New Surround Formats

     Now you're completely surrounded! But hold on -- where are those extra channels coming from? You look on the video shelves for 7.1 channel DVD's, and there don't seem to be any. You check the www.dolby.com Web site and they don't even mention anything beyond 5.1 channels. What's going on?
     Dolby does in fact have a 6.1 channel system for cinemas, but not for DVD. The extra channels are...well, what's a polite way to say faked? Extracted? Synthetized? One manufacturer says the extra channels are matrixed the same way they are in the Dolby Surround system used on VHS.
     There is an important difference, however. Dolby Surround -- the matrixed system on VHS, usually "decoded" with Dolby Pro Logic -- is first encoded at the studio from a master recording which has five or more real channels on it. For instance, material intended for the rear speakers is recorded on the two main channels, but in opposite phase. The "misphased" material is sent to the rear. No such encoding is used for 6.1 channel or 7.1 channel sound, since this is a playback-only system. So how is the "decoding" done?
     In the case of 6.1, it's not much of a mystery. The single rear speaker is fed a mix of the two rear channel signals. This is actually early surround sound technology. Before Dolby introduced Pro Logic, with its variable gain amplifiers, the centre speaker in a surround system would simply carry the left+right signal. The resulting "phantom" centre channel helped fill in the hole left by speakers with poor stereo imaging. Of course, it also greatly diluted the stereo effect, and the louder it was turned up, the worse the dilution. If you turned it high enough, you wound up with mono.
     This dilution of the stereo image may actually be harmless in surround sound, because humans are much less able to spot sounds sources behind them than in front. But if that is true, then just what is the advantage of that extra channel?
     The 7.1 channel system is much like that, only more so. How does it work? It's a mystery, since these processors use proprietary chips to work their simulated magic. Are the two rear speakers connected as they are in Dolby Pro Logic, reproducing the same signal but in opposite phase? It's probably more complex than that, but not necessarily more effective.

Real 6.1 channels
     Want an extra “real” channel? A system does exist, and it's available now. Read about it in our print issue.

Dolby Pro Logic II
     In the era of "real" surround sound, is there still a place for matrixed (simulated) surround? Dolby is convinced there is, specifically because Dolby Digital has raised the bar...and made Pro Logic seem dull by comparison.
     Pro Logic was hardly new. It was a rejigged version of the "full logic" SQ quadraphonic decoder of 1974 -- rejigged only because there are five video speakers rather than four. The word "decoder" is not truly accurate, since the Dolby Surround signal doesn't contain any code. The decoder correlates the relative phase of the signals in the two channels, and then turns volume up or down on the various speakers to simulate a multichannel system. Inevitably, Pro Logic II depends on the same tricks, but electronics has advanced since the days of quadraphonic. You can read about what it does and how it works in our print issue.

(This is an excerpt from the full article. To read the entire article, just order issue 61 at our secure server.)

Complete articles from this issue:
The Battle of the Super Discs, Cambridge Isomagic Converter, Soundcare Superspikes, State of the Art

Excerpted articles from this issue:
New Surround Formats, Defeating DVD Zoning, Vegas 2001, Audiomat Tempo & Vecteur D-2, Audio Refinement Pre 5 Preamp, Osborn Mini Tower Speakers, Mirage OM-9 Speakers


		(Reprinted from issue 61 of UHF Magazine. To purchase the issue, click here. Or click here to subscribe to UHF) New Surround Formats Got your sleek new home theatre processor installed? Got Pro Logic, Dolby Digital and DTS? Here's bad news.
If you like what you see in these sample articles, get all of UHF by subscribing Do the concepts of high end hi-fi give you a headache? Take 20 minutes for the UHF Hi-Fi Course. Trying to reach UHF? Our contact page tells you how to find us.
		Let's get right down to the main question: are 5.1 channels really enough for surround sound? No? There are some people who agree with you (one of them, THX's Tomlinson Holman, who is working on 10.2 channels). Welcome to the world of 6.1 and 7.1 channels. No, it isn't a joke. You may already have seen ads for 6.1 and 7.1 channel surround systems. As usual the ".1" channel refers to a special channel for low frequency information, usually fed to a subwoofer (though in fact many producers don't use it). The additional channels are for the rear. Are they for real? Not always. Here's the familiar surround sound setup, used for both Pro-Logic and discrete surround sound: There are variations, with the "rear" speakers on the side walls, but most surround systems look like this. There's an obvious gap: the one between the two rear speakers. There's a central speaker at the front, so why not at the rear? Enter 6.1 channel surround. An extra rear speaker is added to the mix, which now looks like this. This looks like bad news for anyone who already has a 5.1 channel setup, because it means you need a lot of extra gear: a new sound processor, an extra channel of amplification, and of course one more speaker. And what if your rear speakers are a model that comes only in pairs? That last point is addressed by yet another innovation: 7.1 channel surround. To get a broader and denser surround field, we move the old rear speakers to the side wall (as they already were on some installations) and add two near rear speakers. Now you're completely surrounded! But hold on -- where are those extra channels coming from? You look on the video shelves for 7.1 channel DVD's, and there don't seem to be any. You check the www.dolby.com Web site and they don't even mention anything beyond 5.1 channels. What's going on? Dolby does in fact have a 6.1 channel system for cinemas, but not for DVD. The extra channels are...well, what's a polite way to say faked? Extracted? Synthetized? One manufacturer says the extra channels are matrixed the same way they are in the Dolby Surround system used on VHS. There is an important difference, however. Dolby Surround -- the matrixed system on VHS, usually "decoded" with Dolby Pro Logic -- is first encoded at the studio from a master recording which has five or more real channels on it. For instance, material intended for the rear speakers is recorded on the two main channels, but in opposite phase. The "misphased" material is sent to the rear. No such encoding is used for 6.1 channel or 7.1 channel sound, since this is a playback-only system. So how is the "decoding" done? In the case of 6.1, it's not much of a mystery. The single rear speaker is fed a mix of the two rear channel signals. This is actually early surround sound technology. Before Dolby introduced Pro Logic, with its variable gain amplifiers, the centre speaker in a surround system would simply carry the left+right signal. The resulting "phantom" centre channel helped fill in the hole left by speakers with poor stereo imaging. Of course, it also greatly diluted the stereo effect, and the louder it was turned up, the worse the dilution. If you turned it high enough, you wound up with mono. This dilution of the stereo image may actually be harmless in surround sound, because humans are much less able to spot sounds sources behind them than in front. But if that is true, then just what is the advantage of that extra channel? The 7.1 channel system is much like that, only more so. How does it work? It's a mystery, since these processors use proprietary chips to work their simulated magic. Are the two rear speakers connected as they are in Dolby Pro Logic, reproducing the same signal but in opposite phase? It's probably more complex than that, but not necessarily more effective. Real 6.1 channels Want an extra “real” channel? A system does exist, and it's available now. Read about it in our print issue. Dolby Pro Logic II In the era of "real" surround sound, is there still a place for matrixed (simulated) surround? Dolby is convinced there is, specifically because Dolby Digital has raised the bar...and made Pro Logic seem dull by comparison. Pro Logic was hardly new. It was a rejigged version of the "full logic" SQ quadraphonic decoder of 1974 -- rejigged only because there are five video speakers rather than four. The word "decoder" is not truly accurate, since the Dolby Surround signal doesn't contain any code. The decoder correlates the relative phase of the signals in the two channels, and then turns volume up or down on the various speakers to simulate a multichannel system. Inevitably, Pro Logic II depends on the same tricks, but electronics has advanced since the days of quadraphonic. You can read about what it does and how it works in our print issue. (This is an excerpt from the full article. To read the entire article, just order issue 61 at our secure server.) Complete articles from this issue: The Battle of the Super Discs, Cambridge Isomagic Converter, Soundcare Superspikes, State of the Art Excerpted articles from this issue: New Surround Formats, Defeating DVD Zoning, Vegas 2001, Audiomat Tempo & Vecteur D-2, Audio Refinement Pre 5 Preamp, Osborn Mini Tower Speakers, Mirage OM-9 Speakers [TABLE OF CONTENTS] [BACK HOME]