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Old 22nd May 2006, 06:45 AM   #1
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Default Dolby surround for ambience synthesis

Does the old “Dolby surround” basically work by: the difference signal between left and right channels is taken, then delayed electrically 15 to 30 ms, then applied to two rear speakers?

Does anyone use this for ambience “recovery” (synthesis) – is it useful?
For some time types of music more than others?

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Old 23rd May 2006, 07:33 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
is that Pro logic or pre pro logic?

I had a surround processor (E1000) that did lots of sound effects trying to mimic halls etc as well as early implementation of Pro logic.

I hated it, consigned to it's box in less than a month, what a waste of time and money.
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Old 23rd May 2006, 11:44 AM   #3
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Andrew

No intention to mimicing halls etc.

I simply want a unit that synthesises ambience for two rear channels (at a "just heard" level), and it was suggested that “Dolby surround” does that, in the manner I described.

Other methods include the 1970s Hafler Dynapod, but that would be hard to locate.

I imagiune it would enhance some time types of music, and harm others, hence my question about diffrent types.
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Old 23rd May 2006, 01:22 PM   #4
Schaef is offline Schaef  United States
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Roughly speaking, yes you have described how dolby surround works. The rear channel is the sum of the two front channels out of phase with each other. The center channel is the whatever audio is the same in phase in the two main speakers.

Now, what this does with music, depends on the music, the producer, the studio and the masters. I have some discs that sound cool in surround and others that have the most bizzare sound you'll ever hear. Things like the main vocals coming out of the rear speakers! (Simply because somebody mis-wired things in the mastering process) Meat Loaf's Bat out of Hell 2 sounds really freaky with surround turned on. Main instruments come out of the rear channels and things like that.

If you're looking to do your own music and want to throw some stuff in for an ambiance effect for fun, then that's what you're after.
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Old 23rd May 2006, 01:47 PM   #5
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I wansn't going to use a center channel , just rears.

I suspect much pop is mastered not well, and could sound weird.

I'm hoping for benefits in acoustic and some electronic material.

Bear in mind that adding a processor & rear speakers will add only maybe 6-8% to my system cost, so it's a modest expereimnt. It's second hand, so if it doesn't work, I sell it and lose little or nothing, but had the experience.
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Old 23rd May 2006, 02:01 PM   #6
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Not totally what you are looking at but worth a mention anyway. My receiver has Prologic II, a function of which is supposed to be to digitally ambient-ise normal stereo recordings. It sounds utterly rubbish.
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Old 23rd May 2006, 07:27 PM   #7
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rick57,

I think you are beating your dead horse to death (sic). You can't get blood from a turnip. Ambience must exist in the recorded material or you won't get it. Years ago I used to make time in my schedule to listen to the live weekend broadcasts of the Boston Symphony Orchestra and used only a quality stereo amp with the Hafler ambience rear speaker connection.

As I mentioned before, these rear speakers were placed as far away as possible, in rear corners, facing the ceiling. This way I got as much time delay in the ambience as possible. It sounded excellent. The BSO was recorded with only 3 microphones with minimal mixing or other sound engineering tricks. The live quality of the sound was carefully maintained.

Dolby pro logic only uses the phase shifts inherent in the differences of sound sources recorded, be they direct sound or sound after it has bounced of the walls and ceiling of the recording hall. When these multiple reflections are recorded and played back in a pure manner you can extract a lot of great ambience. However, if the music is recorded in a studio with multiple equalizations and mixes to tailor a finished product much of the original subtle phase relationships are washed away.

When you play with the Hafler Quadapter ambience connection you will discover what I am talking about. Some recordings will sound great, some will sound terrible. You will discover that simple recordings made in live venues are the best. A good example of what I refer to are the 2 or 3 Jazz at the Pawnshop albums still available. A good ambience set up with them will knock your socks off!

http://www3.nbnet.nb.ca/fass/
http://home.indy.net/~gregdunn/dynac...QD1/index.html
http://www.enjoythemusic.com/Magazin...99/decoder.htm

I have used processors that created, electronically, time delay, and most of them sounded garbled and "tinney." The more modern PLII (a digitally processed enhancement to the orginal DPL) is not too bad on a lot of material. But, now, you are talking some serious $$. If you want some sometimes pleasant ambience enhancement go for the Hafler setup using two rear speakers connected in series with each other but out of phase to each other, and with the + terminal of one speaker connected to the left channel's positive terminal and the + terminal of the other speaker connected to the right channel's positive terminal.

Remember, ambience and 3-D effects depend on phase shifts and phase relationships amongst the speakers so it is important that all speakers be as similar as possible, especially in their crossovers. The wrong mix of different model speakers from different manufacturers can wash out ambience reproduction or make it sound terrible. Crossovers introduce a lot of phase shift in the sound.

You can't get blood from a turnip and if ambience is not recorded in the original material you can't retrieve and if it is manufactured it often sounds bad.
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Old 24th May 2006, 02:58 AM   #8
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richie00boy

> It sounds utterly rubbish

How did you use it - relative volume, through two rears, what sort of music?


Dick

You know a lot about it.

> However, if the music is recorded in a studio with multiple equalizations and mixes to tailor a finished product much of the original subtle phase relationships are washed away.

Yes, I expect it to only work on some material, eg largely acoustic material.

Thanks for the links ~ there’s no email on the www3.nbnet.nb.ca/fass/ - any idea how much their Ambisonic surround processor costs?


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Old 24th May 2006, 07:18 AM   #9
dnsey is offline dnsey  United Kingdom
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Look here for plenty of information on ambisonic decoding, including DIY and software decoding.
you can always do it the way we used to, with the rear speakers across the +ve outputs, with a bit of resistance to attenuate the signal , protect the amp, and minimise crosstalk on the fronts. Actually works quite well with the right material!
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Old 24th May 2006, 07:39 AM   #10
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I used PL-II just like normal surround. Put in some music CDs and had a listen. My system is set up so that all channels are the same loudness, and timbre matched.

The most noticeable thing I found was the front soundstage was appaling. This I put down to too much coming out of the centre speaker, so I told my system I didn't have one. This helped but still, switching from stereo to PL-II the stereo one gave a much more natural and coherent sound.

What was coming out of the rears just seemed to detract from the front stage. I only tried pop music though, but it was a mixture of styles with some stuff like REM for the more acoustic stuff, not the best though I know.
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