Sony G90 manual THX-Certified Controllers, Bass Management

process takes about eight minutes, can be per-

formed with the AVP installed in your system,

and doesn’t erase your set-up and configuration

settings.

New software can add capabilities such as

DTS or MPEG decoding (by changing the DSP

code), refine the user interface (by updating

the operating system), or configure the unit to

accept formats not available when the product

was designed (by changing the input-receiver software).

The AVP’s Proceed input receiver (the chip that receives,

identifies, and decodes the incoming bitstream) is custom

made, which allows the AVP to work with future formats

whose interface protocols have not yet been established.

Updating software in this way reduces the likelihood of

needing expensive hardware changes.

Another method of heading off controller obsoles-

cence is “modular” construction. A modular

controller is built like a PC, with a mother-

board and smaller circuitboards that fit into

slots on the motherboard. If a new technology

comes along or better digital-to-analog con-

verters become available, as examples, you

simply swap out a circuitboard to bring your

controller up to date. Some controllers com-

bine the ability to update software with modular

construction for the ultimate in upgrade flexibility.

Bass Management

An important controller function performed by the DSP

chips is bass management, the subsystem that lets you

selectively direct bass information in the soundtrack to the

main loudspeakers or to the subwoofer. Bass management

allows a controller to work correctly with a wide variety of

THX-Certified Controllers

ome controllers are “THX Certi- Sfied,” meaning they incorporate Lucasfilm signal processing – a technology that Lucasfilm believes better translates film soundtracks created for theater playback into the home. THX-cer- tified controllers must also meet a set of technical performance criteria established by Lucasfilm. If the product correctly implements the THX technologies and meets the performance criteria, the unit can be branded “THX Certified”The. man- ufacturer then pays a license fee to

Lucasfilm on every unit sold.

The goal of Home THX is to re-create as closely as possible in a home-theater system the sound that the mixing engi- neers heard on the film-dubbing stage. THX-certified controllers employ four processes that Lucasfilm has found to improve the home-theater experience: surround decorrelation, timbre matching, re-equalization, and the subwoofer crossover. Let’s look at each of these.

Surround decorrelation makes the monaural surround signal slightly different in the left and right surround channels by varying the time and/or phase of those signals. This technique prevents the “in the head” localization of surround signals, and “smears” the surround signal so that we feel a greater sense of envelopment in the film soundtrack. With the advent of 5.1-channel formats with separate left and right surround channels, THX surround decorrelation has taken a new twist, called “adaptive de-correlation.” Adaptive de-correlation turns off the de-correlation circuit when the two surround channels carry different information, but smoothly turns it on when the surround channels are identical. Most 5.1 soundtracks still have mono surrounds most of the time, so this is a useful feature. (See the side-

bar to the Denon AVR-5700 review in Issue 25 for more on surround decorrela- tion.)

Timbre matching makes it possible for sounds arriving from the sides to have the same perceived timbre as sounds arriving from the front. This makes pans (movements of sounds) from front to rear more realistic, because the perceived timbre doesn’t change with movement.

You can easily demonstrate for yourself how perceived timbre changes with direc- tion: Snap your fingers in front of your face, and again to the side of your head. The sound is “sharper” to the side. THX timbre matching compensates for this difference with signal processing in the controller.

Re-equalizationis a treble cut applied on playback to make soundtracks mixed for movie theaters sound natural when played in the home. Mixers intentionally make soundtracks bright for several rea- sons. Theaters are usually full of absorbent seats, drapes, and people, all of which roll off high frequencies to a greater degree than midrange and bass frequencies. In addition, the long distance between the audience and loudspeakers tends to selec- tively attenuate treble. Consequently, the soundtrack has a natural tonal balance in the theater, but excessive brightness on a home-theater system. The answer is to equalize the soundtrack during playback so it sounds correct in the home.

But how much treble cut is correct? And what should the equalization curve look like? To find out the correct THX re- equalization curve, THX’s inventor, Tomlin- son Holman (THX stands for Tom Hol- man’s eXperiment), asked a series of top- level film-sound mixers to listen to their films on a home-theater system. The mixer had an equalizer in front of him, and was asked to adjust the equalizer until the

soundtrack sounded “right” on the home- theater system. Holman averaged the equalization curves created by the mixing engineers (which were remarkably close) to generate the patented THX re-equaliza- tion curve.

To save money, some budget con- trollers license only the re-equalization part of THX processing, not the entire sig- nal-processing suite. Other controllers not licensed by Lucasfilm may employ a selectable treble cut, often carrying a

name such as “Cinema EQ.”

Finally, the THX subwoofer crossover standardizes the crossover characteristics (cut-off frequency and slopes) that split the frequency spectrum into bass for the sub-

woofer and midrange/treble frequencies

for the main speakers. The THX crossover frequency is 80 Hz, with fourth-order low- pass and second-order high-pass slopes. The subwoofer-out jack on a THX-certified controller thus carries a precisely defined signal. When decoding 5.1-channel Dolby Digital or DTS (so-called THX 5.1 mode), the subwoofer output carries the Low Fre- quency Effects (LFE) channel, plus the bass from any number of the other five channels. When decoding Dolby Surround, the THX subwoofer output is a mix of the front three channels’ bass below 80 Hz, assuming that the front speakers are small satellite types.

You may have recently seen the desig- nations “THX Select” and THX Ultra” replace plain old THX. THX Select products have relaxed performance standards, and are designed to allow products suitable for smaller rooms to benefit from THX pro- cessing. The more rigorous Ultra perfor- mance level corresponds to what used to be simply called “THX” and is built on the assumption that the room involved may be

3,000 cubic feet or larger. RH