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.