of Microprocessors

LOW-COST SYNTHESIS TECHNIQUES 759
generators can, with proper programming, produce a complete range of note
frequencies. In many cases, three or four simultaneous notes are possible and,
in rare cases, timbre variations as well. For popular computers with minimal
native sound generation capacity, there are usually a number of independently
manufactured add-ons available. These typically consist of a plug-in
synthesizer board and matching music software. The sophistication of these
boards ranges from 3 square-wave voices to 16 fully programmable waveform
generators. Frequently, the software includes a suitably simplified interactive
graphics score editor and perhaps an instrument definition editor as well. A
couple of products even include a music keyboard for live as well as
programmed performance.
Techniques for Low-Cost Synthesis
The techniques used in low-cost sound-generating devices are not really
differen.t from those studied so far. Instead, the emphasis is shifted from
considerations of flexibility and quality to those of cost and distinctiveness.
In many product designs in which sound generation is just a small part, free
choice ofa synthesis technique might not be possible; available resources may
have to be used instead. Thus, strange and ingenious circuit designs with
limited general applicability sometimes evolve. Here, a few of the more
straightforward low-cost synthesis techniques will be explored.
Interface Chip Timers
Many microprocessor-driven devices use one or more "peripheral
interface chips" (PIC) in their design. A high-volume, relatively simple
product may even use an all-in-one-chip microcomputer that also includes
PIC functions. Most modern PICs include one or more programmable
timers. These are normally used to simplify timing-related programming but
can also sometimes be used as oscillators for sound generation.
Such timers are usually programmed in one of two ways. A single-shot
timer for example typically has a 16-bit "counter register" that may be
written into under program control. After writing, the register counts down
at a fixed rate and generates an interrupt and perhaps an external pulse when
it reaches zero. The timing reference is usually the microprocessor clock,
which sometimes is passed through a programmable prescaler where it can be
divided by a power of two before clocking the register. The counter register is
normally readable as well so that the microprocessor program can determine
how much time remains in the current interval. A free-running timer is
similar except that it has an "interval register" that is separate from the
counter register. When the counter reaches zero, it is automatically reloaded
from the interval register and starts timing a new interval. Thus, variations
in interrupt response time do not affect cumulative long-term accuracy such
as keeping track of the time of day from I-sec timing intervals.