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Whisper -- Online Lab Notebook  

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Whisper Overview

Simply put, the term "Whisper" refers to an approach to tracking. More specifically, it refers to a type of acoustic tracker that we believe will allow accurate body tracking in the presence of occlusions.

Nick Vallidis, a student whose dissertation focused on Whisper, provides the following summary.

The full title of my dissertation is "WHISPER: A Spread Spectrum Approach to Occlusion in Acoustic Tracking". That should give you some more ideas on what I'm pursuing. Whisper is just a name that reminded us (any time I use the plural in this document I am referring to my collaboration with Gary Bishop and/or Greg Welch) of how this system sounds when it is working. It uses a wide-bandwidth acoustic signal that is audible to people. WHISPER is not an acronym, but I write it using small caps (which I can't do here) to signify that it's the name of the system. The sound is not really like a whisper, well, unless you were to take out all the sounds that make the words and leave the hiss. Tune a radio to a channel where there is no station and you've got a pretty good idea of what it sounds like.

Taking the next set of important words we have spread spectrum. It is the spread spectrum ideas that are responsible for creating the wide-bandwidth signal. Except for a commercial system (that might not even exist) and a master's thesis that only used spread spectrum acoustic ranging as an application of the work, I don't know of any spread spectrum acoustic tracking systems. I have seen numerous references to this idea underwater (once again in the form of commercial systems that I can not prove the existence of), but not in air. However, this is not the area where my work is most valuable.

Occlusion is the problem I'm interested in [looking at] with WHISPER. There has been a large amount of work done to develop head-tracking devices to allow a computer to generate appropriate images for use in a head-mounted display. In fact there are two very good systems (UNC's HiBall and Intersense's Constellation) in existence that accomplish this goal. Now there is a rising interest in tracking the rest of the body (specifically the hands and legs) in order to draw accurate avatars in virtual environments in addition to many other uses. The big issue in this problem space is occlusion. The human body is capable of such a range of motion that there is no possible placement of transmitter and sensor pairs such that they would not be occluded at one time or another. Traditionally, this problem has been dealt with by either ignoring it, using magnetic tracking devices, or mechanical tracking devices. Optical and acoustic (all of which have been ultrasonic) systems have ignored this problem. They simply do not provide tracking results when there are occlusions. Magnetic tracking devices tend to have low update rates and high latency (due to the filtering they have to do) as well as accuracy issues in environments containing metals or ferrous materials. Mechanical devices have their own troubles including difficulty to don and doff, and tiring the user or at least altering their motions when using heavy mechanical arms.

The [approach WHISPER takes to] occlusion is the use of the diffraction of sound. By using sound of lower frequencies (audible frequencies), WHISPER is able to continue tracking a target when there is an occluder between the speaker and microphone, although with reduced accuracy.

-Nick (4/23/2002)

The term "Whisper" also refers to the prototype hardware system built to test this approach. More information about the prototype hardware can be found on the hardware page.

Two excellent sources of information about Whisper are Nick's defense slides and his dissertation. See below.

Defense Slides:  PowerPoint  |  pdf            Dissertation:  pdf

Last updated on:
Wednesday, July 17, 2002

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