Derived Center Channel

[Klewless]

Current issue of Stereophile has a discussion of 2 channel vs multi-channel, which I found very interesting. The statement was made that even in the early days of stereo development 3-channel was shown to be superior to 2-channel.

My question to this forum:

Is using today's 2-channel format to sum the L/R to get C REALLY equivalent to an encoded 3-channel (L/C/R) ????

My very limited knowledge of encoding leads me to believe that:

L=L + some C

R=R + some C

Summing them then gives you C. But if you play back the result then you'll have:

L + some C

derived C

R + some C

Could someone who really understands the coding standard help explain this? My instincts tell me that to get the original intended 3-channel one whould have to derive the center and then remove the "center component" from the left and right signals.

Thanks in advance!

------------------

John P

St Paul, MN

[Ray Garrison]

No, summing the L/R signal to get a derived center signal is not the same thing as recording the source using three separate microphones to record separate L/C/R channels.

Recording the channels separately captures the timing differences between the three mics. Combining two channels to produce a derived center does not. Simple thought experiment, using completely arbitrary numbers...

Assume we are recording a small ensemble (so we can talk about the distance between the source and the mics without getting into problems with multiple different distances depending upon where on the stage the performer and instruments are). We have three mics; left, center and right. The mics are on a plane 5 feet in front of the band. The left and right mics are set 24 feet apart, so each is 12 feet to one side of the center mic. Simple trigonometry tells us that the distance from the group to the left mic is 13 feet, the right mic is 13 feet, and the center mic is, of course, 5 feet away. There will be a time difference of 0.01038961 seconds between the arrival of a signal at the left or right mics, and the center mic. This is a 180 degree phase difference for a 200 cycle tone. If you record using only the left and right channels, then sum to mono for the center, a 200 cycle tone (bit more than an octave below middle "c" on a piano) will be 180 degrees out of phase compared to that same channel using a dedicated center mic. The ear uses phase shifts (among other things) to identify the spacial relationships between different sound sources.

So, in a nutshell, it'll sound different. I don't know enough about psychoacoustics to even begin to guess what would happen, but it'll sound different. Laypersons opinion - the depth of stage perceived on the recording would be altered.

Keep in mind that things become rapidly more complex with larger, and more spread out, sources. But the priniciple is the same.

Ray, always an expert at providing over-engineered solutions to simple questions.

------------------

Music is art

Audio is engineering

[Klewless]

Ray,

Thanks for the reply. It makes sense. So what does that say about deriving a center channel from current stereo sources?

Anything useful to be gained by it?

Does that mean that the new multi-channel (as opposed to Home Theater) is the only real way to get a valid center channel. I do understand that the ultimate goal of Home Theater and music reproduction may be different. Then of course we would be at the mercy of the recording engineers (as if we weren't already in that position!) to generate "good" stereo?

------------------

John P

St Paul, MN

[jdm56]

If I may butt in here...PWK originally recommended a derived center channel for use between a pair of klipschorns if the 'horns were in a room which placed them far enough apart relative to the listening position to create a "hole" in the front soundstage. Or at least that is my understanding of it. The beauty of this set-up is it allows for more proper reproduction of different sized (width) acoustics, even with 2-channel material. A recording of a soloist should sound appropriately small, and an orchestra should sound appropriately wide. The acoustic width accuracy won't be as good as it could be with three discrete front channels, but still a lot better than conventionally placed 2-speaker stereo.

------------------

JDMcCall

[Stu Pidass]

James:

I believe your correct about PWK and teh centre channel. I understand that's what he originally developed the Corwall for and that might explain why the Cornwall uses the same drivers as the KHorn.

[Stu Pidass]

Geez. looks like I need a stell specker

[Ray Garrison]

Okay, we actually have two separate questions here.

Question 1: Is there a difference between a derived center channel and a dedicated, separately encoded center channel?

Answer 1: Yes. And, a dedicated center channel is better. See my previous post.

Question 2: Is SOME center channel, whether derived or dedicated, better than NO center channel?

Answer 2: Most folks say yes. Both John and James are correct - PWK was (is?) a BIG proponent of a center channel. Also, many DSP units, notably Meridian and Lexicon, have a three channel mode with a derived center channel. I have never read a single review in which the reviewer wasn't gushing all over about how much better stereo recordings sounded using the three channel mode - John Atkinson, who tends to be somewhat against phony derived multi-channel stuff, was falling all over himself about how good Meridian's system sounded, as I recall...

Ray

------------------

Music is art

Audio is engineering

[WMcD]

Dear Klewless (are you a sail person?)

There is a long appreciation of derived center channel from two channel recording.

There is also a long appreciation of discrete center channel where there are three recording, storage, transmission channels.

Consider that if you make a two channel recording with two spaced mikes in front of an orchestra, each channel is going to have common, in phase, information coming from the center location of the orchestra.

An alternate way of doing this is to have a third center mike, recording in a third channel. But perhaps most of that information is already in the L and R. That information can be exploited.

Also, in the old days, two channel was a major step from single channel mono recording. Three channel recording was not impossible, just another layer of complexity and could only be done on tape. The stereo LP was a breakthrough. It was a mechanical technique of modulating each edge of the record groove. It wasn't going to be possible to get another, third, channel without a lot of cranky multiplexing tricks. E.g. Quad.

Derived center channel playback is also called "phantom channel". Phantom because it was not a discrete recored channel. By using a few resistors as a voltage adder, it was possible to add L + R from a pre amp output and send it to another amp. Then feed the amp to a center channel speaker. A type of analog processor which we find something similar to in our HT receivers.

Question 1. Doesn't this resistor based processor lead to bleed thought of L channel into R, and vice versa.

Answer 1. No. For technical reasons I will not go into, this doesn't happen in a well designed system. The center derived channel is L plus R, but there is not significant cross talk induced into the L and R. Dolby steering systems try to ascertain what is truly common and bump up the gain.

Question 2. Assuming two channel recording, doesn't this mean that a musical instrument which is hard left (or right), now gets into the derived center channel when it should not. This is a type of bleed into the mike acoustic pick up at the mike itself. Isn't it best to keep things hard left and hard right?

Answer 2. Yes, a bit. However, the musical instrument which located in the center of the orchestra makes a contribution to both left and right channel mike pick ups and the recording of it in the two channels is in phase.

With good mike placement, the center located musical instrument makes a bigger contribution to L and R than the off center musical instrument. This is why we hear a central image with two channel stereo, when things are working perfectly. (You have to contempate this for a while. The sound stage can be better with medium spaced mikes because the central stuff is not ignored.)

The problem is that two channel stereo has variables. The recording theater is not the same size as the listening room. The mike placement is not the same as the speaker placement. In the listening room we may wish to place the speakers apart as much as possible to get left and right stereo effects (in a way, avoiding cross talk in what we hear). However, then the center illusion doesn't work quite as well. It creates the "hole" in the center.

But the use of a third center channel speaker solves many issues. The L plus R can be added and fed to it. This makes a tremendous difference. Now the information about what is center in the recording room really gets to a center located speaker in the listening room.

Some of these issues remain with pop recording where it is not a matter of just putting two mikes in front of a classical orchestra. A singer may be mixed center, which means that the mix down from a multichannel master tape is fed in phase to L and to R. Of course this also happens with movies. Actor's voices are mixed center. Derived center channel helps here again.

Three discrete recording channels, and three speakers probably work best, but phantom channel can work very well. Certainly better that two channel.

This is by no means a new art. It is just re- discovered by every generation. PWK's writings implies that he reintroduced it to listeners at Bell Labs, even though Bell Labs did the intitial reseach which he relied upon in the first place. Coals to Newcastle.

Question 3. Why hasn't this caught on in past.

Answer 3. The cost of equipment in the past is something. Now, HT receivers should do it with ease.

Also, it can take some fiddling with the "gain" of the center channel depending on the recording, itself. PWK suggests center playback should be -6 dB. I find it varies. A fixed setting can put too much level into the center on one recording and the side channel suffer; too much is center. Other times the center channel gain has to be bumped up to fill the hole.

I'd be pleased to send you (or any one else) PWK's and Bell Labs publications if you give me your postal address. Also, check out the Wendy Carlos site on the web.

Long post. I didn't have time to make it short.

I worry sometimes. Am I too pedantic? I sure don't want to offend.

Gil

[jdm56]

Well Gil, I think you covered that topic pretty well!

I'm still chompin' at the bit to find a belle klipsch to plop down 'twixt my cornerhorns so I can finally have the "full monty" across the front of my music room and enjoy firsthand the benefits of that summed center channel. It should tide me over quite nicely 'till SACD or DVD-A gets established (and affordable).

------------------

JDMcCall

[HornEd]

Pedantic? Give yourself a break, Gil, there are legions of lurkers and a few good members that appreciate a well thought out reply over a cryptically succinct, by jingo, one-liner. I mean there is enough "fruit salad" to go around, and a meat course now and then is hardly inappropriate. Especially when it's as "right on" as yours tend to be.

Allow me to illustrate a practical example of what you described. In the fuzzy period just before Dolby ProLogic was introduced, many companies developed derived center channel approaches and "audio clue" based surround schemes... and even crossover based isolation of subwoofer fare was readily available. In fact, a well thought out system of ten, or so, years ago could give most HT installations a surprisingly good run in psychoacoustic appeal.

Eastman Kodak had built a first class, 60 seat, theater in their InfoMart showroom (Dallas, TX) with an 18' x 9' state-of-the-art suspended screen and huge Altec Lancing theater speakers imbedded in the walls on either side of the screen. Their purpose was to show off Kodak's slide projectors using a computer controlled multiple array with integrated sound. When I took over the 12,800 showroom for my research company and bought all of Kodak's fixtures and equipment except the slide projectors and $3,500 each stereo Altec Lansing theater speakers... since they could not measure up to the cutting-edge quality our company needed to project.

IMHO, the wedge-shaped theater with fabric covered acoustic dampening 14' high walls did not provide a suitable extension for a pair of K-horns (like the ones in my home). The big problem in updating the theater was the enormous "hole" in the middle of the large soundstage given the somewhat shallow depth of this showroom theater. Bringing the subwoofer impact to the audience was covered by building a 15" floor-firing sub into a custom lectern... and the side and rear channel "ambient" ceiling speaker installations were fed by a second Yamaha "surround sound" amplifier (both of which have been covered by me in other posts).

No matter how large or small, successful sound systems must stimulate the areas between your ears in such a way that the believability of what your eyes see is enhanced. A big hole in the center of the soundstage when your eyes have an 18' wide feast... was a challenge. My first thought was to buy the then top-of-the-line Yamaha and feed a center channel to fill the hole. Surprise, now I had two holes, one between the Left Main and Center and the other between the Center and Right Main... and merely cranking it up just caused audience discomfort... the holes remained.

Fortunately, in this case, the Yamaha pre-amp had a proprietary provision that allowed a derived phantom center by manipulating available soundtrack information. In effect, if a sound is supposed to emanate from a spot 1/3 in from the left edge of the screen, two-thirds of the sound was sent to the left main and one-third sent to the right speaker. As Ray Garrison might opine... simplistically, the effect was the "art" ... engineering the "audio" was the complex part.

The hole was gone and the sound was equal to the visual feast a $91,000 state-of-the-art Ikegami projector could provide from a Laserdisc in those days. Frankly, it was not until comparatively recently (with multiple Klipsch Legends, SVS reference level bass, and discrete 6.1 + front effects) that I have been able to enjoy a better HT system at home.

The "point" is two fold. IMHO, the highest level of "phantom channel" technology for HT has already passed us by... and as Gil correctly points out, this analog dinosaur was quite similar in effect to the HT amps we have today. The second point relates to Gil's factual comments about arbitrarily setting the center channel to -6dB, digital engineering of the HT source is so varied and discrete center channel dependent that good audio demands more tweaking of the center channel to match source material than most of us would prefer.

Yes, indeed, to Boa's appropriate plea for better bass management tools (with which I wholeheartedly agree!) may we add (from Gil's insightful comments) a better (or at least easier to tweak) industry-wide balance between what is too often center channel dominance with highly diminished mains. When you look at what we consumers spend on mains and compare that to the usually downsized capacity of our center... which provides upwards of 65% of our HT sound... kinda makes me stop and think about where we have been "taught" to put our priorities.

Thanks, Gil, for your "raw" thoughts... sometimes we can polish our prose to succinctness that passes understanding. At least I am not too often accused of the "succinctness" part.

Enjoy what we have... put press the industry for the next best thing from our "ear-perspective"... and, maybe, we can build a constituency with enough purchasing clout to make manufacturers sit-up, take notice... and do something about! HornEd

This message has been edited by HornEd on 08-04-2001 at 08:45 AM

[Cruzer]

You guys have an amazing amount of subject expertice. Keep those great post comming, because I want to keep on learning.

Sincerly,

------------------

Cruzer

[WMcD]

Attached is a Dope From Hope on the Bridged Center Channel Loudspeaker. Note two schematics for making a mixer.

I note the use of volume controls.

Regards,

Gil

[Randy Bey]

Gil,

where do people get these "Dope from Hope" things?

There seems to be a lot of good info in them.

[WMcD]

I got mine from Hope, long time ago.

In those days they'd give you a collection of papers by PWK and others, plus a collection of DFH for 10 bucks.

DFH was an occasionally published flyer. I think it was discontinued several decades ago.

All you can do is phone Trey Cannon and see what he says about availability.

Gil

[Klewless]

Thanks gang for all your responses.

One of the reasons for asking the question lies in my need for a center channel because of room problems. It is too narrow for the problem to be one of "hole in the middle" but one where the sound is bouncing around too much, resulting in a terrible image. Image as in NO CENTER. The presence of a center channel Belle Klipsch would have the three speakers (flanked by Khorns) almost touching each other.

So I was wondering whether to derive a center or go with a multi-channel setup. Still have little or no interest in home theater.

Thanks again.

------------------

John P

St Paul, MN

[Stu Pidass]

OK this has tweeked my interest. Stupid question but, exactly how does one hook up the centre speaker? I mean there are no "Centre" binding posts.

On another related topic, I remember way back in the early eighties reading an article on hooking up four speakers to in a "daisy chain" type of style. My memory is foggy about this but I seem to recall that article talking about some kind of "hidden" track that can be heard this way. Anyway, the trick was to have your left & right mains connected as normal. The other two speakers would be facing the listener from the sides just like surrounds do in a HT setup. Now here's were it got interesting. You hooked a speaker wire from the positive of the left main to the positive of the left surround and from the positive of the rt main to the positive of the right suround. With me so far? Now the odd thing (at least it seemed odd to me) was the negatives of the rt & lt surrounds were connected together. I remember needing to adjust the distance of the surrounds from my ears until all speakers were equal. When it was adjusted the sound was absolutely remarkable.

That was my first "surround" experience.There was a term for this phenominum but I can't recall what it was called. Anyone know anything about it?

[Al Klappenberger]

HI Guys,

Well, since I have the classic PWK style center channel I figured I would comment. This is real and makes a major improvement in stereo image especially with a wide speaker placement like I got. Click on the little house icon below and you will see the little box I use to combine L + R to get the center. It is the "Hope from Hope" method Gill posted but the level pots are on the amps. By the way, I just swaped the center Cornwall in the pictures for a third Belle Klipsch.

AL K.

[dubai2000]

Hi Al,

I've admired your photos before so just for the sake of info, how wide apart are your Belles so that you found a centrer speaker 'necessary'?

Wolfram

[Kevin S]

FWIW, my McIntosh pre/pro has a "Music 3" mode that sends a mono signal to the center channel, at a reduced level (I would guess it is about the 6db down described above) and then sends a difference signal to the surrounds. I think this is essentially similar to the old Hafler surround circuit from, I think, the 60's. It is by far the best compromise I have heard between stereo and surround sound for music.

------------------

L/C/R: Klipsch Heresy II

Surround: Klipsch RS-3

Subwoofers: 2 HSU-VTF-2

Pre/Pro/Tuner: McIntosh MX-132

AMP: McIntosh MC-7205

DVD: McIntosh MVP-831

CD Transport: Pioneer PD-F908 100 Disc Changer

Turntable: Denon DP-72L

Cassette: Nakamichi BX-1

T.V. : Mitsubishi 55905

HDTV: RCA DTC-100

Surge Protector: Monster Power HTS-5000

[Al Klappenberger]

Wolfram,

The flank speakers are about 16 feet apart. The Kicker is that my listening chair is only about 9 feet back from the center. The three speaker set up makes up for the strange proportions nicely.

Al K.