Sound-on-film is a class ofsound film processes where the sound accompanying a picture is recorded on photographic film, usually, but not always, the same strip of film carrying the picture. Sound-on-film processes can either record ananalog sound track ordigital sound track, and may record the signal eitheroptically ormagnetically. Earlier technologies weresound-on-disc, meaning the film's soundtrack would be on a separatephonograph record.[1]
Sound on film can be dated back to the early 1880s, when Charles E. Fritts filed a patent claiming the idea. In 1923 a patent was filed by E. E. Ries, for a variable density soundtrack recording, which was submitted to the SMPE (nowSMPTE), which used the mercury vapor lamp as a modulating device to create a variable-density soundtrack. Later,Case Laboratories andLee De Forest attempted to commercialize this process, when they developed an Aeolite glow lamp, which was deployed atMovietone Newsreel at theRoxy Theatre in 1927. In 1928,Fox Film purchased Case Laboratories and produced its first talking filmIn Old Arizona using the Aeolite system. The variable-density sound system was popular until the mid-1940s.[2]
Opposite with variable-density, in the early 1920s, variable-area sound recording was first experimented on by theGeneral Electric Company, and later was applied byRCA which refined GE's technology. After the mid-1940s, variable-area system superseded the variable-density system, and became the major analog sound-on-film system until modern day.
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The most prevalent current method of recording analogue sound on afilm print is bystereo variable-area (SVA) recording, a technique first used in the mid-1970s asDolby Stereo. A two-channel audio signal is recorded as a pair of lines running parallel with the film's direction of travel through the projector's screen. The lines change area (grow broader or narrower) depending on the magnitude of the signal. The projector shines light from a small lamp, called anexciter, through a perpendicular slit onto the film. The image on the small slice of exposed track modulates the intensity of the light, which is collected by a photosensitive element: a photocell, aphotodiode orCCD.
In the early years of the 21st century distributors changed to using cyan dye optical soundtracks on color stocks, eliminating the need for a separate and difficult to control black-and-white emulsion on the soundtrack portion of the film. Because traditional incandescent exciter lamps produce copious amounts of infra-red light, and cyan tracks do not absorb infra-red light, this change required theaters to replace the incandescent exciter lamp with a complementary colored redLED or laser. These LED or laser exciters are backwards-compatible with older tracks.
Earlier processes, used on70 mm film prints and special presentations of35 mm film prints, recorded sound magnetically onferric oxide tracks bonded to the film print, outside the sprocket holes. 16 mm and Super 8 formats sometimes used a similar magnetic track on the camera film, bonded to one side of the film on which the sprocket holes had not been punched ("single perforated") for the purpose. Film of this form is no longer manufactured, but single-perforated film without the magnetic track (allowing an optical sound track) or, in the case of 16 mm, utilising the soundtrack area for a wider picture (Super 16 format) is readily available.
Three different digital soundtrack systems for 35 mm cinema release prints were introduced during the 1990s. They are:Dolby Digital, which is stored between the perforations on the sound side;SDDS, stored in tworedundant strips along the outside edges (beyond the perforations); andDTS, in which sound data is stored on separatecompact discs synchronized by atimecode track on the film just to the right of the analog soundtrack and left of the frame[3] (sound-on-disc). Because these soundtrack systems appear on different parts of the print, one movie can contain all of them, allowing broad distribution without regard for the sound system installed at individual theatres.
Almost all sound formats used with motion-picture film have been sound-on-film formats, including:
