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. 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. The projector shines light from a small lamp, called an exciter, through a perpendicular slit onto the film. The lines change area (grow broader or narrower) depending on the magnitude of the signal. 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 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. After the mid-1940s, variable-area system superseded the variable-density system, and became the major analog sound-on-film system until modern day.
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. The variable-density sound system was popular until the mid-1940s.
In 1928, Fox Film purchased Case Laboratories and produced its first talking film In Old Arizona using the Aeolite system. 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. 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. Sound on film can be dated back to the early 1880s, when Charles E. Earlier technologies were sound-on-disc, meaning the film's soundtrack would be on a separate phonograph record. Sound-on-film processes can either record an analog sound track or digital sound track, and may record the signal either optically or magnetically. 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.
Finally, to the far right, the timecode used to synchronize with a DTS soundtrack CD-ROM is visible. These are generally encoded using Dolby Stereo matrixing to simulate four tracks. The two tracks of the analog soundtrack on the next strip are bilateral variable-area, where amplitude is represented as a waveform. The outermost strip (left of picture) contains the SDDS track as an image of a digital signal the next contains the perforations used to drive the film through the projector, with the Dolby Digital track (grey areas) with the Dolby Double-D logo, between them. Edge of a 35mm film print showing the soundtracks.
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