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The Knowledge : THX

THX-rated
For the ultimate in sublime surround sound, there's only one place to turn...
THX stands for 'Tomlinson Holman's eXperiment' and is named after a past technical director of
LucasFilm, the production company behind Star Wars and many other blockbusters. It was to him
that George Lucas turned in 1982 when it became apparent that not all was well with cinema
sound. The company defined a performance specification for professional cinema components.
Only venues with Lucas-approved equipment could claim the THX logo.

That philosophy eventually came to home cinema. Domestic AV manufacturers build their products
to THX's' tight specifications. Once part of LucasFilm, THX has since become a standalone
company, with investment from LucasFilm, Creative Labs and others. But it still claims to represent
the high-end of home cinema sound.

HOW IT WORKS

Although THX isn't an encoding format it delivers some dramatic differences compared with
conventional home cinema. LucasFilm claims that it addresses four main problems.

Firstly, movies customarily have their sonics balanced exclusively for a large auditorium, with a
specific treble boost. In the home, where speakers double as music monitors, the treble boost can
sound too bright. Additional filtering called re-equalisation compensates for this.

Secondly, there's timbre matching. This ensures that there's no change in perception of sound as it
moves from the front of the listener to the sides and behind. Again, an equaliser in the THX decoder
processes the signal - in this case, as it is panned.

The third area concerns the surround signal. With regular THX as applied to Pro-Logic signals, the
single effects channel is split into two non-correlated outputs for left and right. This process
involves frequency-shifting and enhances the surround effect. So, unlike Pro-Logic where the same
signal is heard from both surround speakers, with THX the rear left and right carry differently
processed signals.

Systems using Dolby Digital and THX decoding use a modified version called THX 5.1. Here,
de-correlation is dynamic - it changes according to the source. If there's good stereo separation, no
de-correlation is applied, but on a movie where the surrounds are near-mono, the THX difference
signal is introduced.

The fourth THX tweak is low frequency filtering. A high-tech electronic crossover splits bass from
the front speakers and delivers it through a tightly specified subwoofer. All this processing is
performed within one box - the THX controller. It employs a Dolby or DTS decoder to separate the
various signals that make up the soundtrack - left, centre, right and left/right surround effects, plus
a subwoofer channel). Then THX processing is employed, producing six signals. These are routed
to dedicated (THX-approved) amplifiers.

THX ULTRA, SELECT AND... ULTRA 2!

In recent years, there have been a few changes in the requirements for THX that split the standard
into two categories known as THX Ultra (the original THX spec) and the lower end THX Select. A
new spec, THX Ultra 2, has also been introduced. This has been designed to replace the original
Ultra spec, and accommodate recent developments like 5.1 and 6.1 discrete digital.

The aim of THX Ultra is to accurately reproduce movies in large rooms. As a result, the front
speakers have special directivity that beams the sound at the listener and reduces reflections off the
floor and ceiling.

THX Select is more flexible over directivity and is designed for smaller rooms where you are likely
to sit closer to the speakers. THX Select models are generally smaller and easier to place, yet have
good tonal balance and encompass a much wider range of speaker design philosophies and sound
types.

With THX the effects speakers should sit on the rear side walls, this is because THX-approved
surround speakers are dipolar designs. In layman's terms, they project sound along two axes with
opposite polarity. The resulting cancellation between these planes means that the sound level at right
angles to the speaker is very much lower than in front (or behind) the speaker. According to Lucas,
this allows surround effects to be non-directional. The vast proportion of the sound is fired
forwards and back, and a correctly seated listener is unable to detect the exact position of the
surround speakers.

None of the kit comes cheap. Notable controllers are the £2,500 Tag McLaren AV32R and £5,600
Lexicon MC1. Unlike integrated amplifiers, these controllers only deliver preamp outputs. You must
add six channels of amplification.

THX Ultra speakers are usually supplied as complete outfits, but can be purchased separately if you
wish to mix and match. Models include the KEF reference series comprising the dipole TDM34DS
surrounds, main TDM23F, centre TDM23C and TDM45B subwoofer. The entire installation sells
for around £2,700. The THX Select standard also benefits from appropriately designed packages,
such as the M&K 750MkII outfit.

The recently introduced THX Ultra 2 is an advanced specification for receivers, amplifiers,
processors and speakers. It replaces the original Ultra specification first introduced over ten years
ago. Ultra 2 also incorporates some future proofing, and should cover formats yet to come. The
new specification, says THX, caters for music as well as movies, meets the demands of digital
soundtracks, is influenced less by room characteristics, is easier to use and will deliver a better
experience over the widest listening area. Only one speaker layout will be required for all
programme material.

THX Ultra 2 surround speakers use the dipole configuration of the original specification. Centre
speakers are also unchanged. Ultra 2 front speakers, on the other hand, have a revised vertical
directivity characteristic that ensures there are no sudden changes in frequency response at angles
well away from the normal listening axis, making them equally suitable for both music and movies.
Subs that meet the Ultra 2 standard have an extended low frequency response, and greater amp
power. And there are more speakers, thanks to developments like Dolby Digital EX and DTS ES.
Ultra 2 specifies a single fixed 7.1 speaker layout, consisting of left, centre and right at the front,
dipolar left and right surrounds, two back-surround dipoles situated closely together behind the
listener and an active sub.There are also changes to the decoders. Ultra 2-compliant decoders will
have a THX Ultra 2 Cinema mode, which auto detects an EX flag and then configures the 6.1
decoding mode. If no such flag is detected special processing is applied to the surround channels so
that ambient information is reproduced by the surrounds, and directional cues provided by the
back-surround speakers. For mono surround tracks, the signal is fed directly to the back surrounds
and, after de-correlation, to the left and right surrounds. THX Ultra 2 gear is currently thin on the
ground. One of the first products to arrive in the UK is Pioneer's hugely impressive VSA-AX10 AV
amplifier, offering just about any facility you could want.

The wonderful world of 6.1

As the last millennium drew to a close, THX and Dolby Labs foisted 6.1 surround on an
unsuspecting public, with Star Wars, Episode 1: The Phantom Menace. And now EX is available in
the home. Originally exclusive to THX gear, Dolby EX modes can now be found on many
mid-market and high-end processors and amplifiers - look for the EX logo on the front panel. So
how did it all come about? LucasFilm subsidiary Skywalker Sound wanted more creative flexibility
when creating movie soundtracks. It craved the ability to position sounds directly behind the
audience - effectively to make them jump out of their seats.

Together with Dolby Labs, Skywalker Sound came up with an enhancement for Dolby Digital
surround that introduces an additional sonic channel (centre rear). Using EX, moving effects can
fly from front to back more smoothly. Dolby Digital EX works by matrixing the additional channel
within the rear left/right pair.

The full EX decoding found on THX processors and amplifiers is known as THX Surround EX. As
with the professional decoders, this uses matrix decoding to separate the back information from
surround left and right, feeding it to two channels of amplification which then drive an extra pair of
speakers placed at the back of the room.

Under LucasFilm's requirements, THX Surround EX receivers and AV amplifiers may include
power amps for both surround back outputs or merely provide preamp outputs for external
amplification. THX licensees are also encouraged to include their own proprietary DSP modes for
the two surround back channels to enhance the audio for 5.1 music recordings. LucasFilm
recommends that Surround EX systems should use two sets of THX Ultra-certified dipole
speakers. One pair should be placed along the side walls to handle surround left/right and the other
placed together on the rear wall for surround back.

The 6.1 story doesn't end there, though. Now DTS has introduced its own 6.1 variant - DTS ES
(Extended Surround). There are, in fact, two variants of ES. The first, Matrix, is essentially the
same as Dolby EX. In other words, the back surround channel is matrix-encoded onto the rear
surrounds - processors with compatible modes will be able to retrieve this extra channel. The other
variant, Discrete, launched in 2000, is a true 6.1 system. Here, the extra channel is buried in the
DTS bitstream. Theoretically, it offers better performance than matrix systems. DTS is also
responsible for Neo:6 - a proprietary full-bandwidth 5.1-surround-from-stereo mode that essentially
does the same as Dolby's Pro-Logic II.


Ref:
http://www.homecinemachoice.com/articles/hccarticles/buyingguide/200208Knowledge_THX.p
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