Historical

Once the surround signal is mixed into the stereo signals, it can never be completely separated out again. Instead of appearing behind you, surround effects might appear some where between where you're sitting and where your front speakers are, and sounds that belong in front will tend to "leak" into the surrounds. To improve the front-to-back separation, Dolby Stereo uses what's called logic steering. (The home version is called Dolby Surround.)

Logic steering measures the signal content in all three channels, compares each one with the others, and "decides" on the basis of their relationships whether the sound belongs in the right, left, or rear channel. Special circuits then subtract the unwanted energy from each channel, to steer each signal exclusively to the speaker it should be coming from. The result is (usually) what sounds like total channel isolation.

No such processing is needed for the subwoofer. Because bass energy tends to be more or less identical in the two front channels, it is simply drawn off from them, filtered to make sure bass is all that's there, and sent to the subwoofers.

Center Channel Stabilizes Soundstage
Larger home theater systems also have a center-channel speaker. This isn't necessary iflisteners will be sitting about midway between the front speakers. But when the seating area is wide, people at the sides will hear sounds off-center, so that an on-screen speaking voice will be heard to come from one side of the screen. A center-channel speaker stabilizes the "soundstage," keeping on-screen sounds where they belong.

The center signal too is a mixture of the front left and right stereo signals, but if it is loud enough to stabilize the image properly, it will have another detrimental effect: It will impair the separation between the front speakers, making them sound as if they're too close together for decent stereo. Again, logic steering is used to cancel the stereo information from the center channel, preserving the full width of the original stereo "stage."

The term "Dolby Surround" applies to all consumer surround decoders that extract a matrixed surround signal, including those that don't use logic steering. "Pro Logic" is the deluxe version of Dolby Surround, and it includes the steering plus a built-in test signal to facilitate balancing-out the volume levels of the various channels. A few years ago, George Lucas of Star Wars fame launched a movie theater quality-assurance program called THX, which was intended to ensure high standards for sound and picture quality. More recently, THX came to home theaters.

THX isn't a product, a system, or a brand name. It is nothing more than a set of technical specifications that define minimum standards of acceptable audio or video performance. The idea was that, since so many directors sweat blood to get good sound into their films, it would be really neat if consumers were able to hear what all the fuss was about. THX gives them a better than even chance of doing this. Home theater products that are developed, in consulta tion with Lucasfilm, to meet or surpass the THX standards, receive certification that allows them to wear the THX logo on the front panel and feature it in their advertisements.

THX Reflects Concern for High Standards
The standards are fairly stringent but not unreasonably so. Most of them just reflect traditional pro-audio values about such things as frequency response smoothness and range, distortion, available power, and loudspeaker sensivity. Others pertain to the way the loudspeakers cover the listening area; still others are innovations specific to the home listening environment. For example, Home THX mandates the use of something called decorrelation, which expands the non-stereo surround signal so as to provide an illusion of real space, like that in a movie theater.

THX also requires the surround speakers to have a "figure-8" radiation pattern, which directs the sound towards the front and back of the room. This makes the whole room a spatial surround field, but puts the listener in a "null zone" to the side of the speakers so he won't be able to tell where their sound is coming from.

THX ensures that all certified products are more or less interchangeable. Although some do sound better than others, you can mix and match different brands knowing that they are at least mutually compatible and suitable for the job—something you cannot take for granted with non-THX components. (An incompatible combination might be a low-powered amplifier and insensitive loudspeakers, which won't be able to deliver enough volume for satisfying movie listening.)

On the other hand, some manufacturers who elect not to pay THX royalties, or whose products may not qualify for certification, nonetheless incorporate some aspects of THX (such as front-and-back-firing surround speakers) in their designs, and they too are worthy of consideration. Some offer what is essentially THX performance at way-below-THX prices.

Should you buy THX or Dolby Pro Logic? The choice isn't that clear-cut, because Pro Logic is the foundation THX was built on. Pro Logic separates out the multi-channel signals; THX just determines to a great extent how they will sound. If you don't care for that sound, you don't have to buy a Pro Logic decoder with THX provisions. Many decoders don't offer the THX option, and all the ones that do offer it also allow you to switch it off.

Raindrops, Noisy Crowds & Closing Doors
The only real choice with respect to surround speakers, other than basic sound quality, is whether or not to go with THX's figure-8 surround speakers, because most decoders don't allow you to operate these as "point-source" speakers. Generally, the figure-8s do a better job of reproducing room sound and ambient surround effects like rain and crowd noises, while point-source speakers, which radiate from the front only, can provide more natural reproduction of localized sound effects like a voice or a closing door. If you think the difference will be important to you, audition both before deciding.

While home theater stresses movie sound, a surround system can also greatly enhance music-only reproduction. Some decoders have the ability to add artificial room sound, to simulate the soundspace of a large hall, a cathedral, a stadium, or a small room. These may be useful if the record ing itself has little or none of its own room sound, but when the recording has all the reverberation it needs (which is usually the case), adding more will make the sound too reverberant and distant, and will obscure a lot of detail in the sound. Generally, it's far better to extract the room sound that's on the recording and feed that to the surrounds, and this can be done with any Dolby Surround decoder. The improvement can be dramatic.

If the recorded reverberation was real rather than electronically synthesized, what you hear from the surrounds is the original recording space. No longer are you sitting on the outside and listening to the performance through a wide window; you are inside, where it's all happening. Large performing spaces such as concert halls and opera houses often have their own unique and recognizable "sound," and this can be surprisingly well reproduced by a surround system.

There are other benefits, too. Individual instruments have more immediacy and more body, as if they've taken on an almost palpable third-dimensionality. And many rock recordings are a revelation in surround, with all sorts of spatial effects that are only suggested through two-channel repro duction. Things whiz past, zoom around you, and pop up here and then there. Realistic? No, but lots of fun.

Trouble For Pro Logic
Not all recordings reproduce well in Pro Logic surround, though. Ones that are not specifically encoded with a surround channel can confuse the logic steering, causing some sounds to hop randomly around the room when they shouldn't, and producing a "pumping" effect that sounds a like severe distortion. For this reason, most surround decoders have at least one so-called music surround mode that has rather more benign steering characteristics, or no front/back steering at all.

A few years ago Dolby Labs developed AC-3, a digital "bit-reduction" system that allows vast amounts of digital audio to be stored in a relatively small space. First used for multi-channel movie sound, it is now being considered as a means for delivering six completely independent sound channels to consumer's home. This will overcome the remaining weaknesses of Dolby Pro Logic, such as its tendency to become occasionally confused and its inability to reproduce several different sounds from different directions at the same time.

Pioneer has already demonstrated a working prototype laserdisc player with AC-3 sound, and the system has been adopted for use with tomorrow's high-definition television systems.

There are still questions about AC-3's possible detrimental effect on sound quality. Only a few people outside of Dolby Labs have heard a direct before-and-after comparison between an original recording and its AC-3 reduction, and they weren't necessarily impressed. (Movie patrons have generally been pleased with what they hear from it.) It is certain, though, that some system, if not AC-3, will be bringing us uncompromised multi-channel surround sound within a few years. Personally, I can't wait.