of Microprocessors

590 MUSICAL ApPLICATIONS OF MICROPROCESSORS
Analog Module Replacement
In many cases, digital techniques and circuitry can replace analog
circuitry wi th the substantial advantages that have been described earlier.
One of the biggest potential advantages in this role, however, is the ability of
digital logic'to multiplex itself among numerous channels. Modern logic is so
fast that in many cases it would just be loafing when used to implement a
single function. The surplus speed along with a small amount of memory can
instead be used to simulate several independent modules using only one set of
somewhat more complex logic. The per-function cost is then reduced, often
considerably below that of an equivalent quantity of analog modules. Digital
oscillators, which will be discussed extensively in the following paragraphs,
lend themselves well to multiplexed operation. It is not difficult to have one
logic board perform the funnion of 16 or more functionally independent
oscillators!
Simple Digital Oscillator Module
Enhanced frequency accuracy and greater waveform variety are the
leading advantages of a digital oscillator over a conventional voltagecontrolled
type. The oscillator we will be discussing initially accepts a
single-word digital input that controls frequency and produces a single
analog output. The oscillator may be used as a stand-alone oscillator module
or may be part of a larger voice module.
The most fundamental part of the oscillator is the variable-frequency
source. Basically, all digital oscillators share one common trait in this area:
they take a fixed, crystal-controlled, high-frequency clock and divide it down
to a useful range under the control of a digital word. There are at least four
distinct ways of doing this, each with a set of advantages and disadvantages.
For the purposes of illustration, we will assume that the goal is to generate
frequencies in the audio range with infinite frequency resolution and perfect
short-term as well as long-term frequency stability.
Divide-by-N Frequency Generator
The most obvious frequency generator is the divide-by-N counter. The
circuit merely accepts the fixed frequency reference, F, a digital number, N,
and produces an output frequency of FIN Hz. Any number of logic schemes
can be used to implement the divide-by-N, and they are all quite inexpensive
and easy to understand. Figure 17-1 shows one that requires only counter
blocks (plus one two-input gate) and is indefinitely expandable. The idea is
to load N into the counter at the beginning of an output cycle and count up
to all ones on successive clock pulses. When the counter overflows, N is
loaded again and the cycle repeats. The actual division factor is 2 M - N , where
M is the number of counter bits. If the twos complement of N is supplied,
however, the circuit indeed divides by N.