Music in the Round #4

For audiophiles who love multichannel music, the center-channel speaker is a problem second only, perhaps, to that of bass management. In many ways, the rerouting of bass to a subwoofer or to the front left and right main speakers is dictated by room acoustics or the bass limitations of the other speakers. On the other hand, most of us had no real need for a discrete center-channel to fill in the middles of our traditional stereo systems. In fact, one of the glories of really good stereo is the simulation of a convincing central image, such that there's a seamless soundstage from beyond one speaker, across the room, to beyond the other speaker.

But that's not all there is to it. Even plain old stereo benefits from a three-speaker array across the center. The research supporting it goes all the way back to Alan Blumlein in the 1930s, and was impressively demonstrated by Paul W. Klipsch at audio shows in the late 1950s and early '60s. In a large demo room, Klipsch placed a pair of his massive Klipschorns in the corners at each end of a long wall; a smaller Klipsch speaker, such as a Belle or Cornwall, fed by a summed signal from a separate amp, was placed in the middle of that wall. With judicious level-setting for the central speaker, a huge but remarkably seamless and stable stereo image was created, one so impressive that most listeners probably remember the experience to this day.

More contemporary and convincing approaches along these lines include the Meridian TriField synthesizer, which I generally preferred to two-speaker stereo, and Jim Bongiorno's Trinaural Processor, a highlight and award-winner at the 2003 Consumer Electronics Show. Both of these methods are more sophisticated and effective than a simple summed center channel. Summing reduces separation by combining different signals, and creates phase errors by blending nearly common signals with different arrival times. Both of these newer methods greatly enhance the palpability of central images and voices without distorting or displacing them. They also permit wider spacing of the L/R speakers without a "hole in the middle."

A different argument is made by such record labels as Chesky and MDG, which use the six channels of the DVD-Audio format in an innovative way. Such releases have no center-channel signals, but rely on the stereo performance of the main L/R speakers to provide a full frontal soundstage. They also eliminate the dedicated low-frequency effects (LFE), or subwoofer channel, and rely on the bass-management facilities of the user's system to derive that channel, if needed. The freed-up channel space is used to provide discrete left and right signals that are higher and/or more to the sides than the main left and right channels. Set up carefully, such an arrangement provides an even more enveloping and surprisingly natural spatial effect.

The height information literally adds another dimension to the spatial representation. The side information, however, is the important contributor to the overall illusion, because the human ear-brain, which can synthesize a central signal from L/R inputs, is not nearly so good at synthesizing a lateral signal by combining front and rear inputs. That's one reason many of us are disturbed by the apparent jumping of sounds from front to rear on some multichannel recordings, rather than feeling as if the music surrounds us on all sides.

If this six-channel, or 2+2+2, arrangement has such advantages, why do only MDG and Chesky release recordings in this format? The answer, in great part, is home theater. Multichannel music technology did not evolve from first principles but rode into the market, and users' homes, on the back of home theater, a medium whose needs are defined differently. One of these needs is for a specialized LFE channel to permit the reproduction of the nonmusical sound effects so important to cinema. The other need is to anchor all the voice sounds solidly to the central screen. Audiophiles already know how much careful setup work is needed to get that center image just right. Pop a big-box TV in the middle and all bets are off. In fact, considering that most home theaters and nonaudiophile stereo systems are still determined more by the needs of décor than by anything else, it's a wonder that anyone gets anything better than Ping-Pong stereo.

Where does all this leave the multichannel audiophile? Keep in mind that virtually all multichannel releases, with the exception of remasterings of older quadraphonic recordings (such as the Pentatone RQR series), were mixed and mastered to have a discrete center-channel signal. There are other exceptions, and there is often a fair amount of phantom center signal mixed into the main channels, but outside of some purist recordings, we can't know how the recording was made and must assume that, if there is a center-channel signal, it contains unique information not common to the LF and RF channels.

Consequently, setting your player and/or preamplifier-processor or receiver to create a "phantom" center channel will result in cancellations and phase errors complementary to those that result from combining the left and right signals. You can't simply add half of the discrete center signal to the mixer's phantom signal and expect to get the intended effect.

With most multichannel recordings, then, we're forced to have a center-channel speaker. If you trust Blumlein, Klipsch, Stuart, Bongiorno, et al, that's undoubtedly a good thing. The obvious requirement is that it match the left and right speakers in timbre and attack, and that it integrate its dispersion with that of the L/R pair. But rarely does a "matching" center-channel speaker look anything like the main pair. And when you listen to what happens when a white-noise signal is played sequentially over the left, center, and right speakers in a multichannel system, the L/Rs can sound pretty much the same, given roughly symmetrical placements, but the center almost always sounds different.

Why? Because the center speaker is in the center. Each main speaker is significantly closer to its sidewall than the center speaker is to either. In addition, the center's probable proximity to a massive monitor further influences its performance. Also related to the presence of that monitor, the center is rarely at the same height as the L/R speakers. Many manufacturers deal with these problems by constructing their centers as horizontal arrays that control lateral dispersion. Some even add switchable internal filters that modify the dispersion in the vertical plane to compensate for placement above or below the height of the main speakers.

Nonetheless, I have never heard multichannel sound reproduced as seamlessly and coherently as when done with identical or nearly identical speakers and without a boxy video monitor in the way. On that list are demos with three large Meridian DSP8000s, three Wilson WATT/Puppys, and four Magnepan MG-20.1s across the front. (Because the Maggies are asymmetric, an angled pair was used for the center channel.) Even my experience with two Meridian DSP6000s and a DSP5500C stands apart from the usual multichannel demo in terms of seamlessness across the front soundstage.

I wrote to and spoke with five manufacturer-designers of high-end speakers (costing from under $1000 to more than $100,000/pair) about the center-channel issue. Each believed that identical speakers are ideal, but each promoted what he saw as a "real-world" solution. All—save the one who does not (yet) make a dedicated center-channel—justified their different approaches to dedicated center speakers, the designs of some of which accounted for the presence of a video monitor, with the implication that the use of identical speakers was unlikely in most homes. Some emphasized the necessity for a holistic approach to the array of speakers, some the fact that achieving matched output might require that the center have a significantly different conformation, and some that their center speakers could be effectively used for left and right with success—which, of course, begs the issue.

My own limited experience leads me to believe that careful vertical placement of the center and main speakers' mid- and high-frequency drive-units is an important factor in speaker matching. With my Magnepan Home Theater System speakers, this was made simple by wall-mounting the front main pair. With my resident Paradigm Reference setup, this was not possible—the center-channel sits atop the TV, the L/Rs on the floor. The only valid sonic solution was to dump the TV to make room for a proper center speaker in its proper place. (Is there a plasma or projector in my future?)

If I needed any more impetus for this than the matters discussed, SACD reissues of the original Mercury Living Presence classics are promised for this spring. These will come from the three-track master tapes that Wilma Cozart and Bob Fine mixed down into some of the most celebrated sound spectaculars. Now, for the first time, we'll get to hear them in three discrete channels. Paul Klipsch would have loved that.

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