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From: good@pixar (Blink twice, son, if any of this is gettin' through.)
Newsgroups: net.video,net.audio,net.movies
Subject: Re: Movie sound formats
Message-ID: <2946@pixar.pixar>
Date: Thu, 17-Jul-86 14:38:43 EDT
Article-I.D.: pixar.2946
Posted: Thu Jul 17 14:38:43 1986
Date-Received: Sat, 19-Jul-86 03:06:47 EDT
References: <14889@ucbvax.BERKELEY.EDU>
Reply-To: good@pixar.UUCP (Blink twice, son, if any of this is gettin' through.)
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In article <14889@ucbvax.BERKELEY.EDU> albert@kim.Berkeley.EDU.UUCP (Anthony Albert) writes:
>I've been wondering about this for a long time. What are the characteristics
>of: 6 track dolby stereo, 4 track dolby stereo, THX sound, and dolby surround
>sound? Is 6 track dolby stereo 6 distinct tracks? What about 4 track stereo?
>Is "surround sound" an artifact of home video; I mean is this one of the other
>formats adapted for home video? I hear that surround sound is stereo in front
>and one mono track in the rear. This would make it 3 track stereo, right? And
>I know nothing about THX. Are there others I've left out? And does anyone know
>the history of multitrack film audio?

Having just finished the final mix of our SIGGRAPH reel, maybe I can shed
some light for you.  The only "Dolby stereo" formats I know of are the optical
one, used on 35mm film, and the mag stripe used on 70mm releases.  The latter
is the 6 track Dolby you hear about.  More tracks are available on mag stripe
than an optical sound track which explains that part of the difference.

In the case of normal Dolby stereo, a box (called a DS-4 I think) takes the
LT + RT (Left Total, Right Total) channels and derives left, center, right,
and surround channels.  The center channel is easy to get by summing L and R,
and completely out of phase information gets sent to the surrounds.

When you watch 70mm they can give you left, center, right, a stereo pair
of surrounds in the middle of the theatre, and the back surrounds.  I'm
pretty sure I have that right.  Anyway, the far back surrounds in either case
would be mono.  Most people don't realize that there is a center channel on
the screen which is why they get confused counting tracks on their fingers.

THX is part of the playback system, and not a format in itself.  The heart of
it is the crossover designed by Tom Holman.  Thus, in addition to the obvious
"THX 1138" reference in the name, it stands for the Tom Holman Xover.  Tom's
system also specifies a set of JBL theatre horns and a subwoofer (yes, it's
a two-way system!), the "wall" it is mounted in behind the screen, and the
amplifiers (last I heard he was using the Hafler 500).  It's really the first
time theatre playback has changed since the 1940's.  And, as Tom says, with
a THX a theatre sounds almost as good as a cheap home stereo...  If you've
never heard a good sound picture in a THX room you really should.  It makes
quite a difference.

Getting back to 35mm vs 70mm, the biggest difference is often the sound
quality.  Besides the better channel definition, the mag stripe buys you
a much larger bandwidth and better S/N.  I *always* opt for the 70mm show
when I have a choice, even if I have to drive farther.  Hope this answers
some of your questions.


-- 
		--Craig
		...{ucbvax,sun}!pixar!good