of Microprocessors

SIGNAL ROUTING 275
if all subsystems are purchased. The first question to ask is: What digital/
analog interface boards are available to plug into my computer and are they
cost effective? Probably the best system from this standpoint is the S-lOO
type closely followed by the Apple- and IBM PC-compatible computers.
Unfortunately, many analog interface boards emphasize analog-to-digital
conversion more than the digital-to-analog, which is needed for synthesis
work. At the time of writing, there was at least one cost effective 16-channel
DAC board available for S-lOO systems. As microcomputer-controlled
synthesis gains momentum, boards with more channels at a much lower cost
per channel are sure to become available.
Another feature that is virtually mandatory in a control computer is a
so-called real-time clock or interval timer. This hardware feature allows the
computer to be cognizant of the passage of time without regard for the
execution time of the software. This capability is necessary for accurate and,
above all, repeatable timing of the music performance. If multitrack
recordings of the music are being made, it may be desirable to have a realtime
clock that can be externally driven or synchronized. Since many systems
use the ac power line as the timing reference, it is not difficult to record
power line hum when the first music track is recorded and then use playback
of that track to synchronize subsequent recordings. The 6522 type of parallel
I/O interface IC found on most 6502- and some 68000-based computers can
easily be programmed to generate a timing reference signal output and later
accept it as input to drive one of the two timers on the chip.
As anybody who has done real-time control programming knows, it is
not possible to have too much speed. An adequately fast computer simplifies
programming and allows many otherwise important time factors to be
ignored. An amply fast machine allows normally inefficient but convenient
programming techniques such as high-level languages to be used for many of
the control functions. Use ofa 16-bit computer would certainly be desirable.
Computer Inteiface Box
The biggest varIable in the system, however, is the computer interface
box itself. This would be expected to consist of a number of DAC channels,
each connected to a standard jack.
The first question that arises naturally is: How many DAC channels are
required? A maximum number is easily arrived at by totaling the control
inputs on all of the modules in the synthesizer. Thus, it would be possible for
the computer to manipulate every control input in the system, which would
be the ultimate in flexibility. Of course, many modules have two or more
control inputs connected essentially in parallel so computer control of multiple
parallel inputs would be redundant. For example, many VCOs have three
control inputs: a primary frequency control input, a secondary one for injection
of vibrato, and a third normally used for a constant transposition voltage.
Typically, the computer would supply the primary and transposition