analog synth intro

[Kent Lundberg]

Hey People,

I help teach an analog laboratory class at MIT, and I am trying to put
together a little handout to steer students in the right direction for
building synths.  This is what I have so far...  If you've got a minute
to read this, I would appreciate it greatly.  If you have any comments,
good or bad, let me know.  Thanks for your time.

--> Kent Lundberg.

So you want to build an analog synthesizer?

Great.  This is a really cool analog lab project, involving wide range
of interesting circuits, but there is a right way and a wrong way to
spend your (valuable) time doing it.

Here's the deal:  There are four characteristics that make sounds
interesting from a musical point of view: pitch, volume, polyphony, and
timbre.  Everybody knows what pitch and volume are.  Polyphony is the
ability to play independent voice parts at the same time.  Timbre is
the quality given to a sound by its waveshape, envelope and overtones.

Since most of us were introduced to producing music through piano or
guitar, we often consider polyphony to be more important than timbre.
Besides, if we want to play harmony and chords (as we're used to), we
will need a polyphonic synth.  Often, this thinking leads to students
building projects that consist of 40 identical oscillator circuits, one
for each independent note.  This is not a terribly challenging,
interesting or efficient use of your time.

The quality that actually makes sound interesting to listen to is
timbre.  The difference between a single note played on a piano, a
trumpet, a violin, a snare drum or a vacuum cleaner is the timbre.  To
get maximum flexibility out of your synthesizer, what you really want is
not a synth that can play a lot of different notes at once, but a synth
that can produce a wide range of timbres.

Thus the prefered method for building an analog synth is to build a
single voice, modular synthesizer, that has maximum flexibility to
adjust the timbre of the sound.  This is how the "classic" analog synths
(MiniMoog, ARP 2600, Roland SH series, etc.) were designed.  A good,
sound producing oscillator is key, but you also want modules that can
filter and shape the sound, and process the signal in weird and
interesting ways.

The most common modules in analog synthesizers are VCO's, VCF's, VCA's,
and ADSR's.

VCO's are voltage controlled oscillators (the output frequency is
voltage controlled, usually from the keyboard) and are the primary
signal source for your synth.  You will probably want more than one, for
a couple of VCO's running at slightly offset frequencies (offset by a
fifth, octave, or even a major or minor third) can create very rich
textured sounds.  The output waveform is sometimes sinusoidal, but is
usually chosen to be a waveform rich in harmonics, like square,
triangle, or sawtooth.  Sawtooth oscillators are usually preferred
because they produce both odd and even harmonics of the fundamental.

VCF's are voltage controlled filters, used to filter the VCO output, and
can often be configured as low-pass, band-pass, band-cut or high-pass
filters.  The cutoff (or center) frequency and the filter resonance are
controlled by the input voltage, and that dynamically changes the
harmonic content of the note as it is played.

VCA's are voltage controlled amplifiers and are used to create an
evolution in volume as the sound is played.  ADSR's are envelope
generators (named for their function: attack, decay, sustain, and
release) that are used to control the VCF and VCA modules.

Other modules that you may be interested in building are LFO's, noise
generators, ring modulators, and glides (portamento).  LFO's (low
frequency oscillators) are used to modulate the control voltages to the
other modules (VCO, VCF, and VCA) to provide tremolo or vibrato effects.
Noise generators are used as additional signal sourse, usually mixed
with the output of the VCO to add in the woosh of a wind instrument or
the the crash of a cymbal or drum.  Ring modulators are basically analog
multipliers that create very weird effects, but are important for
synthesizing things like bell sounds.  Glides are used on the inputs of
VCO's to provide trombone-like slides from note to note.