Sound-on-film is a class of sound 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 an analog sound track or digital sound track, and may record the signal either optically or magnetically. Earlier technologies were sound-on-disc, meaning the film's soundtrack would be on a separate phonograph record. History 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 (now SMPTE), which used the mercury vapor lamp as a modulating device to create a variable-density soundtrack. Later, Case Laboratories and Lee De Forest attempted to commercialize this process, when they developed an Aeolite glow lamp, which was deployed at Movietone Newsreel at the Roxy Theatre in 1927. In 1928, Fox Film purchased Case Laboratories and produced its first talking film In Old Arizona using the Aeolite system. The variable-density sound system was popular until the mid-1940s. Opposite with variable-density, in the early 1920s, variable-area sound recording was first experimented on by the General Electric Company, and later was applied by RCA 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 After the mid-1940s, variable-area system superseded the variable-density system, and became the major analog sound-on-film system until modern day. Analog sound-on-film recording The most prevalent current method of recording analogue sound on a film print is by stereo variable-area (SVA) recording, a technique first used in the mid-1970s as Dolby 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 an exciter, 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, a photodiode or CCD. In the early years of the 21st century distributors changed to using cyan dye optical soundtracks on color stocks instead of applicated tracks, which use environmentally unfriendly chemicals to retain a silver (black-and-white) soundtrack. Because traditional incandescent exciter lamps produce copious amounts of infra-red light, and cyan