OTHER TECH

Testing a piece of gear for THX certification takes between 15 and 60 hours, with cabling and amplifiers taking the shortest time and processors and receivers the longest. The pass/fail report on a receiver, for instance, involves more than 1,000 measurements and is over 100 pages long. Equipment that meets with THX approval falls into either the Select or the Ultra2 category. While Select-certified gear tends to be less expensive than Ultra2 gear, the labels aren't based on quality or price but on the room size the component is best suited for. THX Select gear works best in smaller rooms or in rooms where you'll be sitting closer to the speakers, while Ultra2 gear is suited for larger rooms (approximately 3,000 cubic feet or more) where you are likely to be sitting farther away from the speakers.

Ultra2 was introduced in 2001 to replace the 10-year-old Ultra specs, which were the same as the specs for THX certification before the program was split into the Ultra and Select categories. The main mission of the Ultra2 program is to simplify and improve film and music playback over 5.1- and 6.1-channel systems.

To receive certification, receivers and controllers (preamp/processors) must include THX's patented Re-Equalization (ReEQ), Timbre Matching, and Decorrelation features. Since soundtracks are mixed to allow for the acoustics of a typical theater, the ReEQ circuitry compensates for smaller home spaces by rolling off the treble to keep the sound from becoming overly bright. Timbre Matching uses equalization to correct for uneven sound pans from left to right and from front to back. Decorrelation creates a stereo surround effect, which heightens the sense of spaciousness when you're playing older Dolby Surround soundtracks that have a mono surround channel.

On the performance side, THX receivers and controllers must meet stringent requirements that call for very low noise, low crosstalk between channels, and generous headroom in the power-amp section. They must also include bass-management facilities (to ensure that all of the deep bass can be redirected to the subwoofer in systems that use small satellite speakers) and time-alignment facilities (to compensate for the different distances between the listening position and each of the speakers).

Ultra2 receivers and controllers must have HDTV-capable component-video switching, and they must let you hook up two back surround speakers. They also include the new Ultra2 Cinema and Music modes. Both modes use Boundary Compensation bass equalization, which helps to keep a big subwoofer from sounding boomy in a small room, and Advanced Speaker Array post processing, which creates a smooth blend between the surround and back-surround channels. THX-certified amplifiers must have low noise and distortion, high power output, and plenty of headroom to avoid clipping distortion during loud peaks. To earn the THX badge, a power amp must also be stable into impedance loads as low as 2 ohms and have a fixed input sensitivity, which makes it easier to set the reference (85 dB) volume level.

Speakers must have an even frequency response both on- and off-axis to ensure smooth sound for listeners who are sitting off center, high sensitivity to make the most of the available amp power, and an ability to play very loud (105+ dB) without distortion. Certified speakers must also have wide horizontal dispersion, which helps ensure a broad listening area, and controlled vertical dispersion to reduce ceiling and floor reflections, which improves imaging and dialogue intelligibility.

Subwoofers must have flat in-room frequency response down to 20 Hz and high output capability (105+ dB) without distortion. DVD players must be immune to lip-sync errors and have an easy-to-use interface, wide and flat luminance and chrominance response, and a high video signal-to-noise ratio. They must also be able to pass the PLUGE (picture line-up generation equipment) test pattern to ensure that the darkest parts of an image will be correctly reproduced.

Cables must follow a consistent color-coding scheme and interconnect cables must have low resistance and capacitance. Packaging must also identify the appropriate use and the recommended maximum wiring length. Projection screens must be acoustically transparent, with perforations that are invisible from the seating position. The screen must provide a neutral surface (one that won't alter the color of the projected image) and be uniformly bright from side to side and top to bottom. Equalizers must have low noise and provide one-third-octave adjustments for the front channels (with a 6-dB boost/cut range) and at least one-sixth-octave adjustments for the subwoofer.

- J.S.