Background

Star Wars Episode I: The Phantom Menace broke new ground in the field of cinema audio with the introduction of Dolby Digital Surround EX co-developed by Lucasfilm and Dolby Laboratories. At the time, no home digital audio decoder offered the capability of dematrixing the rear channels. Since the aural addition of a center surround channel adds considerably to the viewing experience, I was motivated to devise a way to decode the new channel using a spare external decoder, an obsolete Shure HTS-5300 analog Dolby Surround Pro Logic decoder. It had been retired when I acquired my first B&K digital decoder/preamp in ’97. But since rear channel dematrixing for EX is essentially identical to Dolby Pro Logic front channel dematrixing, I was able to extract the center surround information from both EX DVD tracks and non-EX fully discrete 5.1 DVD tracks using the Shure. The results were very satisfying and worked well; I used these components in this adlibbed interconnected configuration right up to the time I bought my second B&K decoder, which is EX-capable.

How HD On Disc Resurrects This Old Problem

More recently, I’ve been contacted by several readers who are frustrated to have discovered that even though their decoding receivers support EX dematrixing of digital bit streams, they do not dematrix signals derived from analog inputs. This is significant because the overwhelming majority of BD (and HD DVD) players, on which most of us depend to decode advanced audio CODECs, have only 5.1 analog outputs, not 6.1 or 7.1. That means that those owners are stuck with the difficult choice of having to choose between internal player decoding for best fidelity sound without a center surround, and a 6.1 or 7.1 presentation dematrixed by a receiver or decoder from digital bit streams that can only pass lossy, compressed digital audio bit streams that are not as sonically accurate.

But if you have an old Dolby Pro Logic decoder or receiver lying about, you may have the solution in hand.

Quick Dolby Pro Logic Refresher

For the sake of this discussion, I’ll ignore the Pro Logic surround channels. Simply stated, if the sound mixer wants to place a sound in the front center channel, he encodes in-phase, equal amplitude signals in both the left and right stereo channels. When such a matrixed stereo program is passed through a Dolby Surround decoder, those in-phase, equal amplitude signals are sent to the center channel by dematrixing and steering logic that enhances separation. By manipulating the characteristics of sounds in the two front channels, they can be made to smoothly pan among the front speakers. This is fundamentally the way rear channel processing of Dolby Surround EX works.

Pseudo-EX

Signals in a discrete digital soundtrack placed in-phase at equal amplitude in the left and right surround channels of a 5.1 mix are intended to produce sound effects that seem like they are either directly overhead or directly behind the listener. This so-called “phantom” image is positioned spatially between the two surround speakers. (Actually, if you’re equidistant from surround speakers positioned on either side-wall, it sounds like it’s located inside your head, just like listening to monaural material on stereo headphones.) EX is designed to move that phantom image out from between the two surround speakers to behind the listener on a center surround speaker or speakers. As described in the previous section, in-phase, equal amplitude signals matrixed into the left and right stereo channels of a Dolby Surround mix are intended to create sound effects that come from the center front channel. So it should be possible to extract center surround information when Dolby Surround front channel processing is applied to the discrete left and right surround channels of a 5.1 mix. And as I quickly discovered in ‘99, that’s exactly what my Shure decoder can do. So let’s get to how you can take advantage of this technique with a BD (or HD DVD) player that decodes advanced audio bit streams and outputs the analog audio only in 5.1 form.

The Connections And Setup

Connect the left analog surround channel output from the player to the left front input of your spare Dolby Pro Logic decoder or decoding receiver.

Connect the right analog surround channel output from the player to the right front input of your spare Dolby Pro Logic decoder or decoding receiver.

Connect the right front output of your spare Dolby Pro Logic decoder or decoding receiver to the right surround input of your newer EX-capable decoder or decoding receiver.

Connect the left front output of your spare Dolby Pro Logic decoder or decoding receiver to the left surround input of your newer EX-capable decoder or decoding receiver.

Connect the center front output of your spare Dolby Pro Logic decoder or decoding receiver to the center surround input of your newer EX-capable decoder or decoding receiver.

Connect the subwoofer, left, center, and right front outputs from the player to the subwoofer, left, center, and right front inputs of your newer EX-capable decoder or decoding receiver; these are normal connections.

Use a test disc, like Digital Video Essentials, that has 6.1 channels of band-limited noise to adjust the gains of the three surround channels to match the gains of the three front channels. This can be done in one of several ways depending on what you have available: adjust the individual output levels of the HD disc player; adjust the input sensitivity of your spare Dolby Pro Logic decoder or decoding receiver; adjust the output levels of your spare Dolby Pro Logic decoder or decoding receiver; or, adjust the input sensitivity of your newer EX-capable decoder or decoding receiver.

If your newer, EX-capable decoder allows the adjustment of each individual channel’s delay, set the center surround for the right range. (You should have set all the other channels for proper delay previously, when you first installed the decoder or receiver. If you hadn’t set those delays, read the manual and adjust the setup accordingly. It helps with sonic imaging.) Setting delays is the one potential pitfall of the scheme; some processors won’t compensate for propagation delays for analog input signals. Even so, the addition of the center surround may be worth it. (I’ll assume you have the installation and connection of the center surround speaker and any required external amplifier under control.)

That’s it. You now have duplicated very closely the EX dematrixing process. And if the installation of a center surround is new to you, I think you’ll be quite pleased with the results.

6.1 or 7.1

All my speakers are M&Ks. I happen to have three spare front satellites, so I used one of them for the center surround channel. THX recommends installing two speakers along the back wall (the left and right surrounds are, of course, located on the walls to either side of the viewers, not behind). I installed only one speaker. I want to avoid the comb filter effect that occurs when you listen to an identical signal from two speakers that are at different distances from one’s ears. (A more detailed discussion of acoustical comb filter effects may be found in my Mr. Blandings series). By using M&Ks with identical drivers all around, I achieved a good timbre match among all six loudspeakers.

Listening Impressions

My home theater audio system runs in the 6.1 mode by default. I don’t care if the disc is mixed for EX or not, pans and discrete sounds are still placed in the 180 degrees of the surround field properly… with one exception. If the surround channels are derived from an older 4.0 or 4.1 mix, the surround channel signals will be identical and the surround sound will collapse to the center surround. It’s only then that I set my audio system for the 5.1 mode. Note that this effect can occur whether you use this Pseudo-EX technique or you use a decoder or decoding receiver that generates the center surround signal internally.

When I first tried this technique in my home theater, I listened to a number of sequences from a variety of non-EX DVDs. Included were the conversation between Dragonheart’s Bowen and Draco as the dragon flies in a circle around the viewer, the hijacking sequence in Air Force One, the opening pullback in Contact, the opening sequence in Mask of Zorro in which the viewer is immersed in the crowd, and the guerilla camp firefight sequence in Predator (in an older release).

For those tracks that had discrete surround channels the effect was quite startling. Sounds that had previously panned through my head (the phantom image) as they moved between the surrounds now clearly imaged behind me. The rear soundstage was better defined, particularly for my theater’s off-center seats. Sounds were more evenly distributed across the sides and back of the room. Draco’s voice panned perfectly around the theater. Various radio and television transmissions in Contact moved to a variety of surround directions. The gunshots in Air Force One came from a variety of directions. And Zorro’s crowd seemed more realistic as I was immersed in the voices.

As I expected, the rear channels in Predator collapsed to the center surround. That Dolby Digital 5.1 mix was derived from the original Dolby theatrical soundtrack with a monaural surround channel, essentially a 4.1 presentation. In this case, the Pro Logic decoder correctly (electronically speaking) dumped all the sound from the left and right surround channels into the center surround. But by simply selecting a bypass or stereo mode on the Pro Logic decoder, it disabled the center channel output and restored a normal surround presentation.

Parting Thoughts

So there you have it. If you have the spare equipment lying around, you have nothing to lose by trying this technique to create a Pseudo-EX configuration of your own. It may work well for you or it may not. If you hear problems that diminish your enjoyment, put things back the way they were. Enjoy.