4.4.1 70mm Magnetic (six-track stereo) (Motion Picture Sound Formats - release prints intended for projection)
Description
This article is from the rec.arts.movies.tech FAQ, by Scott E. Norwood snorwood@nyx.nyx.net with numerous contributions by others.
4.4.1 70mm Magnetic (six-track stereo) (Motion Picture Sound Formats - release prints intended for projection)
This system is capable of carrying six separate tracks on four wide
magnetic stripes on the film. It is usually set up to reproduce left,
left-center, center, right-center, right, and surround tracks. This
was long considered to be the premier film-sound format, prior to the
advent of digital, because the tracks were relatively wide, because
the film runs through the projector at a slightly higher rate of
linear speed than 35mm film, and because the sound is recorded in
discrete (separate) tracks, rather than being `matrixed.'
As mentioned above, in the late 1970's (beginning with Star Wars)
through the late 1980's, it was common for distributors to produce
70mm blowup prints of films shot on 35mm in order to improve sound
reproduction in the movie theater. With the introduction of digital
systems, which are capable of reproducing higher quality sound at a
lower cost than a complete 70mm projection system and 70mm print
rental, exhibitors no longer saw much reason to show blowup prints,
except for special `one-time' shows. In the future, magnetic striping
(a major cost of making 70mm prints) may be eliminated, in favor of a
digital soundtrack (currently, DTS (tm) has been used for 70mm
prints). This may encourage the printing (and 65mm original
cinematography) of more films for 70mm exhibition.
Unlike other formats, where the soundtrack runs ahead of the picture,
with 70mm, the sound runs behind the picture, as the magnetic sound
heads are placed before the picture head. Thus, the 70mm print runs
through the magnetic soundhead, picture head, then around the 35mm
optical soundhead, then to the takeup reel or platter. When 35mm films
are run in a combination projector, they are simply loaded through the
70mm magnetic soundhead, without difficulty.
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