of Microprocessors

620 MUSICAL ApPLICATIONS OF MICROPROCESSORS
Fig. 17-20. Digital synthesizer organization
with a "universal interface" that can easily connect to any kind of control
computer. Furthermore, interpretive high-level languages, such as BASIC,
can be used to control the oscillators and other intelligent digital music
modules.
Modular Digital Synthesizer
Up to this point, we have been discussing the digital implementation
of analog modules in which an analog output is retained. These outputs are
then interconnected and processed further in the analog domain just as if they
had originated in an analog module. In a modular digital synthesizer, all
signal generation, processing, and interconnection are done in the digital
domain. Only the final two- or four-channel audio output is in analog form.
One advantage of an all-digital synthesizer is the elimination of vast quantities
of DACs. Another is that multiplexed operation of the component
modules makes available very large numbers of module functions at an
attractive per-module cost. The biggest advantage in the author's mind,
however, is complete interconnection flexibility. As we saw in Chapter 8, a
generalized analog-switching matrix for even a moderate number of modules
can become very large indeed. An equivalent digital matrix using read/write
memory is compact and economical. At this point in history, there are as
many different digital synthesizer organizations as there are digital synthesizers
in existence. The organization in Fig. 17-20, however, represents an
ideal that most of all them approach to some degree. The entire system
revolves around two major memories. The control memory contains all operation
parameters for all of the modules, including interconnection information.
The control computer writes into the control memory, while the modules
read from it.