I work at Skywalker Sound North, and the debate here about THX has made it
clear that few people (even the self-styled experts) understand what it is. In
the interest of education, i spoke with my friend Laura who works at THX, and
asked her to help out. The following is her response, intended for the net:

WHAT IS THX?

Hi, I'm Laura Ackley, the THX System Designer.  I'm responsible for the design
of all professional and commercial THX Sound Systems, so I thought I'd try to
clear up some of the confusion regarding THX.

THX is a set of implemented design and quality control standards for theatre
sound presentation.  First, the actual physical theatre must meet certain
acoustic criteria:

It must have low background noise, (no loud air conditioners or projectors),
good isolation (so the lucky patron doesn't get to hear the gunfight in the
theatre next door), and specific reverberant characteristics based on the
volume of each individual theatre.  Also, the theatre can't have any nasty slap
echoes or reflections.

When an individual theatre meets these standards, it means that the dialogue
will be more intelligible, the high frequencies will reproduce clearly, the
bass will not build up excessively within the room, etc.

Most of the sound one hears in the movies comes from behind the screen, from 3
channels usually.  (Occasionally 5, but THAT'S a different lecture!)

Because of this, the theatre must also have a good length/width ratio.  (i.e.,
No bowling alleys.)  From the visual standpoint, this is of course desirable,
and from the audio standpoint, it means having a good "stereo image" between
the left and right channels, so that the patron is actually hearing in stereo
-- in every seat in the house!

Once a theatre is accepted for the aforementioned preliminary THX criteria, the
owner must choose the equipment to be used from a list provided by us.  This
list includes amplifiers, processors, screen and surround speakers.  They can
choose from a number of different products by various manufacturers, but they
all the products have been objectively tested here to conform to very high
standards.

These high-quality components assure a wider dynamic range and more faithfully
reproduced sound, plus adequate power for the room size.

Incidentally, the ONLY Cinema Processors which were accepted until quite
recently were made by Dolby, so any THX theatre was by default a "Dolby"
theatre... and ever-so-much more!

Thus, we're not in competition with Dolby, we're a COMPLEMENTARY TECHNOLOGY to
Dolby Processing.  Recently, Ultra-Stereo also had a processor approved for use
in THX Theatres.

Also, we're not in the business of selling any equipment, just assuring high
quality hardware in THX theatres.  The ONLY piece of hardware that is
specifically "THX" is an electronic crossover network, which is LEASED, not
purchased by the theatre.

Our design office takes the architectural plans for every theatre, and designs
the placement of the screen speakers and surrounds.  The screen speakers are
set in a special baffle structure behind the screen.  This structure has
certain acoustic properties, and the speaker placement within the structure
is coordinated to best match the projected film image.

The surround speaker array is designed to provide optimum sound pressure and
audience coverage.  (and to integrate with the interior design whenever
possible).

We coordinate all this with the architects, owners, and equipment installers.
Once the theatre has been completed or successfully retrofitted, our field
technicians visit the theatre to align and "tune" the sound system, then they
measure the acoustical characteristics to make sure that they meet our
requirements.  This test data is then evaluated, and if the theatre passes on
all counts, it becomes a "certified" THX site.

This, of course, entitles the theatre to use the THX name and show the THX
trailer.  (You know, "The Audience is Listening.")  The theatre is re-tested
every 6 months to ensure that the quality of sound remains high.

Our "philosophy" is to recreate the film soundtrack mixing environment as
closely as possible so that the audience hears the soundtrack as the film-maker
intended.  (This is all tied up in a DIFFERENT lecture on the history of film
sound....)

In conclusion, we're NOT a method of recording, encoding, OR processing movie
soundtracks!  We're a design and quality control system for the PLAYBACK
ENVIRONMENT.

This means that EVERY FILM will sound better in a THX theatre, not just a select
few.  The premium quality of a THX theatre is often well-demonstrated by
"special-effects-extravaganza" films, because they are mixed with a very wide
dynamic range and a lot of "loud" or "low" sounds that might distort when
played back through a lesser system.

THX theatres are especially well-equipped to handle the demands of the new
digital theatre soundtracks (like "Cinema Digital" and "SR-D") because of their
high performance and headroom.

So, that's the scoop on the THX COMMERCIAL THEATRE and PROFESSIONAL MIXING ROOM
system.  The new Home THX system consists of a family of licensed products that
various licensees have developed to meet specs. designated by our engineers.
The pricing of the home system is dependent on the licensee.  These are sold
through dealers like regular home A/V equipment, and we do not design these
systems.

Any questions about THX (serious only, please)  should be directed to:

Julie Petersen
Director of Marketing
THX Division
P.O. Box 2009
San Rafael, CA 94912

Thanks for reading, and I hope this helps clarify things!
