Hi Richard. Good to hear from you. (More below.) On Feb 28, 2011, at 3:39 PM, Richard Lee wrote:
> I had a page on ambisonia.com with a lot of what has been discussed and was > trying to find it before Jerry, who provided much of the info, noticed our > discourse. > > Unfortunately ambisonia.com seems to be unobtainium. I know this has been > asked before but is ambisonia.com archived anywhere and will it be available > again? > > I'd like to thank Jerry for posting even more details of the history of this > fascinating subject. Transaural (ie Jerry) are definitely the originators of > the 2 most important concepts; > > speakers close together > and > having the bass units further apart. > > These turn xtalk cancellation from a fancy technical exercise to something > which promises better performance. > > Robert Greene was the first to suggest a physical barrier but I don't think > having your nose stuck to a vertical mattress is as elegant as the Greene/Lee > neckbrace. > > One of the points I made on my webpage is that x'talk cancelled "binaural" > over speakers is a SURROUND technology needing only 2 'speakers' but the > encoding is complex. Answers to the encoding question are invited. > > I'll second his reminder of Dr. Duanne Cooper's role in the birth of > Ambisonics. His name appears on the patents and Michael always spoke highly > of him and his contribution. > > Jerry, Eric Benjamin mentioned that he'd found a commonly used set of HRTFs > (KEMAR I think) flawed. I think this is why he prefers spherical head > models. Have you a recommendation for a publicly available HRTF set? Unfortunately I can't make a recommendation from the various publicly available HRTF sets because I'm not familiar with them. We were lucky enough to have access to a rather sparse but sufficient set of HRTFs from a private source that I trust. I mentioned in the original post that there were a few obvious glitches that could be sort of "puttied over" but otherwise I guess the proof of their suitability was really in the results. I compared the results using our HRTF set to those of a sphere model. (FWIW, Fortran code for the sphere model was published in an AES conference paper by Cooper and I in around 1979 or 1980 [thank you, Lord Raleigh, for doing the hard work]. There is a sign error in it, however, and the stopping criterion for the series summation is suspect. Later, Dick Duda approached me at a conference and I subsequently gave him corrected Pascal code which he then used in his own research. Also, he and Bill Mertens published a very nice piece on a spheriodal head model, probably in JASA.) My recollection is that the sphere model matches the real-model heads pretty well up to about 2 KHz for a crosstalk canceller, and up to about twice that for a speaker spreader. I think some of the differences between the two models tend to cancel out in a speaker spreader due to similar features appearing in the numerator and denominator parts of the defining equations, especially if the angle differenc e between the real speakers and the virtual speakers isn't too extreme. I used to know Steve Orfield in Minneapolis who used to (maybe still does) run a kind of sound quality testing business. Steve was kind enough to lend me his facilities and a technician for a day so that I could measure HRTFs on his B&K head. The main thing that I learned is that this takes more than a day. We spent a great deal of time trying to get the wobble out of the turntable as measured by a plumb bob hanging from a very tall step ladder. What is the time constant of a plumb bob on a long string? Plus, the turntable was not a fine B&K unit but a consumer (home use) unit with a piece of plywood attached with degree tics in one degree increments. The turntable had a very small amount of wobble but it was enough to make trying to get say < 1/2 wavelength at 20 KHz a real challenge. I never trusted those results and never used them. Some years later I gained access to a KEMAR. I can't remember which model of ear(s) it had but there was a visibly thin area in the conch area that would almost certainly cause accuracy problems. I contacted the Knowles people about this and they sent another ear, but it was exactly the same. I mean, you could see light shining through it, it was so thin. I applied a bit of rubber cement to the back side to thicken it a little. I don't know if Knowles' mold had worn over the years and I wonder how widespread this is. I kind of thought at the time that it could be the source of some odd things that I noticed in the upper frequencies of some KEMAR HRTFs. This is sort of important because the nonminimum phase nature of some HRTFs is due, I believe, to the focused reflection of sounds arriving from generally frontal directions back into the ear canal. In a "normal" near, I suspect that this reflection is stronger than the direct sound which would cause a nonminimum phase respons e, and the frequency range for the quarter-wavelength one-way path difference required to cause a notch is just right for the notch that is seen at around 8-10 KHz. Jerry _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
