vita@gloucester.dab.ge.com (Mark Vita)

GE Simulation & Control Systems
Wed, 30 Jun 1993 00:59:26 GMT

Some info about Digital Theater Sound (DTS)

I recently spent some time on the job with my brother, who installs theater
sound systems for a living.  He's been pretty busy lately installing DTS
systems for Jurassic Park.  Anyway, I had an opportunity to look over some
of the DTS literature and installation procedures.  I see there's been a
lot of curiosity on here about DTS, so I thought I'd pass on some of the
info I picked up.

DTS, or Digital Theater Sound, is a motion picture sound system format that
stores a digital soundtrack on separate CD-ROM discs.  There are two
variants: DTS-S (often erroneously referred to as "DTS-4"), which offers
four matrix-encoded channels (left, center, right, surround), similar to
standard 35mm optical Dolby Stereo; and DTS-6, which offers six discrete
channels, similar to 70mm six-track magnetic sound (typically: left,
center, right, left surround, right surround, subwoofer).  The DTS-S system
utilizes a single CD-ROM, while DTS-6 utilizes two CD-ROMs.  (I'm not sure
how the DTS-6 tracks are broken down onto the two discs.)

The DTS unit itself is basically a rack-mountable PC clone, containing
either one or two Chinon CD-ROM drives (depending on whether the system is
setup for DTS-S or DTS-6).  The cost of the system is about $4000 list for
DTS-S, and $6000 for DTS-6.  The DTS-S system is upgradeable to DTS-6.  For
Jurassic Park, Universal was offering exhibitors a special promotional deal
which allowed them to purchase the DTS system at reduced prices
(approximately $2.5K for a DTS-S system).

The 35mm prints released for DTS films have a normal, conventional analog
optical soundtrack.  The same prints can thus be played in DTS-equipped and
non-DTS-equipped theaters; studios and distributors only need to maintain a
single inventory of prints.  The analog soundtrack also provides a
fail-safe backup for DTS-equipped systems.

The only unusual addition to a DTS print is an optical timecode track,
which is squeezed in between the optical sound track and the film frame.
[My brother has observed that in some theaters with incorrect projector
apertures, it is thus possible to "see" Jurassic Park in DTS :-)]  The DTS
system includes an optical timecode reader which is mounted onto the
projector.  The film leader contains some header information which allows
the DTS processor to check that the correct discs for the film are loaded
in the CD-ROM drives.  Then, for each film frame, a timecode is read off
the print, and the corresponding digital soundtrack info is retrieved off
of the CD-ROM.  Thus the soundtrack stays in sync if there are missing or
damaged frames (a small read-ahead buffer is utilized), or even if the
reels were spliced together out-of-order (!).  If at any time the DTS
processor is unable to retrieve valid digital soundtrack data (e.g. missing
disc, wrong disc loaded, drive failure, invalid or missing timecode), it
immediately switches over to the normal analog soundtrack (and switches
back if and when valid digital data can later be obtained).

The DTS literature does imply that some kind of compression scheme is used
on the digital sound data, but it does not indicate what kind of
compression is used, or even whether it is lossy or loseless.  I think they
were being deliberately vague on this topic.  However, they did go so far
as to malign one of the competing digital systems (presumably Dolby SR-D)
for using "excessive" lossy compression.

One interesting (and somewhat disappointing) note about the four-channel
DTS-S format is that it is recorded on the disc as two tracks, left-total
and right-total, just like regular analog Dolby stereo, and is played back
through the optical projector inputs on 35mm analog Dolby-compatible sound
processors.  This was done for the obvious backward compatibility reasons.
What it means, though, is that what was once probably a nice, discrete,
four-channel digital sound mix ends up getting matrix-encoded, passed
through Dolby A-type analog noise reduction, then matrix-decoded to
retrieve the original four channels (yuck).  Obviously this results in less
channel separation than might otherwise have been possible.  Also, most of
the cheaper Dolby-compatible theater sound processors (such as Smart and
Kintec) lack the ability to easily turn off the analog noise reduction.
The bottom line is that the original digital soundtrack ends up getting
mashed through lots of analog gobbledygook circuitry, which may account for
the somewhat less-spectacular-than-expected sound at some DTS-S equipped
theaters.  It would have been nice if they had been able to record DTS-S as
four discrete digital tracks, but this is unfortunately unworkable since
most 35mm sound systems lack something even so seemingly basic as discrete
line-level inputs for the four channels (!).  In any event, for these
reasons the discrete DTS-6 system apparently sounds much, much superior to
DTS-S (much more than you would expect from just the inclusion of two
additional channels).  Unfortunately it is DTS-S that is being purchased by
the vast majority of exhibitors that even bought into DTS at all (actually
not too surprising since DTS-6 really requires a decent six-channel sound
system to be worthwhile).

Apparently DTS is being regarded as fairly successful so far, achieving
pretty decent market penetration.  This is probably mostly accountable to
the huge success of Jurassic Park, and the fact that Universal is
practically giving the DTS units away at cost in order to promote the film.
The fact that DTS prints are backward-compatible is also a big selling
point; the necessity for maintaining a dual inventory of prints was
supposedly a major factor in the demise of the earlier Cinema Digital Sound
(CDS) process, an digital/optical sound-on-film format.  (Of course, with
DTS there are those pesky CD-ROM discs to inventory/keep track of/possibly
lose.)  Price is also a major advantage; even without the JP promo pricing,
the cost of the system is pretty reasonable at $4-6K.  Supposedly the
forthcoming competing digital systems are much more expensive.  Dolby SR-D,
a digital sound-on-film format that stores the soundtrack data between the
film's sprocket holes, requires a $20K processor.  Sony/Columbia is also
coming out with a digital sound-on-film format that will likely also be
significantly more expensive than DTS.


--
Mark Vita                                        vita@sunny.dab.ge.com
Advanced Systems
Martin Marietta Simulation and Automated Systems
Daytona Beach, FL