What is THX home Cinema

THX is said to be the best actual home theater system available, maybe no more since AC-3 is now released. LucasFilm keeps a great secret on THX decoding process therefore this may not be obvious to understand how THX works. I tried in this page to explain the technological aspects of the process (with examples and figures!) for those who wonder about the real effectiveness of this home theater technology:

What is meant by THX ?

Why Home THX ?

THX Audio Specifications

The De-correlation System

The re-equalization Process

The Timbre Matching Process

Finally Should I invest in THX ?

THX-certified movie titles

-- WHAT IS MEANT BY THX ?

Even in the movie theatre, from the 1950s through the late 1970s, sound reproduction had had a back seat position to picture reproduction. Innovative techniques were continuously employed in image reproduction while the sound reproduction remained wedded to technologies of the 1940s.

That situation changed with the development and proliferation of Lucasfilm Ltd's THX Sound System for the theatre, which complemented Dolby Laboratories' developments for the reproduction of motion-picture sound. George Lucas, the film maker most noted for his Star Wars film trilogy, wanted movie audiences to experience his films as they looked and sounded in a post-production studio. Anything less, he felt, denied the audience a chance to experience the film as its creators had intended. At Lucas's behest, Tomlinson Holman, Lucasfilm's corporate technical director, developed the THX certficated Sound System. T and H for For Tomlinson Holman and X because the first movie to be released by LucasFilm was THX 1138. Today, over 600 theatres worldwide feature and they are periodically tested to ensure that they meet LucasFilm standards for audio equipment, environment and playback of surround-encoded film. The real improvement of this system was the Infinite Charge Speakers. It simply means that in THX theaters, loudspeakers are directly mounted into the walls without any wood box. It gives very pure sounding effects and deep bass as well as clear and crisping trebles. The problem is that THX claimed to be the inventor of the process and used to ask for a lot of money. However this is absolutly wrong, the process was invented previously but with other goals than Movie soudtrack reproduction.

THX is also a sound quality certification for movie soundtracks. THX certified logos on films and recordings indicate that the final DolbyMP-compatible mixdown was done with the recording console and engineer located in the famous Skywalker Ranch in Nicasio, California (part of Lucasfilm ltd). This is intended to ensure that the film audio will playback in a consistent and predictable manner in all THX theatres (and in homes equipped with THX certified components).

- -- WHY THE HOME THX ?

Most movies on video disk or video cassette contain the same encoded sound track information as their 35mm film version. Therefore, every home-video system begins with a nearly equal potential for accurately reproducing any film's soundtrack and preserving the artistry of its creators. However, theatres and home systems are not acoustically equal environments. The original sound was specially processed for a large movie theatre's acoustics, and when reproduced in a large theatre, the sound is balanced by those acoustics. For example, left right and center channels are tonally balanced to give best sounding behind the screen, that is high frequencies are more dynamic and present than medium or bass. Reproduced in a smaller room, the sound will seem unnaturally brilliant or even shrill. In addition, the sound from the surround speakers will be narrower and distracting, and have a different tonal character from the front speakers. These dilemmas, however, are resolved by Home THX technology.

Home THX has a testing and certification process for home audio equipment. It has established a set of technologies and standards for presenting movies in the home with cinema sound results approaching that of the film mixing theatre. Those models which are submitted by the maker, and pass the tests, may exhibit the branding. THX branded equipment provides the promise of effective home theater, but can still sound hideous if improperly set up and calibrated.

Home THX Controllers decode the audio track signals using Dolby's Pro Logic or digital 5.1 channel circuitries. This is precisely how it's done in the dubbing stage monitoring system during the original recording. However, unlike most other systems which simply amplify and reproduce the resulting soundtrack channels, Home THX applies enhancements before these decoded signals are reproduced.

- -- THE AUDIO SPECIFICATIONS ?

In addition to the patented processes of "Re-equalization," "Decorrelation," and "Timbre Matching," Home THX specifications call for exacting requirements in power amplifier design, loudspeakers directivity, and interconnection performance.

The front speaker systems (e.g. front left, center and right) have a frequency range from 80 Hz to 20 KHz ,and must exhibit well balanced, octave-to-octave response. The two surround speakers have a frequency range from 125 Hz to 8 KHz, and the subwoofer's frequency range is from 20 Hz to 80 Hz. All the speakers must produce high sound pressure levels ( 105 dB), without signs of physical or response deterioration.

Smooth and enveloping surround sound is equally important in the home as it is in the theatre. The surround speaker design largely contributes to attaining this objective. The surround speakers are dipolar meaning that each one (right and left) directs sound in (at least!) two directions. The speakers are situated such that viewer/listeners receive no direct sound. Instead, all sounds they hear are the diffuse result of reflections of sounds aimed toward the front and back of the room. The best positionning is three or four feet above the listening point (your ears actually!). Do not mount them behind or in front of you since you may decrease this famous dipolar effect.

Power amplifiers in the THX Audio System must be capable of amplifying the controller's signals to provide high-level, low-distortion outputs suitable for driving the specified speakers to the upper limits of the soundtrack level requirements. Simultaneously, the power amplifiers must exhibit operating characteristics (e.g. stability under any combination of control settings, wide operating temperature range, etc. ) consistent with highly reliable and rugged systems.

Clear dialogue and pin-point localization of sounds is assured, in part, because THX front speakers are designed to aim sound directly at the viewer/listeners and front speakers are exactly the same to provide identical tonal response. Vertical directivity is focused to increase the proportion of direct sound and reduce sound reflections off of floors and ceilings. Horizontal directivity, however, is wide enough to ensure that anyone sitting off center will benefit from the full effect of precise front localization.

- -- THE DE-CORRELATION SYSTEM

Dolby Pro-Logic decoding includes a provision to delay the surround channel output by 15 to 30 ms ensuring that front sounds always reach the listener ahead of the surround channel (which may contain crosstalk from the fronts). Following the Haas effect, we tend to localise the source of the sound from according to the direction from which we first heard it and delaying the surround signal should avoid some distracting surround sounds to occur. However surround output with Pro-logic is mono and the accuracy of the Haas effect varies a lot depending upon the listener position.

The De-correlation technique splits the single mono surround information into two uncorrelated outputs for left and right surround speakers. The objective is to create an enveloping sound field to maximise the effect Haas whatever the listener position. The viewer/listener hears identifiable sounds but does not sense them collapsing into individual speakers. Experiments show that this collapse of surround sound information detracts from a sense of envelopment and distract one's attention away from actions in the front viewing areas.

This process is achieved by introducing a subtle pitch shift in one of the surround channels in regard to the other channel output. The picture may helps you in understanding the process. It shows de-correlation in action as a simple 1 KHz tone (red) emerges unscathed from the right surround channel, while the same tone incurs a slow +- 0.35 % variation in pitch on the left rear-channel (green). In this example the tone wanders back and forth from 996.5 to 1003.5 Hz. This is too small a change to be directly audible but by taking an average over a minute or so, this plot demonstrates how THX de-correlation is sufficient to confuse the positionning of the rear speakers and encourage a more diffuse surround effect. The improvement in this de-correlation technique is that for the first time, frequency domain pitch shifts were used rather than common time domain phase shifts.

The Decorrelation Process

-- THE RE-EQUALIZATION PROCESS

The Re-equalization technique compensates for the effect of reproducing sounds in a smaller room that have been originally processed for a large movie theatre with high-frequency equalization. Both the front and the centre channels were originaly mixed to give normal sound behind a heavy projection screen which is not the case in Home theater. Without this process, the sound levels would be skewed toward the higher frequencies, making the sound seem overly "hissy" or brilliant. Unlike a large theatre, the smaller video-viewing rooms in a home would not acoustically attenuate those "highs." Home THX Re-equalization restores the original flat-response characteristics before the signals are amplified and used to drive loudspeakers.

The centre channel is mainly concerned by this technique since it reproduces dialogs which are present throughout the movie. The original frequency response (green) begins at about 70 Hz at -3 dB and reaching 20KHz at +0.5 dB (this corresponds to the bright sounds). The reality of re-equalisation (red) is a gentle low-pass filter, reducing all high frequencies beyond 1 or 2 KHz and falling at about -5 dB at 20 kHz (the upper limit of your Hearing). This is quite sufficient to dim the glare from the very brightest of movie soundtracks. As a result the listening seams no more tiring and harsh.

The Re-equalization Process

-- THE TIMBRE MATCHING PROCESS

The Timbre Matching technique ensures there is no change in timbre as a sound moves from the front sound areas to the surround area. In a scene where an airplane flies by, moving from the front toward the rear of the room, then seemingly overhead and behind the viewer, the sound of the airplane moves from the front speakers to the surround speakers. Without timbre matching, even if the speakers had identical reproduction characteristics, viewer/listeners would notice a change in timbre. Our ears simply respond differently to sounds coming directly from the front compared with those coming diffusely from the sides. This is simply due to the irregular profile of our ears. The Home THX Timbre Matching circuitry filters the signals as they cross over from front to surround speakers and reduces the perception of a change in timbre.

Timbre-matching is achieved with an in-built electronic filter which modifies the response of the surround channel, ensuring its tonal balance integrates smoothly with that of the fronts and the centre speaker. The green trace shows a standard Pro-logic rear channel while the red trace shows the shape of the THX timbre-matching curve with twin response gaps centered around 3.8KHz and 6.5KHz. presumably, these counteract peaks in our hearing caused by sounds arriving from the rear. Note this is a very subtle effect since the signal decreases to -5 or -6 dBs only.