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An Interview with Dr. David Griesinger, by Philip Brandes
Part Two

Go To Part One
PB: One of your innovations in the current home pre/pro line is Bass Enhance processing, which is a radical departure from conventional THX-style bass management. Because it's so different, Bass Enhance has generated a lot of confusion about its use and purpose.
DG:

Bass Enhance is there because most of the people who mix recordingspopular music, anywaymix the bass in monaural, into the center. What Bass Enhance does is route the mono LF to the sides, and put a constant phase shift between the two channels.

If you have music that was recorded with stereo basswith decorrelated low frequency reverberations, for examplethen Bass Enhance doesn't do much. If the two channels have no correlation, there is no constant phase relationship between them, so Bass Enhance doesn't do anything. The phase shift is only there when you have a very constant monaural bass signal, and then it changes the way that sounds.

PB: You've had Bass Enhance in the software for the DC-1, DC-2, and MC-1 for over two years now. What kind of response have you had to it?
DG:

I frankly don't know because I'm not quite in that loop. Occasionally, one of the marketing people like Bart LoPiccolo will come and say "People really like it." But I haven't heard anythingcertainly nobody's gone out and polled what percentages of the people really like or hate it. Nobody's done that kind of research so I can't tell.

Informally, though, I do experiments with it in demos all the time, and I find that some people don't like it and some people really do. It has a lot to do with what your expectations are for bass. If you're a recording engineer that sits between two tower loudspeakers in a dead room 8 hours a day listening to the kick drum go boom-boom-boom hitting you in the chest, you may not like Bass Enhance because it changes the situation completely. It's a completely different sound. Those people are more likely to say "There's something wrong with the bass" and "It's phasey" And of course they're right.

In the several of the rooms I've demo'd it in, it is quite different. In some rooms it makes a much more dramatic difference than others. And it always makes bass sound less loud, because it's taking energy out of the very center where you probably put yourself and sort of spreading it more around the room. That will make it softer in the center.

PB: But more enveloping...
DG:

But more enveloping. That's correct. I tend to like it, because to me it makes the bass sound more like a live performance. If that's your reference, then you'll probably like it. If it's not your reference, you may not. So it does have to do with expectations I think.

PB: Tom Holman is now demonstrating a very impressive 10.2 systembut where does it end? In your opinion, how many channels is enough for home use?
DG: I know Tom has been playing around with 10.2. With respect to bass reproduction, my work does suggest that having two subwoofers is better than oneif you define a subwoofer as below 80 or 100 Hz. I think it's very clear that phase and tonal properties are audible past 40 Hz in most rooms. That's even true in carsI've done experiments with 50 Hz in carsyou can hear the difference between one bass driver and two in a car. So that's important, and you can say that. Unless you assume that .1 refers to below 40 Hz only, and in that case 1 probably is enough.
PB: What about the number of full-range channels?
DG:

We've been using 7 channels for a long time, and I think it really can be argued that that's better than five. David Chesky is demonstrating a 6-channel system where he has 4 speakers in the front. I listened to that, and it's interesting and worth hearing. [The Chesky 6.0 system uses a conventional 5.1 encode format, but maps the channels into a non-standard speaker configuration for playbackP.B.]

The problem with it is it has very strong sweet spots, and you have to be directly between the speakers to get a central image. I don't think that that should have to be necessary. If you're going to go to the trouble of making a surround system with more than two speakers it strikes me that it really should be a social experience. And I really do think the center speaker helps with that. I think I'm in favor of a 3-channel front or a 5-channel front. And there's no question to me that a five-channel front sounds better than a three.

Now you talk about the rear channels....how many of those do you need? Well, it seems to me pretty obvious that you need four. And the reason for that is if you look at the HRTF functions of human hearing, the difference between 90 degrees and say 150 degrees is actually pretty large. So that it's not really possible with a speaker at 150 degrees to make it sound like it's at your side. In fact you really can't make the pan from the front to the rear by audio panning techniques because the front has a very bizarre frequency response and the rear has a different bizarre frequency response...and there's no way you can blend them together and get the right sound at the side.

So 5-speaker images are extremely unstable. You can't pan on the sides, so it's really nice to be able to put a speaker there. There are 2 reasons for that. If you want a sound to come from the side you need a speaker at the side, that's just an unfortunate fact of human hearing. The other thing is that envelopment is determined by excitation of equilateral modes in the room at low frequencies, and a speaker at the sides just does that better than a speaker in the front or a speaker in the rear. So if you can get your bass frequencies in the side you're really doing better than having them at the center or the rear.

So, two reasons for the sides. The trouble is, there's also a reason for speakers in the rear, and that is because speakers in the rear are very exciting. Above 1 or 2 kHz, if you have sounds coming from behind you, it's more envelopingit actually improves envelopment to have that sound behind you. And that doesn't happen for speakers at the side. So for the upper frequency characteristics, it's really better if you can have them come from behind you, and if you do a pan again from front to side to rear, you can really hear it go to the rear if there's a speaker in your rear.

You can do a pan from say left rear to right rear, if those speakers are at +/- 150 degrees. That works very well, it just doesn't work at the sides. So having four speakers, two at the sides and two behind you is really a very good combination.

PB: Is there a benefit to adding any more speakers?
DG:

Well, there's a whole argument about what to do in the vertical direction. I don't know about it opening a whole new can of worms, but I think there might be some logic to saying a couple of speakers in the ceiling might be nice, particularly in some rooms. I've found that if your ceiling is absorbingthat is to say a commercial drop ceiling with a 50% or heaven forbid an 80% absorption factorputting speakers up there is really very nice, because there's just this sense of something really missing. Likewise, if you have a very small ceiling, getting speakers up there is not a bad ideait can give you a feeling that things are more correct. However, a hard plaster ceiling at a distance of 8 or 8.5 feet picks up a first order reflection off your main loudspeakers which unless they're very directed can actually be quite strong. I did some experimenting with an absorbing ceiling and found the amount of loudness you needed in the speakers that were in the ceiling above you was about what you got from a first order ceiling reflection. So just the reflection of the speaker off a hard ceiling is really not bad. You really notice it when the ceiling is not hard.

Now, whether you need a discrete channel for that or not, I doubt it, because a very simple decorrelated reverberationelectronically taking the front channels, delaying them a little bit, smearing them out a little bit, and putting that over to the ceiling struck me as doing absolutely as well as anything else. I don't think you really need a discrete channel therea little bit of DSP does it just fine. So that's my feeling about the height question.

A lot of the people who are talking about a height channel say you should have only one, and that's really easy to prove doesn't work very well. If you take two speakers and you put them on stands and move them around, then you move them together, the difference in sound is really quite alarming...they don't get better as they get closer! The sort of ideal is about +/- 30 degrees up there in the air, and about 70 degrees elevation. But the separation is really importantthey should be decorrelated, you know, having two different reverberation signals, and that really does sound best.

I work a lot in concert halls with the LARES system and we know in doing LARES design that its really important to get speakers in the ceiling. You can put speakers in the sidewalls but it doesn't sound as good as if you have sidewalls and ceiling. That's what makes a natural acoustic environment. So I think there really is something to be said for speakers in the height directionit does make it more natural.

PB: So what is your ideal speaker configuration?
DG: Five in the front, four in the rear, two in the ceiling. The five in the front being left, left center, center, right center, right.
PB: That's getting to be a lot of channels!
DG:

But the cool thing is, all the extra channels can be matrix-derived.

PB: So the discrete encode format would be...?
DG: 5.1. That's all you need.
Go Back To Part One

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