Audio Research SDP1 multichannel music processor:
Page 3 The center channel was of negligible value, though. When it was loud enough to stabilize center images for off-center listening positions, there was a marked loss of soundstage width and spaciousness (remember that 3dB adjacent-channel separation). I found myself leaving it off all the time, even for movies. Otherwise, the SDP1 did everything it was supposed to do—and did it without any audible degradation of the front-channel signals.

What does surround-sound do for music? I've described it before, but I'll reiterate: What it does, basically, is put the ambient space of the recording around you instead of in front of you. If there's any spatial information on the recording—and there is on every recording miked in real stereo—it wraps around you the way it does in a real acoustical space. Instruments sound more solid and three-dimensional and less like cardboard cutouts, and bass takes on a richness and bloom that are much more like the real things than stereo bass. And surround-sound seems louder than the same measured sound-pressure level in stereo—by as much as 3dB.

That's what surround-sound is supposed to do. I was puzzled, then, by how little surround-sound via the SDP1 was enhancing the bass—until I got a phone call from Audio Research's Mike Harvey, who suggested I try reversing polarity on the L surround output. That solved the bass problem, but introduced another: now the ambient surround field wasn't nearly as spacious as it had been. The reason, of course, is that the surround was now monophonic—both speakers were now getting the same signal.

That's why the ARC-recommended surround-speaker connection is with them out of phase; having one speaker reproducing the L-R information and the other the R-L information gives a very wide, spacious surround. (Front-to-surround phasing is irrelevant, because the rear-channel delay causes several cycles of phase rotation.) But antiphase connection also causes the low end from the surrounds to cancel to some extent. THX processors assume the surrounds are connected in phase and use "decorrelation" to widen the ambient field; but the minimalist SDP1 has no such provision. So, what's the best way of connecting the SDP1's surrounds? There is no best way. ARC recommends trying both and going with the one you prefer, and I concur.

Movies
How did the unsteered SDP1 handle movie soundtracks? Surprisingly well, considering. Recent film soundtracks are a lot cleaner and more transparent than you might guess. Audiophiles who dismiss Hollywood as a hotbed of deaf philistines should take a long listen to the best audio that's coming from LaLa Land these days. With the center turned off, phantom images tracked their visuals across the screen with stunning accuracy, and surround effects were clearly coming from the rear rather than the front.

There was considerable leakage from hard-panned left and right signals into the surrounds, but the delay kept them resolutely up front as long as they contained enough transient energy for the precedence effect to work on. Sustained sounds like rain and rumbles of thunder appeared almost equally at the front and in the surrounds—differing in level by only about 3dB—but it was no big problem; they're supposed to be all around. Steering, it would seem, is rather less important in a typical living room than it is in a movie theater.

Conclusions
If you have any misgivings about getting into surround-sound for your music listening, the Audio Research SDP1 should dispel them. It passes the all-important front channels completely unscathed, it does as good a job as any decoder can with the surround channels (despite their rather high noise level), and its all-solid-state design should reassure anyone who has learned the hard way how often tubes fail in use and the damage they can do when they do. I just wish it had a single level control for the surrounds; since they're mono, separate controls are an unnecessary bother.

The SDP1 has done nothing to sully Audio Research's enviable reputation for sonic excellence, but the surround channels need more work. Their spurious noise isn't high enough to impair listening enjoyment, but the fact that it's audible at all ill befits a product from so venerable and venerated a company.