From: cs_d476@kingston.ac.uk (Leaback P D)
Subject: Re: TECH: of costs and Convolvotrons.
Date: Thu, 12 Mar 1992 19:11:49 GMT
Message-ID: <1992Mar12.191149.2272@kingston.ac.uk>
Organization: Kingston Polytechnic



>Here's a reference for a system that did all this with even less:
>no pinnea delay lines, just lowpass filters.  Subjects easily
>"found" a virtual sound source in a large room, to an error of
>15 cm!  Of course, head tracking was used.

I think you are being (unintentionally) slightly misleading. The paper
you give a reference to concerns itself with seeking out a virtual sound
source. Being able to seek a sound source is not a good test of the
ability to simulate localisation cues. For example, varying the volume of
sound presented over a single earphone is sufficient to seek out a
virtual sound source (if coupled with a head tracking device).

I have simulated a filtering process very similar to the one you
described within software and have found that its ability of generating
localisation cues is inferior to even a very small FIR filter. The reason
for this becomes apparent when one analyses what cues are being
presented.

The filter system you describe fails to simulate

	* The frequency dependant interaural time delay function.
	* The frequency dependant head transfer function.
	* The high frequency Pinna filter function.
	* Shoulder bounce.
	* The inverse of the headphone filter function.

A small FIR has a good stab at ALL the localisation cues. I gave up on
the approximation techniques because I came to the conclusion that even
very modest DSP chips can support large enough FIR filters to produce
effects that are more impressive than the approximation techniques could
ever achieve.

However, one advantage of the approximation techniques is that one does
not need access to a Head Related Transfer Function which are not easy to
come by!

Regards,

         Pete Leaback.
	 
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