Surround Sound: High Tech Meets the Beast Within Us Page 2

Once the surround signal is mixed into the stereo signals, it can never be completely separated out again. Instead of appearing behind you, surround effects might appear some where between where you're sitting and where your front speakers are, and sounds that belong in front will tend to "leak" into the surrounds. To improve the front-to-back separation, Dolby Stereo uses what's called logic steering. (The home version is called Dolby Surround.)

Logic steering measures the signal content in all three channels, compares each one with the others, and "decides" on the basis of their relationships whether the sound belongs in the right, left, or rear channel. Special circuits then subtract the unwanted energy from each channel, to steer each signal exclusively to the speaker it should be coming from. The result is (usually) what sounds like total channel isolation.

No such processing is needed for the subwoofer. Because bass energy tends to be more or less identical in the two front channels, it is simply drawn off from them, filtered to make sure bass is all that's there, and sent to the subwoofers.

Center Channel Stabilizes Soundstage
Larger home theater systems also have a center-channel speaker. This isn't necessary iflisteners will be sitting about midway between the front speakers. But when the seating area is wide, people at the sides will hear sounds off-center, so that an on-screen speaking voice will be heard to come from one side of the screen. A center-channel speaker stabilizes the "soundstage," keeping on-screen sounds where they belong.

The center signal too is a mixture of the front left and right stereo signals, but if it is loud enough to stabilize the image properly, it will have another detrimental effect: It will impair the separation between the front speakers, making them sound as if they're too close together for decent stereo. Again, logic steering is used to cancel the stereo information from the center channel, preserving the full width of the original stereo "stage."

The term "Dolby Surround" applies to all consumer surround decoders that extract a matrixed surround signal, including those that don't use logic steering. "Pro Logic" is the deluxe version of Dolby Surround, and it includes the steering plus a built-in test signal to facilitate balancing-out the volume levels of the various channels. A few years ago, George Lucas of Star Wars fame launched a movie theater quality-assurance program called THX, which was intended to ensure high standards for sound and picture quality. More recently, THX came to home theaters.

THX isn't a product, a system, or a brand name. It is nothing more than a set of technical specifications that define minimum standards of acceptable audio or video performance. The idea was that, since so many directors sweat blood to get good sound into their films, it would be really neat if consumers were able to hear what all the fuss was about. THX gives them a better than even chance of doing this. Home theater products that are developed, in consulta tion with Lucasfilm, to meet or surpass the THX standards, receive certification that allows them to wear the THX logo on the front panel and feature it in their advertisements.

THX Reflects Concern for High Standards
The standards are fairly stringent but not unreasonably so. Most of them just reflect traditional pro-audio values about such things as frequency response smoothness and range, distortion, available power, and loudspeaker sensivity. Others pertain to the way the loudspeakers cover the listening area; still others are innovations specific to the home listening environment. For example, Home THX mandates the use of something called decorrelation, which expands the non-stereo surround signal so as to provide an illusion of real space, like that in a movie theater.

THX also requires the surround speakers to have a "figure-8" radiation pattern, which directs the sound towards the front and back of the room. This makes the whole room a spatial surround field, but puts the listener in a "null zone" to the side of the speakers so he won't be able to tell where their sound is coming from.

THX ensures that all certified products are more or less interchangeable. Although some do sound better than others, you can mix and match different brands knowing that they are at least mutually compatible and suitable for the job—something you cannot take for granted with non-THX components. (An incompatible combination might be a low-powered amplifier and insensitive loudspeakers, which won't be able to deliver enough volume for satisfying movie listening.)

On the other hand, some manufacturers who elect not to pay THX royalties, or whose products may not qualify for certification, nonetheless incorporate some aspects of THX (such as front-and-back-firing surround speakers) in their designs, and they too are worthy of consideration. Some offer what is essentially THX performance at way-below-THX prices.

Should you buy THX or Dolby Pro Logic? The choice isn't that clear-cut, because Pro Logic is the foundation THX was built on. Pro Logic separates out the multi-channel signals; THX just determines to a great extent how they will sound. If you don't care for that sound, you don't have to buy a Pro Logic decoder with THX provisions. Many decoders don't offer the THX option, and all the ones that do offer it also allow you to switch it off.

Raindrops, Noisy Crowds & Closing Doors
The only real choice with respect to surround speakers, other than basic sound quality, is whether or not to go with THX's figure-8 surround speakers, because most decoders don't allow you to operate these as "point-source" speakers. Generally, the figure-8s do a better job of reproducing room sound and ambient surround effects like rain and crowd noises, while point-source speakers, which radiate from the front only, can provide more natural reproduction of localized sound effects like a voice or a closing door. If you think the difference will be important to you, audition both before deciding.

While home theater stresses movie sound, a surround system can also greatly enhance music-only reproduction. Some decoders have the ability to add artificial room sound, to simulate the soundspace of a large hall, a cathedral, a stadium, or a small room. These may be useful if the record ing itself has little or none of its own room sound, but when the recording has all the reverberation it needs (which is usually the case), adding more will make the sound too reverberant and distant, and will obscure a lot of detail in the sound. Generally, it's far better to extract the room sound that's on the recording and feed that to the surrounds, and this can be done with any Dolby Surround decoder. The improvement can be dramatic.

If the recorded reverberation was real rather than electronically synthesized, what you hear from the surrounds is the original recording space. No longer are you sitting on the outside and listening to the performance through a wide window; you are inside, where it's all happening. Large performing spaces such as concert halls and opera houses often have their own unique and recognizable "sound," and this can be surprisingly well reproduced by a surround system.

There are other benefits, too. Individual instruments have more immediacy and more body, as if they've taken on an almost palpable third-dimensionality. And many rock recordings are a revelation in surround, with all sorts of spatial effects that are only suggested through two-channel repro duction. Things whiz past, zoom around you, and pop up here and then there. Realistic? No, but lots of fun.

Trouble For Pro Logic
Not all recordings reproduce well in Pro Logic surround, though. Ones that are not specifically encoded with a surround channel can confuse the logic steering, causing some sounds to hop randomly around the room when they shouldn't, and producing a "pumping" effect that sounds a like severe distortion. For this reason, most surround decoders have at least one so-called music surround mode that has rather more benign steering characteristics, or no front/back steering at all.

A few years ago Dolby Labs developed AC-3, a digital "bit-reduction" system that allows vast amounts of digital audio to be stored in a relatively small space. First used for multi-channel movie sound, it is now being considered as a means for delivering six completely independent sound channels to consumer's home. This will overcome the remaining weaknesses of Dolby Pro Logic, such as its tendency to become occasionally confused and its inability to reproduce several different sounds from different directions at the same time.

Pioneer has already demonstrated a working prototype laserdisc player with AC-3 sound, and the system has been adopted for use with tomorrow's high-definition television systems.

There are still questions about AC-3's possible detrimental effect on sound quality. Only a few people outside of Dolby Labs have heard a direct before-and-after comparison between an original recording and its AC-3 reduction, and they weren't necessarily impressed. (Movie patrons have generally been pleased with what they hear from it.) It is certain, though, that some system, if not AC-3, will be bringing us uncompromised multi-channel surround sound within a few years. Personally, I can't wait.

Kal Rubinson's picture

All that is so much more complex than modern discrete multichannel and, at the same time, so much less efficient or successful in creating convincing surround. It takes the increased data bandwidth that we enjoy today to make real multichannel possible.

eriks's picture

Hey Kal,

I have a background in this subject as I worked for one of Dolby's competitors when Dolby Surround was still being used.

Honestly, the recorded information on Dolby Surround film was _really_ good. The issues with it was not the lack of discrete channels, or bandwidth but rather the steering matrix was far too interested in effects, and how noisy Dolby processors at the time were. I got to listen to prototype discrete decoders that you could adjust this on and let me tell you, the immersion factor could be glorious by just dialing all that down and letting the tracks talk without being constantly micromanaged by the steering matrix.

I'm not saying it's as good as multi-track magnetic, or modern discrete digital tracks. I'm just saying that there was a lot more sound and music in those tracks than most people realize. That it sounded so campy was a choice made by Dolby which recording engineers could not get around.



Kal Rubinson's picture

I will not dispute that but the result, as seen in the open market, was that Dolby Surround had negligible and transient effect on music recordings.

Prior to the appearance of discrete multichannel, I played around with a Meridian system and the experience established my interest in "surround sound" but it was not sufficient to get me to commit to it. Later discrete, albeit lossy, media were a move in the right direction. However, it was not until the arrival of discrete, lossless formats that significant numbers of music recordings became available to justify (for me and others) rebuilding my system from stereo to multichannel.

eriks's picture

If you mean, should you use Dolby Surround for music, I have to agree with you. It was always a system centered on effects, not transparency. I only meant to bemoan how little of the sound on the tracks we would hear because of it.

Have you heard the Neo6 modes for 2 channel music though?

If you ever notice that instruments are brighter at the sides, but less so when in the center, Neo6 fixes this, with none of the downsides of Dolby Surround.

Kal Rubinson's picture

Thanks but I have no real interest in movie sound or HT. In what is probably the reverse of the experience of many, my HT setup is the by-product of my multichannel music interests.

eriks's picture

I understand, but do try Neo6 Music mode for some 2 channel listening. :) It may help you use your center channel more often.

Bogolu Haranath's picture

Have you tried or know about DTS headphone X? ...... Is it supposed to make binaural sound to multi channel surround sound? .........

Kal Rubinson's picture

eriks said:
I understand, but do try Neo6 Music mode for some 2 channel listening. :) It may help you use your center channel more often.

I have but it didn't stick (i.e., not good enough for continued use. OTOH, I do use Auro 2D/3D occasionally.

Bogolu Haranath's picture

My question was directed to eriks ........ Apparently there was a mix up ....... If eriks is reading this, may be he could tell us some thing about DTS headphone X :-) ............

eriks's picture

As you may know, in between Dolby Surround and Dolby Digital there where discrete magnetic tracks for film, both 35mm and 70mm. Not many movies were released in this format, as it required new equipment to read it and the oxide would wear off, and it produced a lot of dust.

The major benefits were noise, extension but also they did away with the DS matrix, so this format was used for musically oriented films like Yentl, The Sound of Music, Amadeus, but also the occasional weird one like The Natural and Caligula. Maybe we should get a BD of Caligula and see what extraordinary musical content they had? :D

tonykaz's picture

...came closest ( am I wrong ? ) to describing a Sonic Holidome experience of replicating "being there", using available gear, making home audio hobbyist's participation a possibility, for the first time that I'm aware of.

Otherwise, isn't 5.1 and it's variations a "sound effects" gimmick?

Mr.KR is the first Serious Audiophile to attempt elevating the genre ( as far as I'm aware of) , there must be something there to enjoy, something otherwise missing.

Sitting in Central Park NY and hearing sounds from all around me, I don't know if I'd like to bring that Sonic Landscape into my home but having the option is thought provoking.

It might even be ADDICTIVE. Phew!!

I'm stay'n tuned.

Thanks for all the reporting on this.

Tony in Michigan

Kal Rubinson's picture

Tony said: Sitting in Central Park NY and hearing sounds from all around me.......

Really? I'm only 4 blocks away!

tonykaz's picture

I was using Central Park figuratively, as a visualization tool, because it has Central as part of the meaning I'm struggling to convey.

I've stayed at the Mark Hotel on the edge of the Park.

I rather love Central Park, NY. if I'm in NY.

Tony in Ice Storming Michigan

Kal Rubinson's picture

Tony said: I've stayed at the Mark Hotel on the edge of the Park.

High tea at the Mark. A delight.

Robin Landseadel's picture

Best "surround" experience—standing the middle of the Berkeley Symphony Orchestra during a rehearsal of Olivier Messiaen's "Éclairs sur l'Au-Dela" during a passage where the woodwinds were performing birdsong—an orchestral aviary.

ednazarko's picture

I was a professional brass player - mostly trombones and tubas - for years, and I really miss how music sounds from inside. Orchestras, where because I sat in the back, my soundstage was reversed from the audience's soundstage. And big bands, where I loved sitting inside back row. The "mix" I remember is also quite different from what comes on recorded music. With classical music I always want the winds mixed louder.

The simulated surround sound function on my pre-pro can get me close to happy from my normal listening spot. I think my taste for dipole plus cardioid speakers (Gradient Revolutions), or open baffle speakers is because the sound envelops more. I find myself dragging a chair in quite close for stereo, or smack in the middle for surround.

I've had some absolutely spooky surround sound experiences with binaural recordings and open back headphones. First time I listened to one album I startled a few times (without thinking whipped around to look) because I heard someone sniffing who seemed precisely placed behind me. I have a handful of binaural recordings, that through the right headphones, beat my 5.1 system for that spooky feeling of being present.

Bogolu Haranath's picture

Some of us also listen to headphones, both closed back and open back ......... May be you could mention few of those binaural recordings? ....... Some of us may be interested :-) ........

ednazarko's picture

Chesky Records (he owns HD Tracks) does a LOT of binaural. Search and find. I recommend Wyclyffe Gordon "Dreams of New Orleans", Amber Rubarth's "Sessions from the 17th Ward", Macy Gray's "Stripped"... and Dunun Kann, drummers from Guinea.

The first binaural recording I ever heard was a poetry reading (via B&W record release.) I had my CIEMs in, on a plane waiting to take off, and that's when I had my near-neck-breaking moment of "someone's got a cold right behind me." Seats in the next two rows were empty... and I had CIEMs on so I'd have never heard that... then it happened again, and "behind me" was the fuselage and window. The effect is there with any headphone, but for me is strongest on open backed.

Chesky records in interesting places - like a couple of churches with great acoustics, so the sense of space is massive, unlike how headphones can present quite a small image. The knock on binaural is that they don't sound great through speakers, but I don't find that to be true. The space and aliveness on Macy Gray's album is just as spooky through speakers.

dc_bruce's picture

stuck on the wall behind you.

It's easy to dismiss these, but Bose was/is on to the central problem of multichannel sound, whether accompanied by video or not. That is, what do you do with all that "stuff" in the room?

Perhaps one reason for the decline of "specialist audio" is its increasing demands on the listening room.

Consider, in the "hi-fi" era, mono reproduction required only one speaker that, perhaps, could be shoved off into an unused corner. And in the early stereo era, before we got all excited about "imaging," "soundstage" and the like, many speakers were just shoved into a bookshelf, or stood on the floor (in a corner!).

Today, it's just about impossible to have a "serious" stereo that doesn't dominate the room, like a sacrificial altar to the audio gods. Even small speakers that used to be called "bookshelf" have been re-labelled "stand mount" speakers that demand to be set on expensive stands, well out into the room.

I think a lot of us, who don't want the stereo to be relegated to the basement and who don't want to cut ourselves off from our family, have come to accept some sonic compromises. And, while the sonic benefits of multichannel are indisputable, for lots of folks those benefits don't outweigh the costs -- financial and otherwise.

ednazarko's picture

My 5.1 system is nowhere near set up the way I'd like, but I was told that I could only have the rear speakers where I wanted them if I wanted a different wife. Sigh. Still, it's good enough for movies, and for the occasional 5.1 mixed album. When she's got a business trip I pull the speakers out to where I want them and binge-listen.

cozmoz's picture

Wonderful details, but the only thing I could not understand the relation between bone conduction and surround sound. How we are going to merge both? One is physical and other one is more extrinsic in nature.

cozmoz's picture

Wonderful details, but the only thing I could not understand the relation between bone conduction and surround sound. How we are going to merge both? One is physical and other one is more extrinsic in nature.