| Cinema Cynergy May 2001
What the Heck is THX?
We have all seen the trademark silver logo with the blue
border and heard the droning trailer played at the local multiplex, which is also included
on many DVDs these days. Yes, "the audience is listening," but what exactly are
they listening to? THX is not a surround format like Dolby Digital or DTS, and it is not a
hardware manufacturer like Onkyo or Yamaha. THX is actually a division of Lucasfilm that
proclaims itself to be dedicated to bringing the sound reproduction of films in both
theaters and in homes as close as possible to that which was intended by the filmmaker.
THX accomplishes this by licensing products that meet their standards for the reproduction
of film soundtracks. They also license other peripheral products such as DVD players,
cables, and even computers and computer speakers. Additionally, they have a program that
ensures the audio and video quality of films recorded on DVD, VHS tape, and formerly
laserdisc. There are now two sets of THX specifications for home-theater systems: THX
Ultra (the original specifications), and THX Select (a less rigid set of specifications
"more suited to the size and listening distances of most living rooms").
Although THX actually encompasses many different programs,
it is their professional (movie theater) sound system and home sound system licensing
programs that have had the most impact on film soundtrack reproduction. These programs
license products that are involved in the "B chain" of the reproduction of the
sound.  These components consist of amplifiers, speakers, and the
secondary processing that is implemented in THX controllers after the surround signal is
decoded.
As previously mentioned, THX attempts to bring what they
believe to be the sound that the filmmaker originally intended to professional theater and
home-theater sound systems. It is assumed that the filmmaker will be using some sort of
standard audio system in the dubbing studio, which is similar in sound to the professional
THX theater system, which in turn is similar to the home THX system. While many dubbing
studios actually use THX-approved systems or systems that are very similar in design, like
most things in life there are no absolutes, and the sound system that a filmmaker uses to
dub a motion picture soundtrack could really be almost anything. Also, THX seems to
suggest that because their systems most closely approximate the filmmaker’s
intentions (in their opinion), THX systems offer the best possible sound. It should be
noted that THX specifications (like most specs) are minimum specifications that guarantee
a certain level of performance, and that many manufacturers, whether THX licensees or not,
can surpass those criteria if they see fit to do so.
Lest you get the wrong idea, most THX systems do actually
sound good. However, like many people, I take exception to the fact that THX promotes
itself as the correct way to play back motion picture soundtracks, and that their systems
offer the best performance.
What makes a system THX?
Although it is difficult to summarize THX systems in just a
few simple paragraphs (and while many of their characteristics are fairly common
knowledge, some of the specifics are still secrets closely guarded by Lucasfilm), I will
describe some of the primary areas covered by the THX program for home-theater systems.
Smooth frequency response
Although this is something that every manufacturer strives
for when designing components, THX systems achieve this through a particular set of design
parameters.
Re-equalization
Movie soundtracks are often mixed to compensate for a
high-frequency roll off that is experienced in large movie-theater-sized rooms. Thus, when
these soundtracks are played back in a smaller home-theater environment where there is no
such high-frequency roll off, the sound will be perceived as bright or harsh. THX home
processors equalize the signal by reducing the high frequencies so that the soundtrack
will play back "flat" in a home theater.
Bass management
THX controllers and receivers route bass from all of the
channels below a certain frequency to the subwoofer output. This relieves the speakers of
the difficult task of reproducing bass down to 20Hz, which is required for movie
soundtracks. The crossover point for the original THX specifications for Dolby Surround
was 80Hz for the front channels (the rear channels in Dolby Surround are already rolled
off at 100Hz), and it is presumably similar for all the channels in the new discrete
digital formats. One of the benefits of this type of system is that the speakers can be
relatively small because they do not have to reproduce a lot of bass. As a result, they
can be placed more easily in a room.
Timbre matching
Because THX specifies different designs for speakers used
in the front and rear of the room (we will get to this shortly), and due to the different
orientation of these speakers to the listener’s ears, timber matching is utilized to
compensate for the perceived differences in tonal qualities of the sound. Timbre matching
is a form of equalization that attempts to correct differences in the frequency response
between the front and rear speakers experienced by the listener.
Clear, intelligible dialogue (front loudspeaker design)
Not only dialogue, but all of the sound coming from the
front speakers in a THX system is intended to be clear and easily localizable. This is
accomplished by requiring the front loudspeakers to have a wide horizontal, but limited
vertical dispersion pattern, which demands that the speaker be aimed directly (in the
vertical plane) at the listener. This is meant to limit the reflection of sound off of the
floor and ceiling so that the listener receives mostly direct sound and less reflected
sound.
 The very narrow vertical
dispersion pattern required for home THX front loudspeakers is very different from the
design of most normal speaker designs. Consequently, manufacturers that design THX front
loudspeakers must usually design these speakers from the ground up instead of basing them
on their existing designs. One of the criticisms that has been leveled at home THX systems
is that reflected sound is rarely a problem in the home-theater environment due to the
proximity of the listener to the speakers and the acoustic properties of a small room.
Therefore, the design goal of having front loudspeakers with controlled directivity may
not necessarily be as applicable to home-theater systems.
Diffuse surround envelopment
Because the speaker systems used in dubbing studios and
movie theaters consist of an array of many surround speakers, the rear channels of a THX
system are meant to mimic this by sounding diffuse and not being localizable (the exact
opposite of what is intended for the front speakers).
Dipolar surround speaker design
The surround speakers are actually dipolar designs that are
intended to be placed to the sides of the listeners. Dipole speakers radiate sound in both
a forward and backward direction while out of phase. When placed to the sides of the room,
the dipole speakers are oriented so that they fire forward and backward along the sidewall
and no sound is radiated directly towards the listener, who hears only reflected sound
from these speakers.
Adaptive Decorrelation
THX controllers also apply a phase and time shift to one of
the surround channels when a mono signal is detected. This will be applied constantly to
Dolby Surround signals that have inherently mono surround signals, but the controller will
also sense when a discrete digital signal is in stereo and switch itself off.
The two sides of The Force
While the THX program strives for many of the same
characteristics in sound reproduction as others in the industry, namely smooth frequency
response, high dynamic range, good imaging etc., it does interpret the sound in a
number of ways.
In order to reproduce the sound as "intended by the
filmmaker," THX home-theater systems are designed to sound like the dubbing studios
that many of them utilize, and like professional THX systems used in movie theaters. This
requires that home THX systems have a direct sound with very controlled vertical
dispersion coming from the front speakers and a diffuse, mostly reflected sound coming
from the surround speakers. The main arguments against this by THX’s detractors are
that this results in a sound that has little depth up front, and that discrete digital
surround effects cannot be localized as some feel they should be. They also criticize the
restrictive nature of the bass management system, which does not benefit by the use of
full-range speakers and instead, directs most of the bass to the subwoofer(s).
I have personally owned two THX-certified amplifiers in the
past and currently use a THX-certified center-channel speaker, and I have been satisfied
with all three of these components. However, I recently enjoyed watching Bridget
Jones’s Diary (an excellent film with a good music soundtrack) in a THX-certified
movie theater and although the sound was quite good, I am sure that my mainly non-THX
home-theater system would sound even better. Of course the economics of scale dictate that
I will be able to obtain higher quality components for my home, which is a much smaller
space. However, I think that THX’s philosophy of bringing the movie theater
experience into the home by reproducing the sound as it was "meant to be heard"
is flawed, and that the standards of professional THX systems are not directly applicable
to home-theater systems. In my opinion, the superior acoustics of the typical home
environment can be used to actually make film soundtracks sound better than they did in
the dubbing studio or the movie theater and this is what we should be striving towards.
While I have directed many criticisms at THX in this
column, there is no doubt in my mind that without their efforts and the efforts of
companies like Dolby and DTS, the quality of movie theater and home-theater sound would
not be anywhere near what it is today, which, up until the late '70s, was mostly
monophonic.
...Roger Kanno
roger@hometheatersound.com
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