analog synth intro

[Magnus Danielson]

>>>>> "KL" == Kent Lundberg <klund at mediaone.net> writes:

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

 --> Kent Lundberg.

 KL> So you want to build an analog synthesizer?

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

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

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

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

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

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

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

>From one waveform one can create other waveforms by simple voltage
processing, this is usually called waveshaping. From the sawtooth
waveform can many other waveforms be created, either directly or
indirectly. Some waveforms may be modulated in order to change timbre.

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

[Note that this formulation can be interpreted as if both the filter
frequency and resonance is being controlled with the SAME input
voltage, this is very rarely the case]

VCF's provide a method to shape and dynamically shape the timbre of
the signal.

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

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

Cheers,
Magnus