■ FIGURE 4. Block diagram of a hybrid
of the keyboard to a MIDI-to-CV converter and take the control voltage,
gate, and trigger signals from it instead.
You may protest that the cost or
complexity of this hybrid approach is
too much to bear. Well, in fact, Nuts&
Volts Magazine published just such a
do-it-yourself version several years
ago placing it well within reach of
most budgets. See the Resources
sidebar for details.
Let’s consider the VCO next. A
versatile VCO should put out a variety
of signals. Refer to Figure 5 which
shows some typical synthesizer waveforms. From top-down these are: sine,
triangle, ramp, square, and pulse. It’s
■ FIGURE 5. Some typical analog
to design oscillator
modules to generate
all of these, and many
different circuits for
them have appeared
over the years.
The tricky part
isn’t the output, but
what happens at the
control voltage input.
Most analog music
follows what’s called a
1V/octave response. In
plain language, each
causes the VCO to double in frequency or to create a sound exactly one
octave higher. Due to this doubling
phenomenon, the response is, in fact,
a base-2 exponential. This is quite
different from the linear VCOs you
may have seen before. Refer to the
table in Figure 6 which compares the
responses for two hypothetical units.
One advantage of going with the
1V/octave control input is that the
resistors appearing in the keyboard
diagram of Figure 3 (or the D-to-A converter implicit in Figure 4) can be equal
valued. Pondering Figure 6, you’ll soon
realize that if you went with a linear
response VCO, the resistors would
have differing values and probably very
non-standard ones at that. In fact, they
would all no doubt have to be trimpots
just so the thing could be properly
tuned to avoid sour notes. Ugh!
By the way, the bottom waveform
in Figure 5 is of noise. This isn’t created by the VCO, but rather by a separate
noise generator circuit. A good noise
source is essential in a synthesizer. It
can be used to create instrument
sounds like drums, hand claps, and
other percussive effects, in addition to nonmusical things like gunshots, explosions, and windstorms.
Voltage controlled filters come in
many different flavors. But for most
musical work, a four pole lowpass
type works well. A number of good
designs have appeared over the years
using discrete transistors; other
versions exploit the CA3080 or
LM13700 transconductance op-amps.
As mentioned earlier, it’s a good thing
if the VCF tracks the VCO, so again
you’ll want to build one which follows
the industry standard of 1V/octave.
VCAs are quite a bit easier to design
and build, since they’re most useful if
the control input obeys a simpler linear
response. About the only point to look
out for is the business of control voltage
rejection. There’s no need to get real
technical here, but essentially a unit
with poor control voltage rejection
tends to “thump” if it goes from low gain
to high gain rapidly. Luckily, this isn’t
much of a problem nowadays with the
ready availability of better linear chips.
By the way, voltage controlled amplifiers
are generally configured around operational transconductance op-amps such
as those mentioned above.
The ADSR module is employed to
create envelopes. The envelope is
generally imparted to the amplitude
(by means of the VCA), but could also
modulate either the VCO or VCF for
other musical effects. An ADSR is
essentially a timing circuit. It accepts
two digital signals — the gate and
trigger — and generates a (typically)
slowly moving control voltage output
in response. Front panel potentiometers let you adjust the timing and
level of the resulting envelope.
Refer to Figure 7 which shows the
relationships among the various signals.
Starting from the left, a gate and trigger
occur simultaneously in response to a
key being depressed. Now the trigger
(perhaps 1mS wide) will always be
generated when a key is struck; in fact, if
you sweep your hand down all the keys
of the instrument domino style, you’ll
■ FIGURE 6. The frequency in Hz of a
linear VCO increases by a fixed amount
for each 1V increment in control
voltage. A base-2 exponential VCO
doubles in frequency for each step.