Sound Track                               

But how does a VR system know where its user's hand is? There are many possible approaches to this; one that is frequently employed in commercial system is the Polhemus. A Polhemus tracker is a small cube - one or two inches in height - that is connected to a heavier ``base station'' with thin cables. The movements of the cube in an electromagnetic field that the base station emits can be sensed and transmitted to a computer in some way.

Once the position of the cube is known, it can be used to control anything: commonly a position of a virtual object or of the camera, but just as well the pitch of a sound, or the hue of a color.

The system I have in mind consists of one or two motion trackers (the lighter and less obtrusive, the better), an audio output device, and a game console with some exchangeable storage medium, like those used for home videogames.  In the console is a small computer than can execute programs on cartridges.

For this small, portable platform, different applications can be bought:

• Musical instruments.  (Think Laurie Anderson rather than Nigel Kennedy.)  The user can ``pick up'' kinds of sounds by moving the trackers in and out of a certain space; after that, pitch and amplitude are modulated in the remainder of the space.  As a next step, the cartridge is replaced by a cable connecting the system to a PC; the PC runs a development system on which different ``instruments'' can be designed and programmed using a visual programming language.

• A training system for Tai Chi, or at least a system that accompanies these moves by appropriately soothing New Age music; or more ``aerobic'' systems where, for example, the speed of the music adapts to the speed of the person exercising.

• A one-player motion puzzle. The object of the game is to advance through a database of sounds.  The player advances by making the ``right'' motions with the tracker.  The game gives hints as to which motions it wants the player to make by making certain sounds, or by modulating base sounds in a certain way; and it gives feedback about the actual motions made.

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