of Microprocessors

LOW-COST SYNTHESIS TECHNIQUES 761
interesting to novices) that it is frequently used as a programming example in
elementary microprocessor technology courses. The basic idea is to provide an
addressable flip-flop, either alone or as one of the bits of an output port,
which an assembly language program can set to a one or a zero at will. The
logic level voltage then is either connected to an on-board transistor switch
driving a speaker or an external amplifier/speaker.
To produce a pulse of a constant frequency, the student is instructed to
write a program to set the flip-flop to a one, immediately set it to a zero, and
then enter a delay loop, which perhaps decrements a register until it becomes
zero. After the delay, the program would jump to the beginning for another
pulse. When the student has mastered this, techniques for making the delay
variable, perhaps by reading the delay parameter from an input port, are then
introduced. The utility of subroutines might be illustrated by altering the
program for square or rectangular output using a single timing subroutine
called with a different delay argument for the high and low portions of the
waveform. Depending on the course, multitask programming might be
introduced by having the student write a program to simulate two or three
such oscillators, simultaneously producing different frequencies.
While this seems trivial compared with what has been covered earlier in
this book, there are legitimate applications beyond being a teaching aid. In
1976-77, three-voice music programs using timed loops were state of the art
in microcomputer music. Even today on many machines, such as the IBM PC
and Apple II, it is the only viable way to produce sound without hardware
add-ons. The musical Christmas cards and car horns mentioned earlier are
really nothing more than minimal 4-bit single-chip microcomputers maskprogrammed
to play square-wave note sequences from stored scores using
simple timed loops. Almost any computer program, even business programs,
can benefit from the judicious use of sound, and usually a few dozen bytes of
extra code are all that is needed.
Besides playing buzzy little melodies, single-bit timed-loop routines
can produce a wide variety of sound effects as well. White noise is readily
generated by programming one of the shift-register random-bit generators
described in Chapter 15 and sending the bits to an output port. Some control
over the noise spectrum can be secured by running the generator at different
sample rates. Laser zaps and police siren effects can be produced by rapidly
varying the delay parameter in a timed loop tone routine.
Figure 20-1 shows a simple but very effective "strange sound"
generation program. The basic idea is to use two 16-bit simulated registers
where the second (REG2) is successively added to the first (REG1). Each time
REG1 overflows, the value of REG2 is incremented. After each inner loop,
the lower half of REG1 is written to an output port. Each bit of this port will
have a different sound sequence and more can be had by modifying the
program to output the upper half of REG1 instead. The sounds produced by
this routine are really wild and well worth the hour or so required to translate