There is a Mac-only Audio Unit (AU) plugin called Volta that’s been out since early 2009 which allows a person to send CV from an audio interface box equipped with DC-coupled outputs.  There is a lot of information on the web about this plugin, many people are using it to control the more common analog modulars like EuroRack formats, but the only information up to this point I’ve read about controlling the Serge is a quick mention regarding the NTO and PCO.  Volta calibrates well with other Serge sections, allowing effective control over a much wider range then the MIDI to CV converter i’ve used.

In MIDI to CV conversion, tuning the oscillators is an intensive process and requires a separate tuner.  Using the scaling and fine tuning adjustments on the converter box I have, I got 3 Variable Q filters to track over 3 octaves and create polyphony.  Overall, it’s a lot of work to create a simple polyphonic setup.  If the temperature in your studio drops or if you change anything in the signal flow, it requires repeating the intensive tuning process, which can take one hour.

With Volta tuning is very easy.  The user just turns up the volume, and clicks Calibrate; Volta detects the pitch of the oscillator and adjusts the voltage accordingly to bring the oscillator in tune.  Some Serge sections can be patched to oscillate but have only “VC IN” (not “1v/oct”).  That’s OK, just turn the knob to the 2 o’clock position and calibrate, volta figures it out; this works for all the sections I’ve tested.

The sections I’ve tested with Volta, and their calibrated range according to Volta are:

VC TimeGen OSC: both right and left side calibrated 5 octaves, however Volta doesn’t detect pitch on the left side easily; I had to either patch the left side’s red output into it’s gate input and take the blue output, or send through the LO of a Variable Q patched to take the same 1v/oct

VC TimeGen Clock: left side calibrated 6 octaves, right side calibrated 5 octaves

Two DSGs: both calibrated 5 octaves

3 Variable Q filters: one calibrates 6 octaves, the other two calibrate 5 octaves

So that’s 9 oscillators that will all track across a much wider range then anything I’ve seen reported anywhere for these same oscillators, and much wider then my observations with MIDI to CV.  I have not verified Volta’s reported calibrated range with a tuner, but they did sound in-tune to me.

Also with Volta I can get new timbres that I haven’t achieved before.  Tuning a variable Q filter then setting it up to receive the same midi notes, but transposed (a midi effect in ableton live can do this) while piping in an oscillator can precisely emphasize a specific harmonic regardless of the note played.  Harmonic distortion can even be added to this concept to have a Variable Q filter emphasize odd harmonics in the same way.

Volta also offers other interfaces to the control voltage like trigger and pitch sequencers, MIDI CC, automation ramps, clock, and lfo.

Here’s a couple noisy demonstrations of Volta being used with a controller and then as a sequencer.

Polyphonic Serge + Monome + Polygome Max/MSP + Volta in Ableton Live 8 + 100W all tube guitar amp

Serge + Volta + Kaossilator