October 30th, 2008 -- by Robert Harley
Source: The Perfect Vision
Home THX is a set of technologies and performance standards established by Lucasfilm that better translates film soundtracks mixed for the movie theater to playback in your home. THX is not a surround-sound format such as Dolby Digital or DTS. Rather, THX works in conjunction with Dolby Digital or DTS to improve the home-theater experience.
A THX Certified A/V receiver or controller incorporates four THX-developed signal-processing technologies, as well as meets a set of performance standards. (THX Certified power amplifiers and speakers don’t include this signal processing, but must conform to technical standards set by Lucasfilm.)
The four THX processing technologies are called reequalization, timbre matching, surround decorrelation, and subwoofer crossover. Let’s look at each of these individually.
Re-equalization is a treble roll-off (cut) applied in your receiver or controller when THX mode is engaged. Reducing the amount of treble in the soundtrack during home playback restores the tonal balance you’d hear in a movie theater. Films are mixed with extra treble energy to compensate for the fairly dead acoustics of a movie theater. In addition, some treble is lost in the relatively large distance between the speakers and audience (high frequencies suffer greater attenuation with distance than low frequencies). The result is that these bright soundtracks sound just right in a theater, but when reproduced in your home, are far too bright.
Determining the re-equalization circuit’s characteristics (how much to reduce the treble, and at what frequency) was solved in an ingenious way. THX inventor Tomlinson Holman (THX is an acronym for “Tom Holman’s eXperiment”) played film soundtracks on a home-theater system for the engineers who originally mixed them. The engineers were asked to adjust an equalizer in front of them until the sound they heard over the home-theater system sounded like what they remembered hearing on the dubbing stage. Holman took note of the equalizer’s settings. This process was repeated with many mixers, who made nearly identical changes to the equalizer. Holman used this information to create and patent the THX “reequalization curve,” which removes just the right amount of brightness from film soundtracks for naturalsounding home-theater playback.
The next THX technology, surround decorrelation, attempts to make Dolby Surround’s monophonic surround channel less monophonic. The process slightly changes the sound (specifically, the time and/or phase between the signals in the midrange and treble frequencies) in the left and right surround channels. This difference between left and right surrounds prevents the “inside the head” localization of surround signals that can occur between two loudspeakers reproducing the same signal.
Surround decorrelation produces greater ambience, spaciousness, diffusion, and envelopment from the surround speakers. Surround decorrelation is unnecessary with those discrete 5.1-channel Dolby Digital and DTS soundtracks that have completely independent left and right surround channels. Keep in mind, however, that many DVDs containing a 5.1-channel Dolby Digital soundtrack still have monophonic surround channels. If the original source had a monophonic surround channel (films mixed in the pre- Dolby-Digital era), the Dolby- Digital-encoded soundtrack will contain identical signals in both surround channels. In this case, the “inside the head” localization problems mentioned earlier will persist.
THX’s timbre matching circuit compensates for the differences in the way we perceive timbre between sounds arriving from the front and the sides. Try this experiment: snap your fingers in front of your head and then again at the side of your head. The finger-snap’s timbre is “sharper” when arriving from the side. THX’s timbre matching ensures that, as sounds move from in front of to behind the listener (or vice versa), their timbres remain constant.
Finally, the THX subwoofer crossover separates bass frequencies from the midrange and treble. The bass is reproduced by a subwoofer and the midrange and treble by the main loudspeakers. Although all AVRs and controllers contain a crossover, the THX crossover is standardized with regard to frequency (80Hz) and steepness of roll-off (24dB/octave low-pass and 12dB/octave high-pass).
In addition to incorporating those four core signal-processing technologies, a THX Certified receiver must meet certain performance criteria, including output power, dynamic headroom (the ability to reproduce peaks without distortion), noise levels, and the ability to drive lowimpedance loudspeakers, among other factors. (Specifically, a THX Certified receiver must be able to deliver a minimum of 211Wpc into a 3.2-ohm load.)
Products with the THX Select designation incorporate the four technologies described above, but have somewhat relaxed performance standards with regard to output power. This allows lower-priced receivers to employ THX processing. The original THX certification is now called THX Ultra to differentiate it from THX Select.
Finally, the relatively new THX Ultra2 designation is a combination of new performance criteria for the video-switching circuits, as well as new signal-processing algorithms for creating 7.1-channel playback from 5.1-channel sources. THX Ultra2 Certified receivers contain sevenchannels of amplification, as well as a Boundary Gain Compensation circuit to reduce boominess caused by speakers being placed close to walls. In my experience, THX improves the sound quality of films and is worth having in a receiver or controller