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the recent 2-channel 3D sound formats and their viability for actual f
BearcatSandor
post Jul 13 2011, 18:54
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(I posted this on the [SurSound] list but i'd like to get more takes on it.)
I'm interested in the idea of 360 horizontal surround sound but more interested in a sphere recreation.
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Folks,

I've been reading up on the various proposals for 3D sound from a set of
stereo speakers. The 3D Audio Alliance is working on such a system.
Astound Surround is getting ready to market, Edward Choueiri is working
on the same idea (see:
http://www.studio360.org/2011/apr/29/adventures-3d-sound/ ) and there
are others. I used to have a Carver pre-amp with Carver's Holography
button but i could never get it to do much.

Has anyone heard a truly 3D/360 surround effect from 2 speakers using
this stuff? Ever heard a fly buzzing around your head, or an object in
the back far-left of you or some such? Can any of this do as good of a
job as Ambisonics? Is all of this just related to head transfer
function mathematics?

I've listened to some of the headphone applications of this like
binaural and whatever these folks are doing here
http://www.3d60.co.uk/index.php That demo on the 3D60 page sounds
really cool, however nothing ever sounds like it's more than a foot from
my head and nothing is ever right in front of me. Why can't they create
an effect of something coming from a long distance away and getting
closer and closer behind me? If it's all related to head transfer
function you'd think you could create any sound your ears can hear.

I'm looking at my audio system building options and I'd love to throw my
money/space decor at 2 really good speakers and a good 2 channel pre-amp
instead of 12 speakers in an ambisonic system with all the associated
electronics.

Any thoughts on all this 3D through 2 channel stuff?

Thanks,

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Further thoughts i've had are that i don't understand how this works given crosstalk, the differences in everyone's rooms, the differences in everyone's hearing, outer ear/head shapes ect. If this is relying on head position to create the same sounds as they would be after HRTF isn't that one hell of a sweet spot?

In review (and i've only read one) of the system put out by the 3D Audio Alliance the person listening to it described hearing the sound of a dog off to his back right. From two speakers. That's really awesome..if it works.

Bearcat


--------------------
Music lover and recovering high end audiophile
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dc2bluelight
post Jul 29 2011, 09:30
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The first problem in 2 speaker surround and 3D audio is that both ears hear both speakers. One common approach (and the one used by Carver, BTW), is to produce a crosstalk cancellation signal that's mixed into the opposite channel. In the late 1970s there was a brief rash of devices that did this, the Carver Sonic Holography Generator was one. Carver used a sample from one channel, inverted it, and delayed it with an all-pass filter network, and some response shaping filters, then mixed it into the opposite channel in hopes it would cancel the sound from, for example, the left speaker arriving a the right ear. It worked quite well, but you had to eliminate all possible early reflections with a combination of spacing the speakers away from walls, and applying absorption on any reflecting surface between you and the speaker. Unless all early reflections were reduced, the effect was inaudible. But when it worked, it worked startlingly well. However, there was no way to place a sound behind a listener in any predictable way, though you did get a sense of 3D audio.

Another approach was to develop the crosstalk cancellation signal by deriving it from an L minus R signal which was then eq'd and delayed. The idea was that the "enhancement" would not be present for mono signals. While that was true, the system had the undesirable effect of placing vertical groove distortion from vinyl records out in the room with you. That negative effect notwithstanding, it also did present a somewhat 3D soundfield, though again, positioning a sound predictably wasn't possible.

Both the Carver concept and the L-R concept had by necessity a very tiny sweet spot, and the 3D effect collapsed if the listener's head was moved even an inch out of center. The L-R idea did recover a lot of hidden reverberation and room ambiance though, even if heard outside the sweet spot.

The creators of Astound Surround claim to use technology that was developed through brain research, that it has a large sweet spot, and works incredibly well for every listener. You can download the post-processor version in plug-in form and install it as a free demo. It works with iTunes. However, in my tests it falls very short of a desirable effect. The plug-in has a control to vary the amount of "effect". In my personal tests, I kept backing the control down until things sounded good, only to find I'd turned it off completely. Also, the demo has a highly irritating "nag" function that will get you annoyed enough to uninstall it, almost without regard to what it does. I felt it had too much impact on the balance of the mix, making radical changes like burying the vocal for example, rather than just providing a surround effect. Their demo clips, which differ in that they were mastered using Astound Surround technology rather than applying it as a post process, are interesting, but not as "astounding" as one might wish.

The big issue with binaural recordings is that they must be made with mics in simulated ears (pinna at very least) in a head with a chest cavity, both of which must be similar in acoustic density to that of a real human. This makes recording cumbersome. Using a real head and ears is possible, but has its own issues. The real problem is that no two human ears are identical, particularly the pinna shape with its folds and curves. This means that everyone has a slightly different head related transfer function, and is why binaural recordings don't always work well from one listener to the next. In fact, creating a universal HRTF (head related transfer function) is probably impossible because of listener to listener variations. Binaural recordings also cannot be played well on conventional speakers, and only work well on headphones, typically open on-ear types.

So far, attempts and two speaker surround have all fallen a bit flat. It's a compromise at best, but if you realize that, some two channel surround can be though of as an improvement over simple stereo. At least there's some attempt at surround placement, and though far from the performance of real surround speakers, can provide listeners with at least an improved experience. But, in practical terms of real surround, most phantom images are fragile at best. "If you want a sound there, you have to put a speaker there."
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