Hi mtry, how are you?
I need to embellish a tad.
There are three mechanisms of adaptation to aural stimulus..
1. Automatic gain ranging..the ability of our hearing mechanism to adjust to the level of the sound. One can easily imagine sitting in a quiet room, listening to soft music, hearing all the details...then, going outside and revving the Harley for five minutes...then, going back into the room, listening to the exact same music at the exact same level, but finding that it takes half an hour for your hearing to become sensitive again to that which you previously enjoyed.. This is also what happens with visual acuity vs light intensity..
2. Frequency based gain ranging..the annoying thing that happens when one sets an eq 15 khz band just a bit higher to make the highs more brilliant...then, an hour later, it isn't as sizzling as it was when you first adjusted it..you have adjusted to the change in level a bit..
3. Localization stimulus interpretation. In the natural environment, we use timing delay and intensity levels to interpret where a sound source is in the space around us. When we are presented a horribly flawed soundfield, as stereo speakers by design project, the human brain is forced to adapt to that incorrect stimulation to "figure out" where the audible images are coming from. Modification of that flawed soundfield forces the brain to adjust it's interpretation algorithms, to re-establish localization.
It is number 3 that I point out is not understood in the world of audio..
So far, I have seen very little attention paid to the complex relationship between those two items and stereoscopic reproduction.
To all who read this..an experiment.
First, you need a soundcard that has only one D/A converter. The output section must mux the analog output. This forces the channels to have an 11 uSec delay..my soundcard has a built in 11 uSec delay on the left channel.
With headphones, listen to either a wave file or directly off a disk...(not an mp3 if you can avoid it, as losses can get in the way..)
Is the image centered??? Mine isn't. It is slightly to the right, an artifact of the 11 uSec delay..(yes, when I reverse the phones, the image does move to the other side..and yes, when I hot wire the channels together to force mono, it is in the middle regardless of headphone orientation.)
To center the image, I have to use the balance control. When I do this, the image indeed shifts to the center, but there is a problem...when I do this, the image goes all to "heck". It becomes less focussed...I use the mouse to adjust it...click on the control, turn your head away, sweep the control side to side, both extremes, then bring it back to center the image..suprise, it always requires the same level shift..and, all the frequencies DO NOT center at the same time..
This is because we do not have the same sensitivity to IID shifts across the audio band. In point of fact, both ITD and IID sensitivity vary across the entire range of human hearing, and they do not track..meaning, it is not possible to use IID to correct for ITD errors on more than one frequency band simultaneously.
Given the fact that a 1 foot diameter source localization error band, center stage ten feet away...is bounded by IID levels of .06dB, and ITD levels of about 5 uSec, do you really think anybody is looking with test instrumentation...what we can hear???
From what I have read so far, from the experts..they do not have much more than a rudimentary understanding of localization issues.
Dunlavey: hmmm..
If I trick an individual into believing that he has seen a ghost...have I proven that ghosts do not exist???
I expect far more scientific work than that. All he has proven is that the "test instruments" can be fooled.
As for papers...hmm..several things are required..
1. A paper explaining the mathematics of localization of a sound source, with emphasis on differential ITD and IID as it applies to image reconstruction. This is consistent with radar and sonar theory. It will establish the boundary math for reconstruction, ie, it will establish the constraints necessary for reconstruction spacial fidelity. For example, to establish a virtual image 10 feet away, on axis, to within one foot, interchannel ITD must be less than 5 uSEc deviant, and IID less than .06 dB deviant. I could plant the 2 axis plots here, but I do not believe anyone will understand them. (my apologies if that is an incorrect statement, I am working to create 3-D graphs which will encompass all of it.)
2. A paper establishing the susceptibility of present electronics to these levels of deviations..this paper has to establish the test methods required to spot these susceptibilities as well as the equipment required, including the equipment specifications. (note that current state of the art load resistors are incapable of presenting the load parameters necessary to do this). I've explained some of these susceptibilities and test methods on forums, and with various individuals over the years...nothing has been done..
3. A paper which properly tests human hearing capability with respect to well defined differential ITD and IID stimulus, over the entire range of human hearing...until these parameters are controlled up to and including the actual cones, the testing done will be meaningless.. garbage in, garbage out.
All of this, is of course, unnecessary for the the vast bulk of the human race, as there is a very small percentage of humans who even care about the coherent soundstage image to that degree.. I, for example, just enjoy the artist, or the movie, without worrying about confining myself to one point in space..others choose to do so, and that is just fine..
Cheers, John
PS..as you can see, the final proof of my assertions takes a long path, requiring advancing human audibility understandings and electronic test methods..I always welcome the "show me proof" stance you take..this is how it should be..
