of Microprocessors

4
Direct COHiputer
SynthesisMethods
Although voltage-controlled synthesis methods are very flexible and very
popular, they are not the last wotd. Even though much work with direct
computer synthesis predates widespread application of the voltage-control
concept, it will be described here as an alternative to voltage control. As a
matter of fact, the earliest extensive direct computer synthesis program
actually used imaginary "modules" that corresponded closely to the voltagecontrolled
modules described in Chapter 3.
Direct computer synthesis and digital synthesis in general runs the
gamut in sophistication from buzzy one-voice "toy" programs to serious
ptOfessional program packages. Of course, this is not a unique situation,
since analog synthesizers and even standard musical instruments find the
same range of application. Throughout this chapter and indeed in all of
Sections II and III, emphasis will be on techniques suitable for high-quality
synthesis. The application of simplified versions of these as well as other very
elementary techniques in low-cost music systems will be addressed in Section
IV.
Limitations of Voltage Control
Let us first describe some of the very real limitations of voltagecontrolled
equipment and techniques. Accuracy is one problem. Although it
has been shown that extreme accuracy in the final result may be undesirable,
accuracy in the intermediate stages, which may then be degraded in a controlled
manner, is very desirable. For example, in the FM patch just described,
if the spectrum is harmonic and frequency components are reflected,
they mix with unreflected frequency components, and the resulting
amplitude is a function of the phase relationship between modulating and
modulated signals. However, since the oscillators are not exact, they will not
be at exactly the frequency ratio desired. The result is that the phase between
the two is drifting continuously, and thus the spectrum is changing continu-
101