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4.5 What are the three commonly used digital sound formats for 35mm release prints, and how do they work?




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This article is from the rec.arts.movies.tech FAQ, by Scott E. Norwood snorwood@nyx.nyx.net with numerous contributions by others.

4.5 What are the three commonly used digital sound formats for 35mm release prints, and how do they work?

4.5.1 General Information

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Digital Theater Systems (DTS (tm) )
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Sony Dynamic Digital Stereo (SDDS (tm) )
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Dolby (tm) Digital (SR-D (tm) )

Digital sound differs from analog sound in that it represents sound by
a series of consecutive `samples' of the sound (each of which is
represented by the digits zero [0] and one [1]), rather than by a
continuous waveform. Digital is neither inherently better nor
inherently worse than analog, but simply a different method of
representing sound (music, dialogue, etc.). In practice, though,
digital film sound almost always sounds cleaner and brighter than
analog, and is capable of greater dynamic range, due to the
limitations of the optical track as a means of recording sound.

Despite the differences among the various digital sound formats, most
people cannot tell a difference in quality, as they all sound
excellent. Perceived differences among the formats are usually a
result of a different sound mix for each format (such as an 8-channel
SDDS (tm) mix versus a six-channel Dolby (tm) Digital mix).

Digital Theater Systems (DTS) (tm)

This was the first digital sound system to come into widespread usage,
with the release of Jurassic Park in 1993. The system was promoted
heavily by MCA/Universal Pictures, which uses it on most of its
prints. The system originally was sold in two versions: a low-end
version which could reproduce four tracks, and a high-end version
capable of reproducing six- tracks (left, center, right,
left-surround, right-surround, and subwoofer. These systems were
referred to as DTS-4 (tm) and DTS-6 (tm) , respectively. The
four-track version has since been discontinued.

DTS (tm) uses a timecode printed on the film between the picture area
and the optical track. The timecode, which looks like a dot-dash
pattern resembling Morse code) is read by an optical reader placed in
the film path, between the platter or reel and the projector's picture
head. This timecode information is fed to a specialized, souped-up 386
or 486 computer which in turn reads compressed soundtracks from a
CD-ROM disk; the compression factor, though, is the least of the three
digital systems. The current systems have three separate CD-ROM
drives: one holds a `trailer' disk which is sent to theaters
periodically, and contains the soundtracks to all of the trailers
currently showing, including trailers from studios which do not use
DTS (tm) for their films; the other two contain disks for the feature.
Shorter movies require only one disk; others require two. Slightly
over four hours of digital sound can be accommodated for a two-disk
feature. There is no provision for mid-show disk changes.

As with all digital sound systems, the film reader can be placed a
variable number of frames ahead of the picture head. This is
calibrated upon installation with a test film. The computer is capable
of accommodating splices within the film, and adjusting the soundtrack
to match. Further, because the soundtrack is not on the film, no
`popping' noise is heard during splices and/or changeovers (unless the
timecode reader cannot read a certain section of timecode, in which
case it reverts back to the standard analog track, causing a small
`pop').

As with all of the current 35mm digital systems, all prints (except
70mm DTS prints) contain a standard optical track (usually recorded in
`DTS Stereo (tm) ,' a system which is compatible with Dolby (tm) -type
processors) as a backup, should the timecode not be found, or be
unreadable for more than 40 frames. The analog track is also used when
the CD-ROM disk does not match with the movie being shown (at least in
theory-there have been reports of theaters' showing one movie with
another's soundtrack).

Sony Dynamic Digital Stereo (SDDS) (tm)

Sony has entered the cinema sound market with the SDDS (tm) system.
Unlike the other two digital systems, SDDS (tm) is capable of
reproducing eight tracks of sound (left, center, right, left-center,
right-center, left-surround, right-surround, and subwoofer),
potentially a great advantage for films mixed for eight tracks, as a
small number are at present. This, of course, requires that theaters
install additional loudspeakers (left-center and right-center) behind
the screen in order to take advantage of the potential of this format,
however.

In SDDS (tm) , the sound is actually recorded on the film itself,
along both edges of the print. SDDS (tm) uses a middle level of
compression of the digital information of the three current digital
systems. Like the other digital systems (except for Dolby), the reader
(which uses an LED to shine through the track) is placed somewhere in
the film path prior to the film's entrance into the picture head (the
offset is variable, as convenience dictates, and is set up at
installation). The reader reads the track, which is then decoded,
decompressed, and processed in a separate processor unit, which
contains custom electronics designed for this purpose. Just as with
analog sound, splices are accommodated without difficulty.

SDDS (tm) is probably the most expensive of the three digital formats,
although actual cost varies substantially among different theaters and
chains. The expense is largely due to the fact that all of the
electronics within the entire processing system are digital, whereas
DTS (tm) and Dolby (tm) Digital are both designed to simply be plugged
into existing analog Dolby (tm) (or similar) cinema processors.
However, the extra cost may be somewhat justified by the extra tracks
and the fact that the marketer of this system also owns companies
which produce many films each year, almost ensuring that there will be
material in this format for many years to come.

Although it is expensive, SDDS (tm) is very popular, particularly in
the AMC, Sony, and United Artists theaters, where SDDS (tm) is or will
be used in most of the theaters. Many technicians like it because it
is the only system with electronic equalization, allowing the system
to be properly set up very quickly.

Dolby (tm) Spectral Recording Digital (SR-D) (tm)

Dolby (tm) Digital, also known as SR-D (when an SR track is used for
the analog backup), is the digital system from Dolby (tm)
Laboratories. Like DTS (tm) , it is capable of reproducing six tracks
(left, center, right, left-surround, right-surround, and subwoofer),
which are read by a reader (which works much like a TV/video camera,
capturing images of the track) placed before the picture head, or, in
some installations, within the standard projector soundhead. Like the
other two systems, the offset can be varied, and is calibrated at
installation. The actual soundtrack on the film runs 26 frames ahead
of the picture.

The actual digital sound information is printed on the film in between
the perforations, generally considered to be a safer location for the
sound information than the edge of the film (where SDDS's (tm) track
lives). Thus, Dolby (tm) Digital is potentially more reliable than
SDDS (tm) , although it compresses the digital information to a lesser
extent than SDDS (tm) does. Like SDDS (tm) , the track is read, and
then decoded, decompressed, and processed by a separate unit. Splices
can create small `pops,' (and will revert to analog if more than five
perforations are obscured, but this is unlikely..

This format appears to be increasing in popularity at this time, both
in terms of the number of theaters installing the system and the
number of prints available in that format. It is also considered to be
slightly more reliable than the other two digital formats, as the
sound is printed directly onto the film in a relatively `protected'
location. All prints still contain an analog optical track (usually
recorded in Dolby (tm) SR), in case the digital system fails, or is
unable to read five consecutive `blocks' (between perforations).

Technically, it is possible to, with minimal cost, print all three
types of digital track (or, in the case of DTS (tm) , timecode), along
with analog optical Dolby (tm) on a single print, and a few films have
been printed this way. These multi-format prints are now quite common
(containing at least two formats), especially on movie trailers.
Similarly, it is possible to have a projection system which can
accommodate all of these formats, without excessive difficulty.

 

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