Home-Theater Audio

Straightforward answers to basic questions about home-theater audio setup and optimization

My speakers have two sets of binding posts for “bi-wiring.” What is bi-wiring and should I use this feature?

Connecting your speakers with two runs of loudspeaker cable (if your speakers have two sets of input terminals) provides a small but significant improvement in sound quality. One pair of the speaker’s input terminals is connected to the woofer, and the other pair to the midrange and tweeter (or just the tweeter in a two-way speaker). In a bi-wired system, the power amplifier “sees” a higher impedance on the tweeter cable at low frequencies and a lower impedance at high frequencies. The opposite is true on the woofer half of the bi-wired pair. This causes the signal to be split up, with high frequencies traveling mostly in the cable driving the tweeter circuit, and low frequencies conduced by the pair connected to the loudspeaker’s woofer circuit. This frequency splitting reduces magnetic interaction in the cable, resulting in better sound.

If you have the ability to bi-wire, it’s worth the cost of an extra run of speaker cable. You can buy bi-wired cables with a single pair of terminations on the amplifier end and dual termination pairs on the speaker end. Be sure to remove the “jumpers” that connect the pairs of speaker input terminals.

What is THX and do I need it in my receiver?

ome 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.