Greetings All, Regarding mobile recording rigs, their seems to be belief that any recording device or preamp that uses rotary pulse encoders in lieu of potentiometers is intrinsically dead accurate across channels. For this reason, a lot of folks avoid using recording devices with conventional pots for their Ambisonic recorder. I personally prefer NOT to bring my laptop with outboard hardware (e.g., MOTU Traveler) into the field unless absolutely necessary. I really like the sound and quality of my Edirol Roland R-4 Pro. Very low noise preamps (purportedly better than the R-4 without "Pro" suffix) and it's super easy to use. But it has those horrible pots we abhor. Actually, the concentric level adjustments on the R-4 have a conventional pot on the inside and what appears to be an encoder on the outside. With the pots fully CW, the gain adjustments are uniform across channels. Gain is displayed digitally on the display when using the outer gain knobs. I confirmed accuracy across channels with a balanced output calibrator I built. Briefly, my "calibrator" is nothing more than a THAT Corp. balanced line driver, tone generator, and low-noise attenuator using Vishay resistors. Low-noise circuitry is needed because we're dealing with mic-level (mV) signals. The balanced line driver precludes the need for a pseudo-balanced (really single-ended) signal feeding the R-4 Pro's balanced (XLR) inputs. All in the name of low noise... even with short cable leads. In the end, this calibrator could be of benefit to those who may not know whether their digitally-controlled attenuators are truly equal across all channels. I've seen researchers assume the voltage or attenuation at a device's port is exactly what their MATLAB (or whatever) codes says it should be; they didn't consider the effects of buffer resistors, loading, etc. These effects are generally not accounted for in the software, on a computer screen, or other digital displays. Hardware calibration isn't a bad thing at all! RE plane waves: I saw the recent discussions regarding waves and wavefield reconstruction. I'm not one of those persons who solves Legendre polynomials in my sleep, so I avoided putting in my 2 cents worth. But I do have a question/comment. Some years ago, I recall seeing articles on psychological warfare and an acoustical "laser" that had commercial application, too. I can't find those references (or my folder with the related JASA article, etc.), but now I question what I read. Anyway, it seems that a sound arriving at the ear from a coherent sound source would have to be "planar" but of fixed, or finite, area. Waves emanating from a source or many sources (such as a water fall) approximate plane waves as they become distant from the source. But the "area" as well as distance of plane waves would be quite large (infinite if we assume the wave is shaped like the surface of a sphere). Because of differences in how planar waves can be generated, wouldn't there be differences in how these waves diffract around and interact with the head, pinnae, etc. In other words, "head shadow" would vary even when the wave generating sources' direction and angle remained the same. In some ways, this would be akin to comparing shadows from conventional light to laser light (assuming no reflecting surfaces to create diffuse light). Just some thoughts... and probably has some bearing on localization experiments performed in near field versus far field listening environments (??).
Thanks to everyone here for your help, ideas, and suggestions that have propelled me along in my research endeavors. My latest explorations have involved deconvolution and swept-sine sources using my TetraMic. (I use a KRK 9000 monitor as my source along with a battery operated "high-end" amplifier. Still trying to create the Holy Grail of "real-world" stimuli for hearing science and cochlear implant research. Eric -------------- next part -------------- An HTML attachment was scrubbed... URL: <https://mail.music.vt.edu/mailman/private/sursound/attachments/20121002/8de425de/attachment.html> _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
