By Lisa Johnston
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The new THX Ultra2 performance standard, devised by Lucasfilm's THX Consumer Division, includes processors that deliver 7.1-channel playback of any 5.1-channel source and, for the first time, a THX mode intended expressly for multichannel music playback.
The spec, which replaces the Ultra spec, also changes LCR speakers' vertical directivity requirements, which some speaker engineers said made it difficult to build Ultra-certified home theater speakers that were also musical.
The spec is called THX Ultra2, which will replace THX Ultra and will appear for the first time in two Ultra2-certified Pioneer receivers at the CEDIA Expo. They're due in October. Also at the show, Snell will exhibit an LCR speaker, in-wall dipole surround, and a powered subwoofer with pending Ultra2 certification.
Ultra and Ultra2 products are intended for listening rooms with volumes of 3,000 cubic feet or more. The THX Select standard remains unchanged and is designed for listening rooms with a volume of 2,000-3,000 cubic feet. Ultra2 and Select designations are designed to signify best-in-class performance in their price ranges.
Although Ultra and Ultra2 products will coexist in the market for awhile, Lucasfilm will certify new products only if they meet Ultra2 or Select standards, said Laurie Fincham, engineering director at the THX consumer division.
"We're encouraging receiver manufacturers to put in seven channels for the greatest flexibility with current and future technologies," he added.
Here's what Ultra2 also specifies for receivers and processors:
Automatic switching to THX Surround EX mode upon detection of an optional flag, found on most newer DVD-Video discs, signifying the presence of Dolby Digital EX-encoded soundtracks. Users continue to have the option to manually switch if a disc lacks the flag.
Video switching that doesn't degrade progressive-scan DVD or HDTV video sources. Although some suppliers contend a minimum of 30MHz with effective phase-response correction will do the job, Lucasfilm takes no position on minimum input and output bandwidths. The only criterion is a lack of visible degradation as measured in standardized tests.
Switchable boundary compensation control, applied to all channels before a bass management setting is selected, to eliminate exaggerated bass response if a listening position is situated within a few feet of a side or rear wall.
For deriving 7.1 channel soundfields from 5.1-channel sources, Ultra2 receivers and processors must also do the following:
If the soundtrack delivers a stereo surround signal, the devices must feed the stereo signal to the left-right dipole surrounds as usual, switch off decorrelation, and feed a portion of the stereo signal to the back surrounds. Signal processing is used to widen the back-surround stage.
If the 5.1-soundtrack delivers mono to the surround channels, the devices must feed the mono signal to the left-right dipole surrounds and to the two back surrounds, and the devices must turn on side-surround decorrelation. The result is that predominantly ambient information is delivered by the side surrounds and direction information emanates from the back surrounds.
The THX Music Mode is intended for 5.1-channel music discs, including DTS 5.1-channel CDs and Dolby Digital 5.1-channel music performances on DVD-Video discs. The mode directs parts of the stereo surround signal to the left-right surrounds and to the back surrounds, with more directional information appearing in back surrounds as with all Ultra2 modes.
Music Mode switches off ReEq for the front channels as well as decorrelation for the side-surround channels.
For speakers and subwoofers, here's what Ultra2 does:
Ultra2 increases subwoofer extension to 20Hz from 35Hz in recognition of the additional bass content in newer soundtracks, said Fincham.
In LCR models, both standards call for the same wide horizontal dispersion, same degree of sensitivity, and peak outputs. Vertical-dispersion requirements have been revised, however.
The former standard required a tight vertical-dispersion pattern inside which response was very flat, but outside that window, response could be "ragged" because of lobing, Fincham said. This could cause audible coloration unless the room is acoustically very dead, he said.
Now, vertical dispersion spec requires tighter control of response at vertical axes well away from the main listening axis so there are no are no sudden changes in frequency response away from the main listening axis, he said.
This change meets objections that Ultra's tight vertical directivity reduced a speaker's music-reproduction performance, said Fincham, who said he's not convinced that that's the case at all. "A well-designed speaker performs well on movies and music," he said.
For side-surround speakers, Lucasfilm continues to require dipole speakers that deliver a diffuse soundfield, but for the back surrounds, the company now recommends direct radiators that sonically match the front speakers, in part because "it's good for multichannel music," Fincham said.
In the previous Ultra spec, Lucasfilm gave users the option of dipoles or direct radiators for the back surrounds. Dipolar back surrounds "don't work well for multichannel music," Fincham said.
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