Disgression 4: Other Surround Options

 Other Surround Options

Dolby Stereo

With several modifications from the above "diamond", weobtain a much more effective plan, one that's at the heart of most motionpicture stereophony, from Fantasia's Fantasound, through theearly 50's CinemaScope films, andending up with DolbyStereo™. The Left and Right channels have been moved back to thefront. In this case they're rather closer together than you'd choose formusic, keeping in scale with the widest screen a motion picture would beprojected on in a room of these proportions. That constraint producesscreen-left and screen right as the widest positions. Everything else noton the screen is suggested by a monophonic "Surround" channel, played onas many speakers as you can manage. There are designs that avoid some ofthe "comb-filter effect" that playback of the same signal on multispeakers will introduce, and other ways to diffuse the signal so that itbecomes omniphonic, hard to locate, just a vague impression of sound fromthe sides and rear, without any accurate positional clues.

 Dolby Stereois not really multi-tracked. All the mix ends up on a standard stereopair, usually called: Lt and Rt. What is to be heardfrom the C channel will eventually end up as an identical signal at thesame phase in both Lt and Rt (the so-called "sum" signal). What is to beheard from the surround channel is mixed into Lt and Rt at the same level,but 180 degrees out of phase (the so-called "difference" signal). As longas only one or two sounds are to be located simultaneously at any givenmoment, a special circuit called "logic" steers what signal goes where,and reduces the crosstalk inherent in all "matrix" methods. More than twosounds at once, and you get a vague blur of sound all around. For theparticular purposes of film sound, especially when the engineers havemonitored through the matrix and can judge the final results, it can do areasonable job of suggesting a real four track experience. For music it'sa tradeoff, often a major one.

"Depth" Quad

No sooner had stereo been introduced to the masses in 1958,there were fools like me thinking about the next steps. Bert Whyte praised three channel stereo. Four-trackequipment came about more easily than three . The question arose: "where do you put the extra channels?.

The microphones are positioned in front of the sound sources in a similar diamond shaped pattern. The left and right channels are moved wider apart thanyou'd use with 2-tk stereo, and the center is filled not once but twice!. For playback you duplicate the positionings as you see here. If a person wereto walk about while speaking, in and around the microphones, there wouldbe an uncanny ability to judge exactly where s/he was at any moment if youlistened with this "Depth" Quad arrangement. It may not work over a verywide angle, it's certainly not as "surrounding" as some of the otherschemes here. But it is a charming way to duplicate a soundfield instartlingly realistic ways.

The close speaker would be best if mounted rather low, so the center distant track will notbe blocked. The mikes don't need the same finesse. It's effective todeepen the positions even more if you have the room. I first tried it witha deeper than wide arrangement, and that was pretty cool.

Diamond Surround Quad" -- a por old idea

Here's another "diamond" arrangement for four channels:Left-Side, Center, Right-Side and Rear (similar to what's called: LCRS). Offshoots of this one have been widely popular, as it isthe basis for the Dolby Stereo matrix that we've all enjoyed many times(Dolby carefully moves the sides up front). Electro-Voice was an earlyadvocate of the above, but this was before "logic steering" circuitssimulated full stereo separation. Sansui used a similar plan at the coreof their decent QS quadraphonic system's "Regular Matrix." But theyfinally adopted the much worse "obvious quad" layout scheme, in a rushback to the corners, feh. Most of the ill-fated quadra-phonies made thesame mistake, although they added logic circuits to help enhance thelimited separation (nothing filled in the big "holes"). CBS/Sony had aworse scheme called "SQ", which needs a bit more space to speak about, sowe'll put that tale on a related matrix-wars page HERE. There's additionaltech background on matrix surround systems HERE.

 Not many fourchannel systems stayed with this "diamond" plan. There were problems. Theangle between adjacent speakers is a rather unrealistic 90 degrees. Everhear stereo with the speakers that far apart? Yep, no "fusion" betweenthem, a hard to ignore "hole-in-the-middle", as it's usually called. Withthe "Diamond Quad" scheme you get four of those black holes, four largesectors in which no sound source seems to be located. One might place"filling in" speakers with fancy logic circuits that derive the best-guesssignals that would be expected when the actual channels are outputting aparticular pattern. Klipsch did this with his Heresy speakers to fill thelarge gap in the stereophony that two corner speakers caused. We spokeabout that earlier, and, yes, my center speaker is one of those Model-Hfor heresy designs: meant only for along-the-wall placement, and not thebass response of the bigger monsters.

But without four morespeakers to try to fill in the holes (eight in all!) this idea doesn'twork too well. Another problem is that when you face forward it'sdifficult to tell what's coming from exactly in front of you versusexactly behind you.

Tetrahedral Surround Plan A – 4 Channels

Now and again the suggestion of"Tetrahedral" channel placement arises, Phoenix like, from the ashes. Itusually goes like this: "Say, if we've got FOUR separate channels, why notcreate a 3-D solid of sound, using a FOUR-sided tetrahedron! We can placeone channel in each of the four corners of the 'hedron, use a microphonewith four directional elements aiming in each of the four directions, mount the speakers the same way. Then we can have sounds come from anydirection at all!" Great! Certainly is a lovely notion on paper. Exceptthere's something worrying here: a speaker in every corner. Haven't wealready seen that no matter how obvious an approach this is, it comes upas an argument with "holes" in it, to turn a phrase?

 If fourindependent channels are insufficient to cover a flat 360 degree plane, certainly there's little hope they can cover more than that, like aspherical 360 sound space. Good grief (you're right), there's no hope atall (they don't), it sounds lousy. A favored configuration is the above"Plan A" (from Outer Space...? ;^). Note how the LF speaker is located down on the floor, then the next channel, RF is mounted up high in itscorner, and around we go, down, up. Neat, huh? Compare the result with thefolly of "Obvious Quad". Only now the angle between each speaker is more than 110 degrees. You liked thebig holes in the middle with 90 degree spacing, you're gonna love it --nearly 120 degrees of pure emptiness! We've destroyed what little "fusion"there existed before in front, as the pair along any wall must span thefull diagonal length of that wall. No surprise to find black holes allover the place.

 

Evenso, there are somebenefits to record with suchfour-element microphones, like the famous Calrec. By matrix manipulationsof the sum and difference type we can "extract" the equivalent of adirectional mike aimed in any spherical direction. You can capture anevent with many recording channels, four per soundfield mike, and thenlater trim and fine-tune the mike aiming points. No, you can't effectivelyreposition the mikes, but it still is a most flexible scheme of eventcapture. If you have enough channels of monitoring, perhaps eight or more (Octophonic Sound, anyone?), and place these into a more modestconfiguration, you might be able to come up with a workable soundspace ofenvironmental sound.

Tetrahedral Surround Plan B – 4 Channels

But if you're stuck with only four channels forreproduction, there's not much more you can do about the up-down, or"third axis". Here's another scheme, Plan B, above which tries to squarethe circle, trisect the angle, invent perpetual motion, and on down tooblivion. Is it just me, or isn't this one kinda nervous making? I mean,would you mind having a large loudspeaker suspended right over your head,aiming down at you? Great for "the voice of God" effects! Of course one ofthe six channels in IMAX theaters does exactly this. At least they havefive other tracks, so the main expanse of the screen is better handledthat the above plan, with only three channels left to define 360 degrees.Yes, that's gonna lead to more of those blackholes, who says we haven'tdiscovered "all the missing dark matter" in the Universe?

In any case, I've put the cart in front of thehorse here. Our hearing apparatus is very weak at detecting up-downmovement and locations. I could have added another experiment to try in Digression II above. It'seasy enough to do. With your two channel stereo turn your head over to theside by 90 degrees, one ear aiming down, one up. Now listen to the twospeakers, one effectively "above" your head, the other "below". What'swrong with this picture? Do you hear much separation? Close your eyes andlisten carefully. Play some "ping-pong" stereo material, or have a friendrotate the balance or level controls so the sound definitely moves backand forth between the speakers. How's it sound? Straighten up and compare.Unless you do have an extra ear on top of your head, I suspect you'll comeaway from this a little less excited by the prospects of 3-D sphericalsurround sound. I was. The test here works better outdoors, where thereare no clues from reflections on walls or ceiling. More honest test thatway, unless you have an anechoic chamber handy.

 The othersuggestion for a test, with a tiny noisemaker, a "cricket "or "clicker"should be repeated here. Have the sounds be moved from below to above atthe same left-right angle. See what differences can be heard. Try itoutdoors. Compare with front rear motion or arbitrary jumps, and near tofar motion and jumps. We have to be sure about what we can easily detectand what we can't. Our eyes will deceive us, both ways: we can hear"phantom center" sounds between two speakers equidistant from us, our eyestell us the space is empty. Our eyes see speakers above and below, but ourears are not so sure it's mostly guesswork. It's by coming up withconcepts that look good to the eye we blunder both ways. We come up withplans that can't be heard well, and never consider the plans our ears willreally enjoy. It's another case of how easily we can fool ourselves, especially if we've invested a lot of time and money in an idea resting onacoustic folly. Please trust your ears as you navigate these rockynarrows. Do everything "double-blind", with verification by others whoseem to have excellent hearing. Find out what works for you, in any case, even when the lights are out, and it's every ear for itself...