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October 2000

Center Channel . . . in the Rear?
by Doug Blackburn

The rear center-channel speaker is the latest development to keep the average man and woman on the street confused. Those who actually have a clue about this audio stuff even have a hard time keeping track of what’s what. You’d think with all the fanfare going around about the center rear channel being the latest coolosity -- launched with Star Wars: The Phantom Menace no less -- that the major players, Dolby Labs, DTS and THX would have their websites just plastered with rear center-channel information. The most I could find was some pretty general stuff, and fairly comprehensive lists of theatrical and DVD movies employing a rear center-channel of one kind or another.

What’s the point of a rear center-channel?

The point is to enhance the feeling of truly being surrounded. Having a center channel at the rear center gives better control over placement of sounds in the rear half of the room. It allows center of the room flyovers from front center to rear center to be even more convincing than if you relied only on the left and right rear surround speakers -- especially if you bought dipole speakers for the rear surrounds after 5.1 discrete soundtracks became the norm. Dipoles were fine for ProLogic where ambience was the most you could really get out of the matrix decoding, but in the 6.1 world, you can nail the position of someone dropping their keys or a bullet-hit precisely anywhere in the back of the room if you are using point source surround speakers. With dipoles, the bullet-hit or dropped keys don’t have localization that gives you precise sonic position cues.

What about the rear center channel then? Dipole or point source? And just what is "dipole" and "point source?" Last question first: a "point source" loudspeaker is like the ones you use for your 3 front speakers. Most of them radiate forward only or forward mostly. They provide great imaging, including great phantom images that can be placed anywhere between any 2 point-source loudspeakers. "Dipole" speakers as applied to surround or rear speakers are those which generally have 2 sets of drivers, neither set aimed at the listener. They produce an intentionally diffuse sound that makes it impossible to retain the precision imaging you can get with point-source loudspeakers. So for the rear channel, point source or dipole? Here again, I vote point source so you can get the sound engineers exact intent with the positioning of the rear effects. I also vote for two rear center-channel speakers so that people sitting together in a row or on a couch can feel like the sound is right behind them. A single rear center speaker will pull the image off center somewhat, for viewers who are off center even a little. A pair of rear center speakers that are separated a bit, not too much, will give a nice wide spread to the center sound yet still allow precision sound placement when the left and right surrounds come into play.

There are 2.1 flavors of rear center channel

Just when you thought that you were actually going to understand this stuff, now I tell you that there are 2.1 flavors of rear center channel. In the Dolby Digital/THX world you have a matrix-decoded rear center channel. This means the rear center channel is derived after the rear channels are converted to analog audio, but before you send these analog signals to the rear speakers. The decoder is more or less an updated ProLogic decoder. Remember those? Anytime 50% of a sound comes from the left rear and 50% of the sound comes from the right rear, the decoder understands that 100% of those sounds should come from the center speaker. Now, of course, that’s a simplification. To keep the sound from being too mono-sounding in nature, the Surround EX decoder allows a little leakage of center sound to the left and right rear-surround speakers.

DTS has both matrix and discrete digital rear center-channel schemes. While information is thin, it appears that the DTS matrix rear center channel is compatible with the Dolby Digital EX decoding system. Discrete DTS 6.1 requires a new or updated surround processor, which derives the discrete center channel from the DTS bitstream.

I’ll go as far as to say that any rear center channel is better than no rear center channel. I’ll take a matrix center channel if I have to, but a digital discrete rear center channel, now you’re talking serious sound technology in the back half of the room.

What do all the confusing names mean?

Let’s crack the EX code first. Dolby Digital Surround EX is the name used for movies shown in theaters equipped with Dolby’s rear center decoding hardware. Surround EX becomes THX Surround EX because THX worked out a deal with Dolby Labs to insure that there are some standards applied to home implementation of Surround EX. So, you will see DVD players, surround processors, and A/V receivers coming out regularly, which claim "THX Surround EX." When you buy a DVD with intentional rear center surround and a Dolby Digital soundtrack, things get a little muddy. So far, most of these discs seem to indicate Dolby Digital Surround EX. Just remember that EX is EX is EX and don’t worry too much about which company’s name comes first.

DTS ES or DTS ES Matrix is DTS’ name for a rear center channel derived from a matrix encoder/decoder, but DTS has upped the ante with true discrete 6.1 digital surround where the rear center channel is derived directly from the soundtrack without having to be matrix processed. DTS seems to be calling this DTS ES Discrete or DTS ES Discrete 6.1. By the way, ES stands for "Extended Surround" just in case you were wondering.

What happens if I play DTS ES or Dolby/THX Surround EX encoded DVDs through my 5.1 decoder/receiver?

You will get the usual great 5.1 sound since no information is lost from either the matrix or digital rear center channel schemes when they are not used or recognized. Go ahead and buy all the DVDs you want which have rear center channel information.

Well, that’s it for this month. Hope this has cleared up any misconceptions you may have had about the rear center channel. You may have one more question though. How do I perceive the rear center channel? Is it a gimmick or a worthwhile expansion of the dominant 5.1 surround system? I’m willing to go as far as to say a rear center channel has definitely had a positive effect on my enjoyment of most any movie with a dense, interesting soundtrack. I’ve enjoyed encounters with the rear center channel, even home-brewed ones. In fact, I thought several 5.1 movies sounded better after having been run through an ES/EX matrix decoder, deriving a center channel that the sound engineers seemingly did not know about when the soundtrack was made originally. So "matrix" rear center mode may have more applications than you might have imagined.

I won’t bother mentioning that DTS also has Neo: 6, "a matrix technology that derives up to 6.1-channel playback from conventional, stereo program material." Where will it all stop? Probably not until there are center-of-the-sidewall channels, more than one ceiling channel and some kind of floor shaking output to allow actual physical motion energy to be applied to your house.


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