Voltage Control

If you have not already done so, check out my section on synth modules before jumping in here.  The information presented here assumes that you already have read the information on VCOs, filters, and so on.

Modular analog synthesizers have remarkable sonic capability but depend on patching various modules together to create the sound.  One of the confusing aspects for the uninitiated user, is "what is all this control voltage stuff and how do I use it?"  Hopefully, I can help you out with that.  Explaining how control voltages in an analog synth work, is a bit like explaining how to tie one's shoe.  It turns out to be a simple process but it requires many words to explain.  I will attempt to be concise.

Before I try to define just what "voltage control" is, it is necessary to understand a few things about an analog synthesizer.  If you slog through the forthcoming text, when I do define what control voltage is, you'll smile and think "I get it".  If you leap directly to the definition it will probably make no sense at all.

STANDARDS  Many, but not all analog synthesizers, have complied with a very old set of rules that govern the kind of voltages that are to be used within the synthesizer.  The "standard" is that one volt of control results in precisely one octave change in frequency.  For example, a Voltage Controlled Oscillator (VCO) has a pitch-control input that changes octaves with each 1.000V change in control voltage.  There are 12 notes in an octave, so each note differs from the next note by 1/12 of a volt (0.08333333V).

Suppose the very first note corresponds to 1.00000V.  The very next note up the scale will correspond to a control voltage of 1.08333V.  The next note up the scale will correspond to a control voltage of 1.16666V.  The last note in that octave corresponds to a control voltage of 1.91666V.  The first note in the NEXT octave corresponds to a control voltage of 2.00000V.  (There is an upcoming table that will help clarify this.)

You probably know that notes spaced an octave apart, are related in frequency by a factor of two.  Note "A1" has a pitch (frequency) of exactly 55.000Hz.  It follows that note "A2" is exactly 110.000Hz.  Note "A3" is exactly 220.000Hz and note "A4" is exactly 440.000Hz.  You probably recognize "A4" as the "A" just above middle-C, which is a common tuning point.  [Other notes don't have such exact numbers for pitch (frequency) so I used "A" to make things easier to write.]

The following table lists a few musical notes, their ideal frequency for an even-tempered musical scale, and the hypothetical control voltages required to cause a VCO to generate the listed frequencies.

If you look at the progression of the entries in the above table, you can see that note A1 has a frequency of 55Hz and a control voltage of 1.0000V.  Note A2 has a frequency of 110Hz and a control voltage of 2.0000V.  Each note higher than the previous note, requires a 0.0833333333V increase in the control voltage.

Voltage Control - Detail

Voltage control is nothing more than some voltage in the range of +/-5V effecting change to one or more parameters of one or more analog synthesizer modules, in real-time.  That rather dry definition does not hint at what might be possible.  Hopefully, I can open your eyes a bit more...

Connecting an envelope generator to a VCA that is fed by a VCO to establish notes, is the most elemental use of voltage control.  This arrangement is shown below:

The above is a sort of mind trap!  To escape this one-dimensional thinking about sound generation, remember: Control voltages can be "audio" and "audio" can be control voltages.  The levels are compatible and can be freely mixed to create most unusual sounds.  Use a VCO as the sound source for the signal input to a filter (normal arrangement).  Now use a second VCO to adjust the "knee" frequency of the filter (not "normal" arrangement).  The filter response is being changed at an audio rate and sounds rather unusual.

The whole point of voltage control is there are essentially no rules and anything can be connected to anything else, in any order.  Some arrangements won't make any sense but others will surprise you.  Control voltage or audio ... what's the difference?  Indeed!

Experiment and you will discover sounds that people have not heard before.