What does THX mean?

There is a considerable lack of information -even misinformation, which is worse- and a great deal of "urban legend" surrounding the meaning of the acronym THX, prominently displayed on the facades of some movie theaters and whose logo we also find printed on the front panels of certain home theater components, both high-end and not so high-end. Of everything that is said and discussed, the most widespread opinion associates THX with a badge of distinction that the front panels of some components simply "show off", for which a far from negligible amount of royalties is paid to George Lucas, the father of the brainchild. As is customary at CEC, we will try to demystify these initials, telling you what lies behind them as objectively as possible, albeit in somewhat condensed form. To explain in depth what THX is would require the pages of a book and documentation to which your humble servant does not have access.

Originally designed for the professional cinema sector, THX is a program of technical specifications and requirements that a THX-certified movie theater must meet, as a guarantee of the best possible sound and picture quality. Before certification is issued, key points are reviewed and corrected, such as external noise and noise from the theater itself (air-conditioning equipment, projector noise, etc.), distorted, poor or reverberant sound, poor-quality images, uncomfortable viewing in some areas, and a long list of other issues. Meanwhile, the SMPTE 202M standard (Society of Motion Picture Television Engineers) sets out the criteria that all sound mixing and dubbing studios must follow, regardless of the technical equipment they use, to ensure that movie soundtracks are produced to the same standards, among which the standardization of frequency response is especially important. However, this standard has nothing to do with the acoustics of some movie theaters, which can ruin even the best production and dubbing work.

Aware of these problems, in 1980 George Lucas, film director and producer of legends such as the Star Wars saga and the Indiana Jones trilogy, brought Tomlinson Holman, an expert from the world of audio, to his Skywalker Ranch through his production company Lucasfilm, to study the influence that movie theater acoustics had on soundtracks and on mixing techniques themselves. As a result of his research, 1983 saw the first THX-certified movie theater, the AVCO Cinema in Los Angeles, and the first movie, Return of the Jedi, better known today as Star Wars Episode VI, also THX-certified. Everything suggests that the initials THX come from George Lucas's first film, THX 1138, although there is no shortage of opinions linking it to an acronym for Tomlinson Holman eXperiment. THX Ltd. was founded in 1983 as a division of Lucasfilm, based in San Rafael, California, but since 2002 THX Ltd. has been an independent company in which Lucasfilm is a shareholder.

We are talking about a time when the dominant sound in movie theaters was based on the analog Dolby Stereo standard (later in its improved version, Dolby Stereo SR), designed by Dolby Laboratories. Its home version is Dolby Surround, which is decoded by the popular Dolby Pro Logic.

Dolby Stereo encodes four channels of information, left, center, right and a mono effects channel -with no subwoofer- on two optical tracks running along the film between the frames and the sprocket holes on one side. Production studios mix soundtracks with a high-frequency equalization known as the X-Curve, a universally accepted standard regarded as very appropriate for large movie theaters. But when they are transferred to a home listening room, smaller than a theater, the sound is somewhat unbalanced and colored. For these environments, THX developed a whole program of specifications for home theater equipment to correct these deficiencies, a standard initially known as Home THX.



FIRST STEPS



The first THX-certified processor-amplifier was the Technics SH-THX10, which appeared in 1991, followed by models from virtually every brand, while the first certified loudspeaker systems also made their appearance, initially with two passive subwoofers, such as the B&W Home Cinema reference models, technologically on a par with the then 800 Matrix series, or the Jamo System One. Today, THX has defined requirements for all the components that may form part of a home theater installation. But THX goes further, with standards for picture transfer, personal computer sound cards and their small loudspeakers, and even HDTV.

Initially, THX-certified components were only found in the higher ranges, owing to excessively stringent requirements. When certification was extended to a wider range of products, THX defined the following selective groups, identified by a specific logo, according to the category and type of component:

THX Ultra. For components designed and built to high quality standards, but whose performance is not affected by the size of the room, such as high-end DVD-Video players, projection screens, equalizers and connection cables.

THX Ultra2. For very high-performance components designed for large rooms, with long distances between the screens -acoustic and viewing- and the seating area. It specifies standards for loudspeakers, receivers, A/V processors, power amplifiers and 7.1-channel systems to optimize the coverage of four surround loudspeakers, as well as for multichannel music listening. It provides technical requirements for equipment incorporating video signal management, progressive scan on DVD-Video and HDTV.

THX Select. Basically, the difference with the previous group lies in specifications applied to components intended for smaller rooms, hence the need for less amplifier power, simplified loudspeakers, etc.

THX Certified Multimedia. Complementary products for listening at very short distances, such as personal computer sound cards and their small loudspeaker sets.

As for the processing of audio signals, we can now make clear -in case there is any doubt- that THX is not a sound encoding and decoding format. It is processing applied to the information coming from Dolby ProLogic (analog) or Dolby Digital or DTS (digital) decoding. Therefore, any THX-certified A/V preamp-processor or integrated amplifier will include a series of circuits to perform the following functions:

Re-Equalization. We mentioned that movie soundtracks are mixed with a high-frequency equalization, the so-called X-Curve, which is very appropriate for large movie theaters. But when the distance from the sound source to the audience is small, as in a home environment, movie audio sounds overly bright. THX uses a re-equalization curve designed for these environments that restores the correct overall balance of the soundtrack.

Timbre Matching. Unlike a movie theater, with its numerous surround loudspeakers around the room for sound effects, the transition between the front and surround channels in a home environment, with only two loudspeakers, is too abrupt and clearly audible. Two corrections, one of timbre and the other of tone, ensure that this transition takes place more smoothly.

Adaptive Decorrelation. With Dolby Stereo decoding, our ears perceive small left-right differences in information in a movie theater's mono surround channel, because of the many loudspeakers used for this channel and the reflections in the room. But in a home environment, with Dolby Pro Logic, its mono character is immediately noticeable. Decorrelation constantly changes the phase and time differences between the two parts into which THX divides this channel, creating a pseudo-stereo sensation. For Dolby Digital and DTS, with two independent channels, decorrelation acts intelligently, only when it detects an identical -mono- signal in both. Hence the adjective adaptive.

Bass Management Electronic Crossover. This circuit allows smaller loudspeakers to be used on all channels by sending the low-frequency signals to a subwoofer, which improves frequency response, lowers distortion and increases dynamic range. For Dolby Pro Logic, only the decoded signals of the two main channels are handled by this filter, whereas with Dolby Digital and DTS all channels are managed.

Bass Peak Level Manager. It protects the subwoofer against overloads caused by the large amount of low-frequency energy present in the LFE channel of certain Dolby Digital and DTS soundtracks. This circuit makes it possible to regulate the subwoofer's capacity to obtain its best performance, without distortion.

Loudspeaker Position Time Synchronization. The ideal listening position is at a point equidistant from each of the loudspeakers, subwoofer included, which in practice is unfeasible in a home. This circuit makes it possible to modify the apparent position of the loudspeakers to obtain the best multichannel image and a flatter frequency response.



LOUDSPEAKERS



Another important group of specifications concerns THX-certified loudspeakers. In a conventional home theater system, homogeneity across the front soundstage requires installing a center-channel loudspeaker from the same manufacturer and range as the main ones, to avoid unpleasant timbral imbalances. THX requires three loudspeakers that are identical in construction and driver layout for the front trio, known as LCR (Left Center Right), with wide horizontal dispersion for greater coverage of the soundstage and controlled vertical directivity to avoid reflections from the ceiling and floor of the room. Such reflections would degrade the sound image and impair dialogue intelligibility. Since THX requires a subwoofer, these loudspeakers have their low-frequency response restricted below about 80 Hz. They must be magnetically shielded.

As for the surround loudspeakers, it is really difficult to achieve a good sound field with just two, compared with the array of surround loudspeakers in a movie theater, which our home theater tries to imitate. THX requires dipole loudspeakers, a special design that generates a diffuse sound field by radiating onto the walls and ceiling of the room, with little direct sound, and none at all in some designs. They have a very flat power response (a more demanding characteristic than frequency response), perceptible anywhere within their null radiation zone which, together with the timbre-matching circuit, ensures a correct sonic transition with the front loudspeakers.

In all the loudspeaker models described, the sensitivity and impedance values must ensure compatibility with well-designed amplifiers and with all THX-certified ones. They must have high power handling, allowing them to generate a sound pressure level of 100 dB for three hours of uninterrupted operation. The specifications for subwoofers are especially stringent, with a frequency response that must reach down to 20 Hz and a sound pressure level capable of reaching 105 dB without distortion. The first THX loudspeaker systems had two passive subwoofers, but since only powered models are made today, one is needed for every 43 m3 of room volume, which is why they usually include a connection that allows them to be daisy-chained.

So that our readers have a more complete idea of THX, or in case any aspect of what has been explained has remained somewhat unclear, we will make a few clarifications that may serve as a counterpoint to, or in some cases confirmation of, certain opinions often heard about THX.

THX certification only guarantees that the certified product strictly meets the technical requirements for its type of component, within its certification group (THX Ultra, Select, etc.), and that it is perfectly suited to a home theater environment. Its strictly "musical" quality, however, will depend on the greater or lesser care that the manufacturer in question has put into its construction.

To give an example, a THX-certified power amplifier is guaranteed to be suitable for driving the far from easy loudspeakers carrying that certification within the same group. Nevertheless, exceptional power amplifiers and other components of the strictest high fidelity are made that are not THX-certified. Certification involves paying royalties, which has an impact on the final price, so some manufacturers are not prepared to take it on.

Since this means entering into considerations of a commercial nature or of product demand, we will only mention that in the US there is great demand for THX-certified components, whereas on our old continent, not so much.

Listening to a good movie soundtrack with a high-quality THX-certified processor and loudspeakers is a formidable, unique experience. But it is less so when only one of the parts, the processor or the loudspeakers, is certified. In the first case, the correction circuits described above will be missing, and in the second, the loudspeakers' special sound radiation properties. Precisely because of the latter, there is no shortage of criticism from hard-line music lovers of THX-certified loudspeakers for strictly musical listening.

THX OPTIMIZER

Some movies come with an interesting extra, the THX Optimizer. This is an attractive tool, developed by THX Digital Works, that allows any user to optimize the sound and picture reproduction of the disc in question for any home theater system.

The system includes different series of tests that make it easy for anyone to adjust the audio and video parameters of their system. The advantage is that these parameters match those used during the disc's mastering processes. Unlike other setup discs, such as Joe Kane's "Digital Video Essentials", which allow a single adjustment for our entire DVD collection, the THX Optimizer is designed to adjust our system only for the disc in hand although, frankly, there are few if any differences between the patterns the THX Optimizer offers on different titles. The system includes settings for loudspeaker alignment, phase tests and the subwoofer crossover; and for video, patterns for contrast, brightness, color, tint and aspect ratio.