fox continued making with single-system cameras due to their ease of mobility.
Due to film grain and possible dust on the soundtrack, optical sound could be noisy or have crackling sounds, especially when projecting worn . In low-volume sections (where the noise would be especially noticeable) noise reduction was originally performed either by partly masking the track or, in variable area recording, narrowing the width of the transparent oscillations. Later, electronic noise reduction was used (e.g. analog Dolby A).
As digital sound became the standard of sound reproduction in the 21st century, 35 and 70-mm films have increasingly included a digital version of the soundtrack on the edges of the film strip. Most films continue to be processed with both digital and analog soundtracks so they may be read by any projection systems in a movie theater.
The Orchestron was a version of the Optigan built by Vako Synthesizers Inc., intended for professional use as an alternative to the Mellotron in the mid-1970s. The Orchestron featured improved recorded sounds over the Optigan, though many professional musicians of note have performed and recorded using Mattel's toy version.
A resurgence in interest in the Optigan has led to a circuit of collectors trading program discs. Though originally marketed as a toy instrument, the Optigan was used by professional musicians to achieve unusual sounds, and the instrument made on recordings by Bruce Haack (1973), Alan Steward (1976), Steve Hackett (1980) and Devo (1981). In the 1990s the Optigan became popular as a vintage synthesizer, and samples of its sounds were released as digital software, making the sounds accessible to musicians not able to obtain the actual instrument. Since then, Optigan music has been used by numerous artists working in popular music, television, film, and is the featured instrument for the band Optiganally Yours.
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