SPOTLIGHT ON "PSEUDO EX" by Dan Ramer

When 5.1 Simply Isn't Enough

I'm confident that you're aware that 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. When I first read a description of the EX audio enhancement, I was intrigued. The left and right surround channels are supplemented with a surround back channel that is matrix-encoded into the left and right surround channels. During playback the surround back channel is extracted using what is essentially Dolby Pro Logic Surround decoding of the left and right surround channels. The recovered back surround channel is then amplified and sent to the surround back speakers. How logical. How elegantly simple.

Now, this technology has arrived for the home with the introduction of THX Surround EX decoders and the first DVD titles encoded with this new audio feature: Austin Powers: The Spy Who Shagged Me, arriving in stores on November 16th; and, The Haunting, arriving on November 23rd. It occurred to me that I still had my trusty old Shure HTS-5300 analog Dolby Surround decoder complete with Pro Logic type steering logic, retired when I acquired my B&K digital decoder/preamp in '97. I should be able to extract the surround back channel information from THX Surround EX encoded DVDs (and possibly non-EX encoded fully discrete 5.1 DVDs) with my existing equipment. Cool.

Sharp-eyed readers will have spotted the rack-mounted Shure in the photographs that accompanied my home theater epic, Mr. Blandings Builds His Dream Theater. But laziness prevailed until Shane, our busy Tech Editor, published his THX EX piece. We began a spirited exchange about the wisdom of a do-it-yourself Pseudo EX configuration. My laziness vaporized in a bright burst of curiosity. I got to work.

How Does Dolby Surround Work?

Simply stated, if the sound mixer wants to place a sound in the center channel, he would encode 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, these in-phase, equal amplitude signals are steered into the center channel. Dolby Pro Logic adds signal processing that enhances separation. (For a more comprehensive explanation, see the Appendix.)

Not all decoders perform equally. Better-executed designs maintain the front soundstage despite a strong signal in the center. Poorly designed decoders may suffer from an audible collapse of the front soundstage to the center channel when the center channel is active at the same time as when there are signals on the left and right channels. In other words, in a poor decoder implementation, signals intended to remain in the left and right channels can be erroneously steered to the center channel, essentially collapsing the stereo image. Fortunately for me, my Shure does not exhibit a tendency to collapse the soundstage.

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 create sound effects that appear to be 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 surround back 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 channel. So it should be possible to extract surround back channel information when Dolby Surround processing is applied to the discrete left and right surround channels of a 5.1 mix. And as I quickly discovered, that's exactly what my Shure decoder can do.

I connected the analog surround channel outputs from my B&K AVP-3090 digital decoder/preamp to the stereo inputs of my Shure HTS-5300 analog Dolby Surround decoder. The front left output and front right output from the Shure were connected to my existing surround channel amplifiers. The center channel output from the Shure was connected to a spare amplifier to drive the Surround back channel. I took special care that the channels were properly connected (don't be concerned if your "front channel processing" of the surround channels sounds reversed with incorrect pans; just reverse the analog input connections to make it right).

All my speakers are M&Ks. I happen to have three spare front satellites, so I used one of them for the surround back channel. THX recommends installing two speakers along the back wall for the back surrounds (the normal surrounds are, of course, located on the walls to either side of the listeners, not behind). I installed only one speaker. I was trying to avoid the comb filter effect that occurs when you listen to a monaural signal from two speakers that are at different distances from one's ears. If the two path lengths differ by any more than a few inches, I'll opt for a single loudspeaker to reproduce a monaural source (for a more detailed discussion of acoustical comb filter effects, see my Mr. Blandings piece). By using M&Ks with identical drivers all around, I achieved a good timbre match among all six loudspeakers. I then balanced the system with the phase test bands on the Avia Guide to Home Theater DVD. I made this seemingly unusual choice because when the surround channels in the 5.1 mix are in phase, the Shure routes the audio to the surround back speaker. When the surround channels in a 5.1 mix are out-of-phase, the Shure routes the audio to the normal surround speakers.

Be Prepared for Ridicule

Now, before I describe how well it worked, I have to be fair and explain that film industry audio professionals take an extremely dim view of these shenanigans when applied to non EX-encoded material. Some of their concern is related to the potential for misbehaving Dolby Surround decoders to collapse the soundstage (in this case the rear) to the back surround speaker. Additional concerns seem to be related to decoding errors that would create audible effects that are either unnatural or unintended by the director or sound designer.

DVD audio was the focus of a recent Panel Discussion sponsored by the Cinema Audio Society and chaired by Tomlinson Holman. On the panel were: Jerald Cole, Manager of Digital Compression Ops of the California Video Center; Phil Jamtaas, a DVD Mixer from Warner Hollywood; John Kellogg, from Dolby Labs; Steve Thompson, Pacific Ocean Post, Managing Director of the Cinram/POP DVD Center; Larry Walsh, DVD Mixer at Universal; and, Robert Winder, Vice President at Post Production Services at Warner Hollywood. Serious guys. Here's a brief excerpt (available materials don't specify the source of the quotes in this exchange):

"Going through the consumer home theater magazines and through the newsgroups, I see a lot of people advocating... a pro logic receiver, let's throw them on the surrounds and see what we get. Are you guys [aware of] that stuff? I mean, is that just a foregone conclusion that people are putting pro logic on their surrounds are going to end up with all kinds of weird stuff?"

"Yeah, if we find them, we shoot them. No." (Laughter)

"That has been kind of going on actually since I think the first evidence that we found of it, at least on the Internet newsgroups was back in about '95. People were buying, you know, integrated home theater receivers. And, you know, one guy gets on and says: Hey, I took that old pro logic decoder I had that I bought eight years ago. And I put it on the surround channels with the DVD and put an extra speaker up, and something happened, you know. It's not anything anybody advocates. You can do that."

(Unintelligible?)...(Laughter)

"You know, some of the home theater people up on the Internet are doing all kinds of wacky stuff. And, you know, when some of these EX encoded sound tracks - you know, things like "Austin Powers" and "Phantom Menace," I mean, we don't know when that will ever come out on DVD, but there is a number of titles that are going to have - essentially, they've got a matrix encoded surround channels that are - you know, when you play them in just a normal 5.1 mode they sound just like they sound. They're fine. And then, you know, if have this extra matrix decoding, EX decoder, and you put it on your surround channels and put an extra speaker in the rear of the room, you essentially get this extra channels of stuff. It's like a center channel in the rear. You know, there's a number of our licensees and THX licensees that are all, of course, racing to make this, you know, the next big thing. And like you're right, I mean, people have been strapping pro logic decoders and putting them in Dolby three stereo mode for some time. And I think once we're in the realm of the consumers living room, they're free to do whatever they choose. If they think it's fun, great, you know."

"What do you think we should do?"

"Well, I'm just questioning whether or not, I mean, how much that stuff is being monitored to find out whether you're getting artifacts and stuff that..."

"Well, if it's not encoded that way, you'll definitely get something..."

"It's definitely artifacts."

"It's an artifact, you know. But whether the consumer thinks that's fun or not is, I guess, up to them."

Ouch. Wacky? Artifacts? Now, as an electronic engineer and acoustician, I have to tell you that this makes me a little uncomfortable, particularly if I'm going to describe something to you that may cause more harm than good. (And besides, I don't want the CAS Police to burst into my home theater and shoot me.)

Listening Impressions

Satisfied that channel levels were all correctly balanced, I listened to a number of sequences from a variety of DVDs. Included were the conversation between Dragonheart's Bowen and Draco as the dragon flies around the theater, the highjacking 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 which the surround channels are monaural).

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 the off-center seats. Sounds were more evenly distributed across the sides and back of the room. Draco tracked 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 it surrounded me with voices.

As I expected, the surround channels in Predator collapsed to the Surround back channel. This DD 5.1 mix was derived from the original Dolby theatrical soundtrack with a monaural surround channel; the resulting "4.1" presentation fed monaural signals into the surround channels. In this case, the Shure correctly (electronically speaking) dumped all the sound from the left and right surround channels into the surround back channel. Simply selecting the Shure's Stereo mode, rather than the Dolby Surround mode, and disabling the center channel restored the normal surround presentation. This is a bit of a bother. You have to keep track of whether or not the 5.1 mix on any particular DVD contains a monaural surround program, and set the mode accordingly (another good reason to read our DVD reviews).

I also identified a potential "artifact" on a non-EX encoded, totally discrete Dolby Digital 5.1 DVD. If the sound designer wants to broaden the score's soundstage, he might mix a little of the score's right front channel into the right surround and a little of the score's left front channel into the left surround. Because the orchestra's instruments are arrayed continuously across the front of the soundstage, instruments that are located in the center of the orchestra might get mixed equally into the surround channels. When this occurs, an analog surround processor in a Pseudo EX configuration will place those centered instruments in the surround back channel. I heard such a Dolby Surround matrix decoding artifact as I watched The Rock for a recent retrospective review. The drum line could be heard in the center front and occasionally at a lower level in the surround back channel, subtle but audible. You'll have to judge for yourself whether this kind of intrusion is acceptable.

There's another disadvantage to this configuration. There's no way to adjust the time delay for the surround back channel. In my theater, the surround back speaker is closer to the listeners than the left and right surround speakers at the sides of the listening area. I would have preferred to lengthen the delay to the surround back channel. I could not.

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 THX Surround EX" configuration of your own. You might save yourself some money on a THX Surround EX upgrade, and you may enjoy listening to some non-EX encoded material with this arrangement. 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. Decide for yourself. So far, my impressions are mostly positive. I'll continue my critical listening before I decide whether or not to upgrade my system to true THX Surround EX. And I'll be back with my EX impressions of The Spy Who Shagged Me and a full review of The Haunting.

Appendix

Dolby Surround 101 - The Matrix

Clearly, the concept is to store additional audio channels on two conventional stereo channels. The technique is to "matrix" the audio channels, a concept originally conceived and patented by Peter Scheiber and subsequently applied by Dolby Laboratories. Simply stated, if the sound mixer wants to place a sound in the front center speaker, he would add that signal at equal amplitude and equal phase to both the left and right stereo channels. If the sound mixer wants to place a sound in the surround speakers, he would add that signal to the left stereo channel shifted by 90 degrees, and add the same signal to the right channel shifted by -90 degrees (so the surround information is of equal amplitude and 180 degrees apart - inverted - on the stereo channels). When such a matrixed stereo program is passed through a Dolby decoder, the sum of the left and right becomes the center. (The center is needed to position certain sounds at the center of the screen for off-center viewers, dialog in particular.) The difference between the left and right channels becomes the rear. I'll remind you that Dolby Surround channels are indeed monaural. The disadvantage of this passive decoding system is that separation between adjacent channels (L-C, R-C, R-S, or L-S) is only 3 dB.

Dolby Surround 102 - Pro Logic

Dolby Pro Logic enhances Dolby Surround by using a number of techniques to increase the separation between adjacent channels. Cancellation subtracts the left channel from the right and subtracts the right channel from the left. This has the effect of greatly reducing sounds intended to be directed to the center speaker in the left and right channels; signals that are common to both left and right are removed. Pro Logic controls this process by detecting sound dominance and its direction. If a sound is found to be significantly louder in any one direction such that it exceeds a predefined threshold, a fast acting voltage controlled amplifier enhances the dominant signal. These techniques may yield adjacent channel separations in excess of 30 dB for static test signals, but may exhibit problems for dynamic program material. (For the sake of this discussion, I'll omit descriptions of such manipulations as phase and pitch shifts to diffuse the surround sound, rear channel bandwidth limitations, and noise reduction - these processes are not relevant to front channel processing of Dolby Digital surround channel information.)

Dolby Surround 103 - Decoders

Early decoders, such as my Shure, were designed with discrete components. As Dolby licensed and approved integrated circuits (ICs) became available, almost all manufacturers used those inexpensive analog chips; exceptions are digital executions of Dolby Pro Logic decoding. Not all decoders perform equally. Better-executed designs maintain the front soundstage despite a strong signal in the center. Poorly designed decoders may suffer from an audible collapse of the front soundstage as the center becomes active. (This may be one source of the panelists' disdain.)

Surround 104 - Alternatives

A less well-known decoding process called Circle Surround applies the same matrix decoding process but enhances separation with active gain manipulation of each channel. Their technique is claimed to eliminate the adverse audible effects of gain pumping by dividing the audible spectrum into various bands and applying faster acting amplifiers to higher frequency bands. There has been some speculation that this technique might be less audibly objectionable for Pseudo EX than Dolby Pro Logic processing. I'll leave it to those who may own such processor to evaluate it for themselves.

Surround 105 - EX Digital Flags

I understand that there's going to be an EX flag embedded in the Dolby Digital bitstream to notify authentic THX EX digital decoders that it should turn on the appropriate processing. But that flag will not be present for normal AC-3 5.1 mixes, nor is not expected to be present for early EX DVD releases, like The Haunting and The Spy Who Shagged Me, each of which had theatrical EX mixes. And I can't predict whether all commercial EX decoders will allow you to override the decoding mode based on the embedded flag to manually select or defeat EX processing. One would hope that you can so you will be able to capture EX encoded soundtracks like the two mentioned above (and experiment with non-EX encoded DVDs).