as a single entity, break it down into
logical units or modules. Not only is it
easier to design and build this way, but
you’ll find troubleshooting to be much
simpler, as well. And then, of course,
there’s the business of upgrades. If you
should happen to come up with an
improvement, it’ll be simpler to swap
out the old module and replace it,
leaving the rest of the instrument
intact. So let’s think modular.
Refer to Figure 2 which shows a
block diagram for a typical analog
music synthesizer. Don’t let the alphabet soup there worry you, for we’ll soon
sort out what the various acronyms
mean. As an overview, let’s take a quick
run around the figure. Afterwards, we
can concentrate on what you should
know about the specific modules.
The heart of a synthesizer is the
voltage controlled oscillator (VCO)
which produces a stable source of
waveforms. Voltage control is what
makes it so useful, allowing other
circuits to easily change its operating
frequency. This is a key notion in the
design of analog synthesizers.
The control voltage (CV) output of
a musical keyboard puts the VCO
through its paces. (By the way, in Figure
2, vertical arrows represent control
voltages while horizontal ones indicate
the audio path.) As you tap out various
notes on the keyboard, switch closures
are somehow translated into different
voltages which causes the VCO to
change frequency accordingly. Notice
that the VCO can also be modulated by
an LFO or low frequency oscillator to
impart a vibrato to the waveform,
analogous to the effect produced by a
violinist wiggling his or her finger
against a string being held down.
The audio output of the VCO is
routed to the VCF, or voltage controlled
filter. The purpose of this module is to
modify the harmonic content of the
waveform. You’ll note one of its control
sources comes from the keyboard, too.
This is done so that the VCF “tracks,”
ensuring that the harmonic content
doesn’t change as we move from note
to note, up and down the keyboard.
But most traditional musical
instruments do in fact change their
timbre over the duration of a single
note; think of a how a harmonica player
cups his or her hands around the
instrument to create a “wah-wah” effect.
So the VCF is also modulated by an
additional control voltage coming from
an ADSR envelope generator. More
about that module in just a moment.
We’ve seen so far how the frequency and harmonic content are manipulated in the instrument by the VCO and
VCF circuits. The remaining parameter
is modulated by the voltage controlled
amplifier (VCA). The amplitude of the
audio signal passing through it (from
the VCF) is boosted or attenuated, once
again under control of the ADSR envelope generator. The letters here stand
for Attack-Decay-Sustain-Release.
These represent the four components
making up a cycle of a musical note,
beginning from silence, on up to full
volume, back down to silence again,
and awaiting the start of the next note.
Observe that the keyboard — in
addition to putting out the control voltage mentioned earlier — also generates
gates and triggers. These two signals
tell the ADSR when to do its thing. We’ll
get more detailed later, but for now just
think of the gate and trigger as indicating when a key has been pressed.
Figure 2 has illustrated just one
simple “patch.” But in a full-fledged
analog synthesizer, hundreds, if not
thousands of other arrangements are
possible. Since each module sports a
variety of jacks, one need merely
rearrange the interconnecting patch
cords to effect a new circuit configuration. Both audio and control voltage
interconnections can be made.
■ FIGURE 3. Block diagram of an
analog keyboard arrangement.
MODULES FOR
MUSIC SYNTHESIS
With this overview out of the
way, we can focus on modules that
you can easily construct to make a
complete analog synthesizer. Let’s
begin with the keyboard. You have two
basic approaches available: analog and
an analog/digital hybrid. Take a look at
Figure 3 which depicts the former.
A constant current source drives a
string of resistors. When a key is
depressed, a switch underneath closes, dumping the voltage corresponding to that part of the resistor string
onto a bus. Note, then, that each key
will produce a different voltage since
each one selects a differing number of
resistors. The bus carries that voltage
on to a sample-and-hold. It’s the duty
of this circuit to “remember” that voltage until the next key is struck. By the
way, the gate and trigger circuitry isn’t
shown here; typically these are generated with additional switch buses. And
if it isn’t clear, the focus is on a monophonic system (playing one note at a
time); you can worry about polyphony
once you get your design chops down!
Such an analog controller interface is quite easy to design and build.
The problem is usually finding a raw
keyboard sporting a switching bus.
Thanks to their popularity in combo
organs, 20 years ago these were a
dime a dozen, even showing up on the
surplus market. Nowadays, however,
they’re far less common.
But there is a reasonable alternative for our present age. Consider the
hybrid arrangement shown in Figure 4.
In this case, you use any MIDI enabled
unit. Virtually all modern electronic
pianos, organs, synthesizers, and so
forth come with a MIDI out jack. This
even includes most bargain basement
synths available at department
stores. You simply feed the output
January 2006 59
