Sunday, 5 December 2010

Fonitronik mh31 VC Modulator

A look at the Euro-prototype of Fonitronik's unconventional ring modulator.

See this thread at the electro-music forum for the background to this project and Kevin Kissinger's excellent demo.

"Harry Bissell once recommended Irvine's design for a ring mod. I changed the resistors to our common values, and added two VCAs to the inputs, thus giving a voltage controlled ring mod. Makes a lot of fun."

Matthias's opening description in that thread might lead one to believe that this circuit is indeed just another ring modulator with the added benefit of voltage control, something akin to Doepfer's A-133 VC Polarizer. Irvine refers to his design as a 'frequency mixer' and, while I'm not sure of the the semantic difference, the sonic results certainly are unlike 'normal' RM.

Here's a comparison between the Cwejman VCO-2RM, the A-133 and then the mh31:


The Cwejman and Doepfer (0:26) exhibit 'classic' RM, with the latter distorting at higher CV levels. The mh31 (0:54) produces a harsher, more metallic tone that, while having RM characteristics, doesn't yield the expected sine-on-sine bell-tone. Here's what the waveforms look like:

Irvine's circuit sums the inputs, then inverts, rectifies and doubles the amplitude of the sum. This offers a clue as to why this circuit sounds different. In the following example, I've modulated an audio signal with an LFO. Using a traditional DC-coupled RM, the amplitude is attenuated as the LFO nears 0V and is then polarized. To the ear, flipping the polarity of an audio signal should make no difference:

(VCO-2RM, A-133 @ 0:11, mh31 @ 0:25)

This last clip highlights the difference in Irvine's approach. Rather than simply changing the polarity, at slow speeds, the waveform seems to 'change direction'. This reminded me of the type of soft sync that is implemented in some VCOs or mixing waveforms in a min/ max analogue logic module like the A-172. Here's what it looks like:

The term 'ring modulator' might be misleading, but that shouldn't detract from the possibilities of this circuit, both in terms of timbre and function generation. The build quality is excellent and, taken as an alternative to the classical DC-coupled ring mod, the mh31 offers new sounds and surprising results when processing control voltages.

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