Brian Eno's Early "Surround" Technique

[Babaluma]

I've revently been collecting Brian Eno's "Ambient" series on vinyl, and on the back of "Ambient 4: On Land" is a diagram showing an early surround sound technique with 3 speakers. It involves a regular L and R pair, plus an ambient speaker which receives the positive + (red) speaker signal from both the L and R speaker outs on the amp.

Now, I'd like to try this, but I only have active monitors. I was wondering if I could achieve the same effect with sum/difference processing digitally.

Can anyone tell me if adding the + outputs from L and R speakers would be the same as creating either the sum or the difference of a stereo signal?

If I knew how, I could replicate the effect with three digital channels and send them to three different active monitors.

Any clues? I would love to hear what this might sound like!

TIA,

Gregg

[jonathan]

Sounds like it would be awesome...

Do you have Ambient 2 on vinyl???

Sorry for the off topic.

[Babaluma]

yes, i have ambient 2, 3, and 4 on vinyl, and ambient 1 is on it's way to me as we speak.

i love 2 (with harold budd - melancholy but beautiful), 3 (with laraaji - side two is incredible), but 4 doesn't impress me so much, it's very dark and scary in places, in a bad way (i mean i love some dark and scary ambient music, nurse with wound, lustmord etc).

anyway, am still wondering about how this surround thing might sound, and am also thinking of trying 2 regular speakers with the difference (or side) signal sent to another ambient speaker further away...

[bluesman]

I used to use the third speaker on my home stereo back in the 70's. The third channel contains information that would otherwise be phase cancelled. It was interesting for a while...lots of reverb signal, some information that would otherwise be so low in the mix as to go un-noticed. I stopped using it when I expanded to a quad system with an sq decoder, which did essentialy the same thing when using it on regular stereo records. Back in the days of quad, everyone wanted it, but no one could afford two stereo systems + the decoder + the quad material to play. The ultimate downfall of quad was the lack of available quad records, also the fact that it was so expensibe to buy a proper quad system for home listening. The third ambient speaker was a viable compromise. In retrospect, I should have put a passive volume control on the third speaker, it got quite loud at times & was kind of distracting. I never tried it as a rear speaker, only as a center channel. Its a fun experiment & cheap enough to try, the size & quality of the third speaker didn't seem to matter much. I tried several & finally settled on a small fullrange bookshelf Radio Shack speaker.

[leigh]

I've revently been collecting Brian Eno's "Ambient" series on vinyl, and on the back of "Ambient 4: On Land" is a diagram showing an early surround sound technique with 3 speakers. It involves a regular L and R pair, plus an ambient speaker which receives the positive + (red) speaker signal from both the L and R speaker outs on the amp.

Now, I'd like to try this, but I only have active monitors. I was wondering if I could achieve the same effect with sum/difference processing digitally.

Can anyone tell me if adding the + outputs from L and R speakers would be the same as creating either the sum or the difference of a stereo signal?

That's clever, I'd never heard of that DIY-surround technique.

In theory, the third speaker would be getting a difference signal. For a given moment, if the L signal is "high" (voltage) and the R signal is also high, then they're pushing against each other (across the 3rd speaker's coil) and they would cancel out. So when both signals are the same, they cancel. But if L is high and R is low/negative, that's a big difference, and essentially then L "pushes" while R "pulls".

In practice, there would be some additional effects, depending on the relative impedance of the speakers. Perhaps, in some situations, some crosstalk and narrowing of the stereo spread in the main L & R speakers?

With active speakers, yes, you could do this. Because that third speaker, being active as well, would have a very high impedance input (like 10k ohms, as an active line stage, rather than 8 ohms for a passive speaker element), it would virtually eliminate possible crosstalk issues.

Cool idea, in any case! I have an old, completely passive, fake surround "synthesizer" from Realistic that I found at a yard sale. Works with the same principle.

cheers,
Leigh

Leigh

[ashcat_lt]

Here's a link to a picture, and some dude's explanation. Despite what dude thinks is going on, I'm pretty sure that leigh is right. It must be a difference signal.

[Babaluma]

thanks for that link, the picture makes it easier to understand than my wordy description earlier.

i too, when i really try to picture what's going on, feel that it's more likely to to be the difference signal.

thinking about all of this has made me realise that i don't really understand what "speaker level" signals actually are. i understand balanced, unbalanced, mic, line, modular, low impedance, hi impedance, ac/dc signals etc, but what are speaker signals? why do there need to be two wires for one signal? does the negative signal "pull" the speaker inwards and the positive signal "push" the speaker outwards? are they both seperate dc signals which make ac when combined?

anyway, i'm hopefully going to give the digital version a try this weekend - send a stereo .wav file to the l r speakers as normal, but also decode a difference signal using the voxengo msed plugin and send that to the ambient speaker.

[infopimpster]

thanks for the new desktop background!

[ashcat_lt]

That'd oughta work just fine. It can be done in the analog world by hacking a couple cables together.

Speaker signals really aren't anything special. The only real difference between the signal on the input side of the amp and the output side is overall power. There's two wires for the same reason as every other electrical circuit. It wouldn't be a circuit otherwise. No current could flow because there'd be nowhere for it to flow to (or from). The voicecoil moves in or out depending on which direction the current is flowing. That depends which of the two wires is more negative at any given time. And that depends on the input signal.

Laying a DC signal across a speaker makes it try to move in one direction until either a) mechanical force from however the cone is attached to the frame of the speaker overcomes the force exerted by the magnet on the coil or; b) the coil moves far enough from the magnet that the magnetic force becomes minimal*. Some people test polarity of speakers using a 9V battery across the terminals and watching which way the cone moves.

If you attach a DC voltage to each side of the speaker the voltages would add, but would still be DC.


* If this happens, it's not likely the coil will ever end up back in the magnet, at least not without your help.

[CraigS63]

If you have Windows Media Player, go into 'Now Playing', Enhancements, and SRS WOW Effects, and crank the WOW Effect slider all the way to the right, this will be emphasing the L - R like the center speaker / Eno / Hafler circuit thing. Things panned dead center (usually lead vocal, kick drum, bass) will be less noticeable, and sounds off to one side (reverb, 'ambience', background vocals and sounds) will be louder.

If you are running 'Rockbox' on your mp3 player, there is a 'Karaoke' setting somewhere that's even more drastic (the L + R is removed and L - R is louder), I think I already posted this in one of those 'things I never noticed on recordings' threads.

[ashcat_lt]

If you have Windows Media Player, go into 'Now Playing', Enhancements, and SRS WOW Effects, and crank the WOW Effect slider all the way to the right, this will be emphasing the L - R like the center speaker / Eno / Hafler circuit thing. Things panned dead center (usually lead vocal, kick drum, bass) will be less noticeable, and sounds off to one side (reverb, 'ambience', background vocals and sounds) will be louder.

If you are running 'Rockbox' on your mp3 player, there is a 'Karaoke' setting somewhere that's even more drastic (the L + R is removed and L - R is louder), I think I already posted this in one of those 'things I never noticed on recordings' threads.

^There are plenty of these sorts of things on consumer gear. Just about every cheapass boombox out there has a "stereo enhancer" of some sort. Damndest thing trying to explain to non-engineers why they shouldn't push that button. Especially when listening to my music.

I've never tried it, but I think having the (L-R) actually coming from a seperate speaker would have to be a bit different. The effect will necessarily be completely different depending on where the listener is compared to the various speakers. Even the specific room geometry could have a great impact on how the effect translates.

[Babaluma]

exactly, i've tried increasing the M to S signal (and vice versa) on a stereo pair many times with interesting results, but what we're talking about here is using another speaker in addition to the L and R, behind or above you, with only the S signal.

[Andy Peters]

thinking about all of this has made me realise that i don't really understand what "speaker level" signals actually are. i understand balanced, unbalanced, mic, line, modular, low impedance, hi impedance, ac/dc signals etc, but what are speaker signals?

"Speaker" signals are just like any other ("mic level" or "line level" or "instrument level"). The main thing is that your power amp is really a voltage amplifier that is capable of driving very low-impedance loads and its power-supply rails (at least for the output stage) is high compared to those of line-level equipment. To get high power output you need high voltage rails and adequate current-source capability.

why do there need to be two wires for one signal?

For the same reason EVERY signal has two wires: it needs a return. This is basic electronics. Now, wrap your head around this: the red terminal is the voltage output, and the black terminal is tied to ground. This means that the voltage at the black terminal is 0 V but the current into it is not. The current there is equal in level and opposite in polarity to the current in the red terminal.

does the negative signal "pull" the speaker inwards and the positive signal "push" the speaker outwards?

No. It's the voltage across the speaker that moves the cone in and out, in the direction determined by the instantaneous polarity of the signal. (Yes, the output at the red terminal can be, and will be basically half the time, negative.)

are they both seperate dc signals which make ac when combined?

No! And if your amp puts out DC at anything more than a minimal level, your speakers will die.

-a

[Babaluma]

thanks for the clear explanation!

[winky dinglehoffer]

I've been perusing old issues of The Audio Amateur. One of their early projects is the Quadpod, which derives L-R & R-L for a pair of rear speakers. Sadly, the project uses 741 opamps, so it's probably not the most hifi sound, but it's a pretty simple circuit that could probably be reworked quite easily with better quality opamps. It's in 1971 Issue 1 if you're interested at all. They actually sell back issues at audioxpress.com