Home-Theater Audio

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.

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.

How can I get the best possible sound quality from my theater system for music listening?

First, choose an AVR or controller with an “analog bypass” mode. This feature provides a pure analog path through the receiver that doesn’t subject the signal to sonically degrading analog-to-digital and digital-to-analog conversions. Second, take sound quality into consideration when buying a DVD player. High-quality video processing has become incredibly cheap recently, resulting in fabulouslooking video performance in sub-$100 DVD players. But there’s been no parallel trend in audio performance. To get good sound, you’ll still need high-quality parts in the audio circuitry, along with a large and clean power supply—all relatively expensive items.

An alternative is to select a receiver with first-rate digital- to-analog conversion and analog-output circuits, which obviates the need for a high-quality DVD player. In this setup, you’re not even using the digital-to-analog converters in the DVD player. Instead, you make a digital connection between the DVD player and receiver, and the job of converting the signal to analog falls to the receiver. If you choose this configuration, be sure to connect the DVD player to the receiver with a coaxial cable, not the optical TosLink connection. TosLink introduces timing variations (called “jitter”) in the clock that degrade sound quality.

Finally, be sure to turn off any unwanted signal processing in the receiver or controller.

What can I do to improve my room’s acoustics?

This topic could fill an entire book, but a few simple techniques will greatly improve your system’s sound. First, avoid having acoustically reflective objects near the front loudspeakers. That’s difficult if you have a television between the left and right speakers. You can, however, reduce the amount of reflected energy by moving the speakers forward of the television. This placement also has the advantage of positioning the three front speakers in an arc, which puts the listener the same distance from each of the front speakers. If your center-channel speaker is on top of your television, position the front baffle (the surface on which the drive units are mounted) in the same plane as the television screen so that there’s a continuous surface. This simple trick confers a large increase in dialogue intelligibility.

If possible, avoid hard and flat surfaces on the side walls between you and the loudspeakers. These surfaces reflect the speaker’s energy to the listening position, degrading fidelity. Hanging rugs, bookcases, CD and DVD storage racks, and any other common items will absorb or diffuse that reflected energy and improve the sound. If possible, use similar material on the left and right walls to maintain acoustic symmetry between the left and right channels. Similarly, the rear wall behind the listening position is a good place for a large bookcase. This diffuses the reflected energy and prevents it from interacting coherently with the direct sound.

Avoid large areas of bare parallel surfaces, which create “flutter echo,” heard as a pinging that hangs in the air after you clap your hands. Flutter echo reduces intelligibility and adds a fatiguing hardness to the sound. A hanging rug works wonders in reducing flutter echo. Remember that you don’t have to treat both parallel surfaces, just one. Scattering the absorbing material (several small rugs) is preferable to one large absorbent surface.