I've just started reading around the subject of ambisonics, not that there is such a lot about it on the net and what I find seems quite old. I'm fascinated and would like to try some experiments but before I invest a pile of time and money into this I wondered what the diyaudio guys had to say about it. What do I find ? Nothing. Well almost nothing. What? all these people, building, experimenting with amps and tubes and DACS and speakers and tone arms and God knows what but nobody playing with surround sound. Amazing.
Is there a taboo against this? Did everyone try it already and hate it? What's up?
An impressions I get are: a) Audiophiles did not like Quadraphonics in the seventies so they gave up on the surround idea. b) Audiophiles don't much like modern cinema style surround systems so again no go. c) Surround can expensive and clumbsey. d) There is no source material.
What I learned is that a) From a theoritical point of view the Quadraphonic system can never produce a high quality spatial image. b) Modern cinema surrround is wedded to the layout of the speakers at the playback end. c) Is no issue for DIYers. d) Source material is still available form the likes of Nimbus records, can be recorded by oneself or composed with the help of a computer.
The theory behind Ambisonics is quite compelling and I'd like to put together something like:
A cubic array of inexpensive fullrange speakers, small Tangband boxes say. Yes 8 speakers for full 3-d.
Eight chip amps to drive it. I alreay have four.
A home made ambisonic decoder.
Has anyone here attempted such a thing? What experiences did you have?
Is there a taboo against this? Did everyone try it already and hate it? What's up?
An impressions I get are: a) Audiophiles did not like Quadraphonics in the seventies so they gave up on the surround idea. b) Audiophiles don't much like modern cinema style surround systems so again no go. c) Surround can expensive and clumbsey. d) There is no source material.
What I learned is that a) From a theoritical point of view the Quadraphonic system can never produce a high quality spatial image. b) Modern cinema surrround is wedded to the layout of the speakers at the playback end. c) Is no issue for DIYers. d) Source material is still available form the likes of Nimbus records, can be recorded by oneself or composed with the help of a computer.
The theory behind Ambisonics is quite compelling and I'd like to put together something like:
A cubic array of inexpensive fullrange speakers, small Tangband boxes say. Yes 8 speakers for full 3-d.
Eight chip amps to drive it. I alreay have four.
A home made ambisonic decoder.
Has anyone here attempted such a thing? What experiences did you have?
I was interested once, but when I looked into the circuits that could be found, and the vague descriptions the web sites provided, I voted against doing it. Also looked at the way thye proposed layout of speakers took no concern about the impedance effects on amplifiers which was another minus point.
The 2+2+2 method from MDG seems quite easy though, which is a setup I wish to try.
The 2+2+2 method from MDG seems quite easy though, which is a setup I wish to try.
Yes there is not a lot to be found on the web, I'm basically getting my info from here:
http://www.york.ac.uk/inst/mustech/3d_audio/welcome.html
http://homepage.ntlworld.com/henry01/ambisonic_decoder/ambisonic_decoder.htm
http://homepage.ntlworld.com/henry0...nic_clock_decoder/ambisonic_clock_decoder.htm
http://homepage.ntlworld.com/henry01/cheap_soundfield/cheap_soundfield.htm
http://www.ambisonic.net/
http://www.geocities.com/ambinutter/AMBISONIC_HOME_PAGE.html
Having read through those a few time the basic ambisonic idea seems beautifully simple:
Capture the three directional components of the sound pressure at the recording position, X, Y and Z. Also capture an overall level signal W. These are recorded with a special multi-capsule mic. (or created artificially, by computer say) These signals (so called B format) contain all information about the sound field and are what is recorded, transmitted, mixed edited etc.
Importantly they relate to the moment of recording not playback. So your speaker arrangements are not dictated by your medium as in Quadraphonis systems or 5.1 Cinema sound etc.
For play back we use some simple trig to map the sound pressure signals to the available speakers. So playback can be mono, stereo, 4 speakers or six etc and of course can include height info from the z channel.
At it's simplest thats all you need and it sounds to simple. But then again normal stereo is pretty straight forward.
Then there are some frequency related compensations added because on recording the mic capsules are not truly coincident so there is an uper frequency limit on this working nicely. On playback there are phase issues as the sound runs around the listeners head. And then perhaps a correction for the fact that the maths of all this assumes plane wave fronts whereas they are of course spherical. But people seem to get by without much of this extra compensation.
The really nasty part is when you encode the B format "master" recording into stereo (UHJ format) such that it can be recorded to vinyl or CD and on playback is decoded into two dimentional surround. I don't really understand how this works yet and it involves some nasty phase shifting circuitry. BUT now a days we don't really need this, computers can easily record the 4 channels of the B format, and DVDs can carry it in their 5.1 channels.
It's that "BUT" that makes me think that now is a good time to get back into ambisonics, no more messy stereo coding/decoding anymore.
soonqsc could you elaborate on what you mean by "took no concern about the impedance effects on amplifiers"? What is the problem here?
Oh and where can I read about "The 2+2+2 method from MDG"?
http://www.york.ac.uk/inst/mustech/3d_audio/welcome.html
http://homepage.ntlworld.com/henry01/ambisonic_decoder/ambisonic_decoder.htm
http://homepage.ntlworld.com/henry0...nic_clock_decoder/ambisonic_clock_decoder.htm
http://homepage.ntlworld.com/henry01/cheap_soundfield/cheap_soundfield.htm
http://www.ambisonic.net/
http://www.geocities.com/ambinutter/AMBISONIC_HOME_PAGE.html
Having read through those a few time the basic ambisonic idea seems beautifully simple:
Capture the three directional components of the sound pressure at the recording position, X, Y and Z. Also capture an overall level signal W. These are recorded with a special multi-capsule mic. (or created artificially, by computer say) These signals (so called B format) contain all information about the sound field and are what is recorded, transmitted, mixed edited etc.
Importantly they relate to the moment of recording not playback. So your speaker arrangements are not dictated by your medium as in Quadraphonis systems or 5.1 Cinema sound etc.
For play back we use some simple trig to map the sound pressure signals to the available speakers. So playback can be mono, stereo, 4 speakers or six etc and of course can include height info from the z channel.
At it's simplest thats all you need and it sounds to simple. But then again normal stereo is pretty straight forward.
Then there are some frequency related compensations added because on recording the mic capsules are not truly coincident so there is an uper frequency limit on this working nicely. On playback there are phase issues as the sound runs around the listeners head. And then perhaps a correction for the fact that the maths of all this assumes plane wave fronts whereas they are of course spherical. But people seem to get by without much of this extra compensation.
The really nasty part is when you encode the B format "master" recording into stereo (UHJ format) such that it can be recorded to vinyl or CD and on playback is decoded into two dimentional surround. I don't really understand how this works yet and it involves some nasty phase shifting circuitry. BUT now a days we don't really need this, computers can easily record the 4 channels of the B format, and DVDs can carry it in their 5.1 channels.
It's that "BUT" that makes me think that now is a good time to get back into ambisonics, no more messy stereo coding/decoding anymore.
soonqsc could you elaborate on what you mean by "took no concern about the impedance effects on amplifiers"? What is the problem here?
Oh and where can I read about "The 2+2+2 method from MDG"?
It's (d), the lack of any but the most limited source material. Ambisonics was a technical wonder (I've heard several demos and was very impressed) and a spectacular market failure. If all you want to listen to is 5 or 6 mediocre performances, lovely, it's just the thing for you. But exactly zero of the music that interests me is available in that format.
Since nearly none of us are in a position to record important artists, Ambisonics will, sadly, remain a tiny niche within the tiny niche of high end audio.
This is an example of how your could wire speakers up with the ambisonics method. Seems very unlikely that you can get the same accurate results in imaging. Also the woofer signal has no reference to it's own ground, what comes out of the woofer is really dependent on what contents the other amplifiers have. It just seems to me that it uses lots of out-of-phase signals to create a synthetic sense of space, having to relevance to where the performers actually are.
Let me see if I can dig up some explanation about the 2+2+2. This site has some explanation but not much.
http://www.mdg.de/indexeng.htm
The basic speaker arrangement is 4 speakers in the front, 2 surround speakers. Two of the front speakers are in the normal listening location, two are higher in order to create image height. The two surround speakers are placed like normal surround speakers. The sound delay for the upper front and surround speakers are set based on distance and possibly angle from the listening location.
If you ask MDG where more information can be found about setting a 2+2+2 system up, they can tell you.
Let me see if I can dig up some explanation about the 2+2+2. This site has some explanation but not much.
http://www.mdg.de/indexeng.htm
The basic speaker arrangement is 4 speakers in the front, 2 surround speakers. Two of the front speakers are in the normal listening location, two are higher in order to create image height. The two surround speakers are placed like normal surround speakers. The sound delay for the upper front and surround speakers are set based on distance and possibly angle from the listening location.
If you ask MDG where more information can be found about setting a 2+2+2 system up, they can tell you.
Attachments
SY you are right of course, not much material. I not imagining to bin my faithfull old stereo.
But you did say you were impressed by some demos so I'd like to invest a few hours and euros into having a little of that experience. Nothing to grand.
Also I know some garage bands and other performers around here so we could have fun trying to record them.
But you did say you were impressed by some demos so I'd like to invest a few hours and euros into having a little of that experience. Nothing to grand.
Also I know some garage bands and other performers around here so we could have fun trying to record them.
Those speaker layouts look like the ones from the old Integrex Ambisonics kit iinstructions. Probably a good idea at the time for saving on amps. Did not like the look of it myself.
My plan would be to have four or six speakers for 2-d surround or eight for 3-d. The decoder would be built (configured) to handle the different layouts. Note the square or cubic arrangements can be stretched front to back or sideways to fit the room and the decoder adjusted accordingly.
Each speaker could be cheap Tangband with it's own chip amp. Not so expensive and no need for confusing speaker connections.
soonqsc: Your comment "It just seems to me that it uses lots of out-of-phase signals to create a synthetic sense of space, having to relevance to where the performers actually are" Seems a little harsh to me. Those out of phase signals originate from the moment of recording the original performers and are faithfully preserved (ideally) so I would not say a "synthetic" sense of space but rather a "shadow" of it's former self. Much like stereo is anyway.
My plan would be to have four or six speakers for 2-d surround or eight for 3-d. The decoder would be built (configured) to handle the different layouts. Note the square or cubic arrangements can be stretched front to back or sideways to fit the room and the decoder adjusted accordingly.
Each speaker could be cheap Tangband with it's own chip amp. Not so expensive and no need for confusing speaker connections.
soonqsc: Your comment "It just seems to me that it uses lots of out-of-phase signals to create a synthetic sense of space, having to relevance to where the performers actually are" Seems a little harsh to me. Those out of phase signals originate from the moment of recording the original performers and are faithfully preserved (ideally) so I would not say a "synthetic" sense of space but rather a "shadow" of it's former self. Much like stereo is anyway.
Yes heater, those are from the ntegrex kit instructions. If I sound stage is to be correctly reproduced, there must be a standard was of allocating the speakers in order to preserve the time relation between the recorded direct performance and the delayed reflections and reverberation. I have not seen any information that explains how the speakers are correctly setup.
I have played around with a few phase shifting circuits that do give you a better sensation, the same feeling when you have two speakers wired out of phase from each other. If you have main speakers in phase and some surrounds out of phase or shifted some what, you will get a good sensation of space to various extent, but the important thing is that the relfection time delays cannot be correctly preserved.
When people talked about binaural recording in the early days, the mathematics was well revealed. With Amisonics, they just show some wording and some unknown matrixes that nobody could verify to be correct or not.
All this does not mean it will sound bad, but it will just not correctly preserve such experience as we are led to believe. Yes, it is the way currently stereo is.
One thing that seems interesting is that there are some software out there that allow you to create a "hall" and place each individual recorded performer in the hall and recreate the "hall effects. This is very interesting indeed and opens the door to lots of possibilities.
I have played around with a few phase shifting circuits that do give you a better sensation, the same feeling when you have two speakers wired out of phase from each other. If you have main speakers in phase and some surrounds out of phase or shifted some what, you will get a good sensation of space to various extent, but the important thing is that the relfection time delays cannot be correctly preserved.
When people talked about binaural recording in the early days, the mathematics was well revealed. With Amisonics, they just show some wording and some unknown matrixes that nobody could verify to be correct or not.
All this does not mean it will sound bad, but it will just not correctly preserve such experience as we are led to believe. Yes, it is the way currently stereo is.
One thing that seems interesting is that there are some software out there that allow you to create a "hall" and place each individual recorded performer in the hall and recreate the "hall effects. This is very interesting indeed and opens the door to lots of possibilities.
That integrex kit is mind bending. Seems to support a lot of different input codings that don't or never did exist and does not support the common UHJ as used by Nimbus I believe.
Not sure I understand what you say about speaker layout, direct sound and reflections. Seems to me that the basic maths of ambisonics tries to encode the 2-D or 3-D components of the sound field at the recoding position, which will then be reproduced at the listening position on playback. So for example a recorded reflection is in there just like any other sound, time delays and all. Why concern ourselves if is direct or reflected? The recorded B' format is then decoded according to whatever speaker array you have, square, hexagonal etc. Better have regular shapes, speakers equidistant from the listener.
I have to say that I have no idea about the shelf filters and distance compensation filters sometimes included to magically fix things up where the theory does not match the practical implementation. Those parameters do seem to come out of thin air! People seem to get by without them. For example this schematic for a cubic decoder:http://www.york.ac.uk/inst/mustech/3d_audio/davedec.htm
There is a lot of maths published about Ambisonics, it's accuracy etc. Just seems to be buried away in academic journals. But again the frequency dependent tweaks seem to have arrived by trial and error.
Not sure I want to get into any old phase shifting circuits for an "ambiance" effect.
Not sure I understand what you say about speaker layout, direct sound and reflections. Seems to me that the basic maths of ambisonics tries to encode the 2-D or 3-D components of the sound field at the recoding position, which will then be reproduced at the listening position on playback. So for example a recorded reflection is in there just like any other sound, time delays and all. Why concern ourselves if is direct or reflected? The recorded B' format is then decoded according to whatever speaker array you have, square, hexagonal etc. Better have regular shapes, speakers equidistant from the listener.
I have to say that I have no idea about the shelf filters and distance compensation filters sometimes included to magically fix things up where the theory does not match the practical implementation. Those parameters do seem to come out of thin air! People seem to get by without them. For example this schematic for a cubic decoder:http://www.york.ac.uk/inst/mustech/3d_audio/davedec.htm
There is a lot of maths published about Ambisonics, it's accuracy etc. Just seems to be buried away in academic journals. But again the frequency dependent tweaks seem to have arrived by trial and error.
Not sure I want to get into any old phase shifting circuits for an "ambiance" effect.
Heater: I think I have just not seen any math that correctly describes preservation of the sound field. If seen some things show show you some fance 3D patterns generated by math, but none that describes the actual math from performer mic location to playback speaker lisenter location. If I have misted something, please by all mean point us in the right direction. I did all the search I could, but nothing meaningfull, only what normally you would see in promotion talk. Is there ans AES publication on this? If there is I would be interested in reading it.
Basic geometry shows us that in order to truly reproduce 3D you need at least four speakers with one speaker elevated from the rest of the three to recreate height allocation. This is the same way with GPS.
SY: I've always heard this kind of talk, but no real convincing expalantion or math. This just puts the dicussion in the "cables differences" category. Perhaps Peter Fellgett can point us to some information that will shed some light?
Edit:
I would like to refer to Wireless World November 1982 "Binaural Recordings and Loudspeakers" by J.H. Buijs. Now this article explains Binaural Recording and playback in a very understandable manner. If there is something on Ambisonics like this, it would let everyone understand how good it technically is.
Of course, if there is anywhere I can go to in this neck of the woods to have first hand experience, it would be even better.
Basic geometry shows us that in order to truly reproduce 3D you need at least four speakers with one speaker elevated from the rest of the three to recreate height allocation. This is the same way with GPS.
SY: I've always heard this kind of talk, but no real convincing expalantion or math. This just puts the dicussion in the "cables differences" category. Perhaps Peter Fellgett can point us to some information that will shed some light?
Edit:
I would like to refer to Wireless World November 1982 "Binaural Recordings and Loudspeakers" by J.H. Buijs. Now this article explains Binaural Recording and playback in a very understandable manner. If there is something on Ambisonics like this, it would let everyone understand how good it technically is.
Of course, if there is anywhere I can go to in this neck of the woods to have first hand experience, it would be even better.
ambisonics tech papers
Here's a short bibliography compiled by the late Michael Gerzon, a co-inventor.
http://members.tripod.com/martin_leese/Ambisonic/biblio.txt
edit: Basic equation of operation at wikipedia:
http://en.wikipedia.org/wiki/Ambisonics
Chemists will recognize the process as being the same one as mixing atomic orbitals, e.g., hybridization.
Here's a short bibliography compiled by the late Michael Gerzon, a co-inventor.
http://members.tripod.com/martin_leese/Ambisonic/biblio.txt
edit: Basic equation of operation at wikipedia:
http://en.wikipedia.org/wiki/Ambisonics
Chemists will recognize the process as being the same one as mixing atomic orbitals, e.g., hybridization.
Actually I found this site that probably provides more information.
http://audiolab.uwaterloo.ca/~jeffb/thesis/thesis.html
I think I'll sudy it a bit.
This makes me remember I was asked to quize a job applicant with a Masters degree in controls, I looked at his thesis and asked a simple question, "Why do you assume the sensor to be a constant?" His answer was "Because that's what they do in the books." That was the last of it. A simple answer would be that it can assumed to be constant within the well within the sensor performance bandwidth.
So lets see what we really understand from this thesis.
http://audiolab.uwaterloo.ca/~jeffb/thesis/thesis.html
I think I'll sudy it a bit.
This makes me remember I was asked to quize a job applicant with a Masters degree in controls, I looked at his thesis and asked a simple question, "Why do you assume the sensor to be a constant?" His answer was "Because that's what they do in the books." That was the last of it. A simple answer would be that it can assumed to be constant within the well within the sensor performance bandwidth.
So lets see what we really understand from this thesis.
Well I was thinking, if I'm going to do this at all it has to have 3-d capability, boring old 2-d is done to death isn't it? Now eight speakers seems to be enough to have to deal with. But an eight speaker cube as here http://www.york.ac.uk/inst/mustech/3d_audio/bdecoder.htm will never be a good fit in my home. Four speakers up is OK but four speakers down would have to fight with my sofa, tables, etc especially in the room corners.
Then the idea, why not tip the cube up on on of it's edges by 45 degrees?. So we have front left and right, back left and right. Then we have up-left and up-right near the ceiling. Then we have down-left and down-right in some available spaces near the floor, which can be easily kicked out of the way when not in use.
The magic of this is that there is a normal "front pair" for stereo (all be it with 90 deg between instead of 60 but they can be slid around a bit. Then a normal square rig for 2-d ambisonic and a 3-d rig with not to much junk on the floor.
So now, how to tweak the decoder parameters?
Hmmm well the wikipedia ambisonics page has the 3-d decoder equation. In to that I plug the angle of elevation of my up and down speakers, which if my rusty maths is correct would be inverse cos (1 / (root 3)) or about 54.7 degrees. Negative for down of course.
Any coments on this idea?
soonqsc:
Here is someone who doesn't want to just accept what the books say about ambisonics:
http://www.speech.kth.se/publications/masterprojects/2004/KarinCarlsson.pdf
Then the idea, why not tip the cube up on on of it's edges by 45 degrees?. So we have front left and right, back left and right. Then we have up-left and up-right near the ceiling. Then we have down-left and down-right in some available spaces near the floor, which can be easily kicked out of the way when not in use.
The magic of this is that there is a normal "front pair" for stereo (all be it with 90 deg between instead of 60 but they can be slid around a bit. Then a normal square rig for 2-d ambisonic and a 3-d rig with not to much junk on the floor.
So now, how to tweak the decoder parameters?
Hmmm well the wikipedia ambisonics page has the 3-d decoder equation. In to that I plug the angle of elevation of my up and down speakers, which if my rusty maths is correct would be inverse cos (1 / (root 3)) or about 54.7 degrees. Negative for down of course.
Any coments on this idea?
soonqsc:
Here is someone who doesn't want to just accept what the books say about ambisonics:
http://www.speech.kth.se/publications/masterprojects/2004/KarinCarlsson.pdf
There might be more material out there than is apparent; a number of people added a Calrec soundfield mic to their multitrack recording, and discovered that it didn't integrate any too well into the "improved stereo" multimic environment (try and record it as it ought to have sounded, but due to local acoustics,, dynamics between musicians, or the laws of physics, it didn't.) These "purist"track couldn't be released, as there was no accepted physical support to sell them on; and they still couldn't , because there still isn't (try and convince people they want a music/video room set up with 5.1 for video and those audio recordings specifically prepared in that format, and a second system with, at most two loudspeakers in common (and, even in that case, with the listening image rotated)
I basically live, work and occasionally sleep in a 5.1 studio, and though it's obvious that the standard was developed for picture support (there's a great big perforated screen in front of me) and is non-optimal for music reproduction (even ignoring the people who use the 5.1 just as stereo was used in the early sixties; effects for effects sake, movements, musicians all around you, I suppose it was predictable) but even so the difference when the music is flattened into stereo is every bit as impressive as the reduction of stereo to mono.
In the various surround sound forums I attend, it's generally accepted that present 6.1 and 7.1 systems aren't the be all and end all, and propositions of 10.2 (and even one Japanese gentleman proposing a 22.2 which could be matrixed down into practically any other standard - don't even ask what it would be recorded onto), in particular for eliminating the sweet spot (like waveform resythesis inside out)
But, while recommending clients to archive material in surround, I can't do as much for release; Most DVD players expect ac3, and the majority are connected to TV sets (neither a guarantee of quality) and, when they do run into a receiver, it's connected to loudspeakers the size of cigarette packets, that you woldn't accept in your car.
So, if there were a market for ambiosonic recordings, I'm sure the industry could be persuaded to release some of those old tapes (nothing they like better than selling something they already own and which will cos a minimum to remaster) but I can see no way of generating the market.
I basically live, work and occasionally sleep in a 5.1 studio, and though it's obvious that the standard was developed for picture support (there's a great big perforated screen in front of me) and is non-optimal for music reproduction (even ignoring the people who use the 5.1 just as stereo was used in the early sixties; effects for effects sake, movements, musicians all around you, I suppose it was predictable) but even so the difference when the music is flattened into stereo is every bit as impressive as the reduction of stereo to mono.
In the various surround sound forums I attend, it's generally accepted that present 6.1 and 7.1 systems aren't the be all and end all, and propositions of 10.2 (and even one Japanese gentleman proposing a 22.2 which could be matrixed down into practically any other standard - don't even ask what it would be recorded onto), in particular for eliminating the sweet spot (like waveform resythesis inside out)
But, while recommending clients to archive material in surround, I can't do as much for release; Most DVD players expect ac3, and the majority are connected to TV sets (neither a guarantee of quality) and, when they do run into a receiver, it's connected to loudspeakers the size of cigarette packets, that you woldn't accept in your car.
So, if there were a market for ambiosonic recordings, I'm sure the industry could be persuaded to release some of those old tapes (nothing they like better than selling something they already own and which will cos a minimum to remaster) but I can see no way of generating the market.
I find this quite a fascinating subject and past-time. My first venture into it was with 2 additional speakers connected in series, out of phase, with a w/w 50 ohm pot to adjust level. The whole lot connected to the + output of left and right.
It can add a true 2 dimensional effect, ie l-r and f-r. (There is no 3rd dimension unless you go up and down)
4ch vinyl with out 4 chan decoders sounded good, due to the fact they were recorded with 4 ch in mind. ie a good a-b mix, not ping pong.
Another scheme I have tried was an analogue decoder, where by difference signals are created, then mixed with left and right information. This allowed for greater control. This setup was used in the days when I was doing the sound for teen discos.
In a large area, this could provide an "on stage" atmosphere.
In a typical listening room, it will provide a good spatial effect, but not true ambience, I think that requires a time delay, or very mild reverb.
I find a lot of modern recordings are too discreet between the channels, and need a blend between left and right.
I haven't tried it yet, however, an extra speaker connected at the junction of the 2 rear speakers in the first example returning to ground or common will privide front centre. NB Don't try on bridged amps!
I have it on good authority that when Allan Parsons engineered "Dark Side of The Moon" he did so with 4 chan in mind, but with several standards for quad at the time, it was released as stereo.
The first track on "the other side of life" (Moody Blues) is also interesting, even if a bit "eighties"
I think one of the big issues with quad, is TWF. It's (was) hard enough to get decent front speakers into the lounge, let alone 2 at the rear or sides, and one in the centre.
It can add a true 2 dimensional effect, ie l-r and f-r. (There is no 3rd dimension unless you go up and down)
4ch vinyl with out 4 chan decoders sounded good, due to the fact they were recorded with 4 ch in mind. ie a good a-b mix, not ping pong.
Another scheme I have tried was an analogue decoder, where by difference signals are created, then mixed with left and right information. This allowed for greater control. This setup was used in the days when I was doing the sound for teen discos.
In a large area, this could provide an "on stage" atmosphere.
In a typical listening room, it will provide a good spatial effect, but not true ambience, I think that requires a time delay, or very mild reverb.
I find a lot of modern recordings are too discreet between the channels, and need a blend between left and right.
I haven't tried it yet, however, an extra speaker connected at the junction of the 2 rear speakers in the first example returning to ground or common will privide front centre. NB Don't try on bridged amps!
I have it on good authority that when Allan Parsons engineered "Dark Side of The Moon" he did so with 4 chan in mind, but with several standards for quad at the time, it was released as stereo.
The first track on "the other side of life" (Moody Blues) is also interesting, even if a bit "eighties"
I think one of the big issues with quad, is TWF. It's (was) hard enough to get decent front speakers into the lounge, let alone 2 at the rear or sides, and one in the centre.
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