Re: HRTF: azimuth to ITD (Brian FG Katz )

Subject: Re: HRTF: azimuth to ITD
From:    Brian FG Katz  <brian.katz@xxxxxxxx>
Date:    Mon, 18 Feb 2008 21:13:34 +0100

Dear John, This is a very important question, and lead to a very fundamental question: what is the ITD? There are a variety of opinions on the matter going around, concerning difference in time of arrival or time delay for the most coherence between left/right signals. The addition of frequency variations is also an important questions. Can you actually say there is sense in a measure ITD between 2 ears at 80 Hz? The wavelength a this frequency is enormous, human localization ability being negligible, and finally the majority of HRTF measurement systems don't have decent response at these low frequencies (as well as the rooms response), so many (like CIPIC) simulate the results. At higher frequencies, the measured ITD values, and contour pattern (spatial function one could say) vary greatly depending on the method used to calculated the ITD. For example, with the max(ITD) azimuth varying from 60° on front to 120° to the rear. The problem, as such, is both a signal processing and psycho-acoustic. While many mathematical methods can be derived to calculate a time-difference, there are in fact very few studies on the correlation between perceived ITD and measured ITD. The few that have been done have focused on the frontal plane, where there are less differences between methods. While I realize that this response doesn't really answer your question, I think it is important to highlight that in fact here is not a real consensus on the issue, even though many take the idea of ITD as a given and simple process. We are currently working on a publication for which much more information will be available shortly. Regrettably, I can not diffuse the results before publication. Cordially, -Brian FG Katz --- Brian FG Katz, Ph.D Perception Située LIMSI-CNRS BP 133 F91403 Orsay France tel. (+33) fax. (+33) direct.(+33) e-mail Brian.Katz@xxxxxxxx web_theme: web_group: ------------------------------ Date: Thu, 7 Feb 2008 13:24:12 -0500 From: JOHN WOODRUFF <woodruff.95@xxxxxxxx> Subject: HRTF: azimuth to ITD i am doing some work that involves head-related transfer functions. i have been using the MIT and CIPIC databases and have noticed some unexpected results regarding interaural time delay. for example, in the case of the MIT database (full,L) at 0 degrees elevation, the time delay associated with the HRTF of 70 degrees azimuth is smaller than that of 65 degrees azimuth for 80-150 Hz. similarly, for 6.5-8 kHz, the time delay for 70 degrees azimuth is larger than that of 75 degrees azimuth. i have noticed similar trends in the compact MIT database and in the CIPIC database. i can imagine that HRTF measurements taken from humans may have slight discrepancies due to subtle head movements by the subject, but can anyone explain why measurements using KEMAR would have non-monotonic functions relating azimuth and ITD? does what i'm seeing reflect physical time delay or is it possibly an artifact from the measurement process or signal processing involved? thanks very much, john ******************************** John Woodruff Perception and Neurodynamics Lab Ohio State University ------------------------------

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