tracks do not absorb infra-red light, this change required theaters to replace the incandescent exciter lamp with a complementary colored red LED or laser. These LED or laser exciters are backwards-compatible with older tracks. Earlier processes, used on 70 mm film prints and special presentations of 35 mm film prints, recorded sound magnetically on ferric 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. Digital sound-on-film formats 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 two redundant strips along the outside edges (beyond the perforations); and DTS, in which sound data is stored on separate compact discs synchronized by a timecode track on the film just to the right of the analog soundtrack and left of the frame (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. Sound-on-film formats Almost all sound formats used with motion-picture film have been sound-on-film formats, including: Optical analog formats Fox/Western Electric (Westrex) Movietone, are variable-density formats of sound film. (No longer used, but still playable on modern 35 mm projectors.) Tri-Ergon, another variable-density format prevalent in Germany and Europe until the 1940s. The US patent rights of this Berlin based company were bought by William Fox in 1926, leading to a patent war with the US film industry lasting until 1935. Tri-Ergon amalgamated with a number of other German competitors from 1928 to form the Dutch-controlled Tobis Film syndicate in 1930, licensing the system to UFA GmbH as UFA-Klang. RCA Photophone, a variable-area format since the late 1920s—now universally used for optical analog soundtracks. Since the late 1970s usually with a Dolby encoding matrix. Encoding matrices Dolby Stereo (SVA) Dolby SR Ultra Stereo Optical digital formats