of Microprocessors

340 MUSICAL ApPLICATIONS OF MICROPROCESSORS
able to "show" the material to the user in several ways, such as music
language statements, standard or specialized music notation, graphs of
parameter variation, and actual synthesized sound. The editing commands
should be simple yet powerful and correspond to the method chosen for
showing the material to be edited. Composition by editing is a uniquely
interactive method for producing the kind of music the user really has in
mind rather than the best approximation that human dexterity or music
language restrictions allow.
Types of Display Devices
Historically, computer data displays have been divided into two types,
alphanumeric and graphic. The former is so named because it is designed to
display strictly letters and numbers, whereas the latter is more generalized,
being capable of displaying line drawings or in some cases gray scale images.
Theoretically, the alphanumeric display is a proper subset ofgraphic displays
because, after all, characters are nothing more than graphic shapes. However,
the very generality of graphic display means t~at character display quality or
quantity is likely to be less for equivalently priced devices. Because of this,
many computer users have the mistaken idea that a graphic display can never
do as good a job on text as can the alphanumeric type. A good-quality
graphic display, however, can certainly equal the performance of commonly
used, less expensive text-only displays with the added convenience of a
single-display device for both types of information.
Many display technologies have been used to display characters and
graphics. Although other methods have claimed to be superior to the cathode
ray tube in one way or another, the CRT remains the undisputed leader and
likely will continue to be until the late 1980s. CRT hardware is cheap
because of heavy usage in television, radar, and oscilloscopes. Display resolution
can be very high; a million resolvable points is routine and upward of 20
million can be done. The most commonly used CRT displays are fast,
capable of being completely updated in milliseconds. Although other
capabilities such as gray scale and color presentations are important for some
applications, the following discussion will focus on monochrome displays
capable of line drawings and alphanumerics.
A graphic CRT display can be thought of as a two-dimensional rectangular
surface. Any point on (he surface can be specified by giving X and Y
coordinat~. However, since digital logic drives the display, there is a limit
to the number of discrete X and Y coordinate values. The smallest increment
possible in either direction is commonly called a raster unit. A low-resolution
display may have as few as 200 raster units in each direction, whereas a
high-resolution display might have 4,000 or more. The electron beam that
creates the spot oflight on the screen does have a finite size, however. In fact,
it is often larger than the raster unit size in a high-resolution display. Herein