of Microprocessors

324 MUSICAL ApPLICATIONS OF MICROPROCESSORS
Fig. 10-2. Two-axis joystick construction. Source: Efectronotes Musical Engineers
Handbook, 1975.
Joysticks can be readily extended to three or more degrees of freedom.
The third axis can simply be twisting of the handle as it is moved left and
right, forward and back. To add such a capability to Fig. 10-2, one merely
replaces the handle with a third pot whose shaft becomes the handle. The
fourth axis could be a mechanism for sensing up and down motion of the
handle. One might even conceive ofa fifth output that would be proportional
to squeezing pressure on the handle!
Graphic Digitizer
Although not a new input device, graphic digitizers have recently been
reduced in cost sufficiently to suggest their use as a dynamic input device.
The digitizer acts somewhat like a two-dimensional ribbon controller and
typically consists of a flat padlike surface perhaps a foot square and a special
stylus that the user holds like a pen. When near the surface, the device
outputs the X and Y coordinates of the pen tip position to an accuracy of
0.01 inch or better. In one mode of operation, X and Yare sent out 50 or
more times/sec, allowing the host system to determine not only the pen
position but its velocity. In another mode, ouputs are only generated when
the user presses the pen on the pad, thus allowing precise coordinates to be
entered. A sheet of paper may be placed on the pad surface and anything
printed on it entered into the system. Digitizers are also available that simply
respond to finger or pencil pressure. These can be very inexpensive (under
$100) but are generally not as precise as those that use an electromagnetic
pickup stylus. The possibilities in a music system are endless, ranging from a
two-dimensional keyboard to a quick method of inputing sheet music. Even
three-dimensional digitizers are available. The position of a pen point in
three-dimensional space (as large as 36 inches on a side) is output as X, Y, and
Z coordinates!