convert v/oct to v/hz

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how would people on this forum go about converting a v/oct cv signal to v/hz?

i’ve got a few metasonix modules that i want to use with a hodgepodge of moogerfoogers and synthesizers.com gear, but i’d like to get a more even response from the metasonix stuff.

for example, if you use the unpatched s/h output from the cp251 as a frequency input for a v/oct oscillator, you’ll get a uniform distribution of notes (and an exponential distribution of frequencies) within the range of the oscillator, but for a v/hz oscillator, the distribution will be off, and you’ll get an exponential distribution of notes (and thus a linear frequency distribution), with more notes in the high range.

in csound, you would use the opcode “octcps” to convert data from octaves to hertz, but as far as i can tell, nobody seems to sell the equivalent analog circuit in the form of a module or stomp box. which is wierd, because every oscillator with a v/oct input would have to have some kind of circuit built-in to convert it to v/hz internally.

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Here’s a link to a schematic drawn by Bob Moog for a cv scaling circuit to be used with a Micro or Multi Moog:

http://machines.hyperreal.org/manufacturers/Moog/schematics/moogfix.jpg

I have thought about this problem before (I have an MS20 with Hertz Scaling) and have considered using this circuit (probably with some modifications) as a possible solution (or step towards a solution). The Micro/Multi doesn’t use hertz scaling, of course, but it’s not quite 1/volt per octave either-- which is why it needs a circuit like this in order to scale properly with another synth. So, it seems like this circuit could be modified to work for hertz scaling.

Korg is the only manufacturer that I know of that ever offered a comercial product for this pupose. It was the MS02, I believe. They’re very very rare now. Although, one sold on ebay last week for $400. I’ve never seen a scematic for it.

I belive that you could also convert the scaling with a CV offset circuit, like the one on the Moog CP251. I’ve never tried it, but it would seem like it would work. I once asked the people at Synthesizers.com if one of their modules with CV Offset would work to convert my MS20’s CV to normal Volt/Octave–and they told me it would.

Kenton used to make an add on PCB for their Pro-Solo for use with Hertz scaled synths, and I once considered buying it, and converting it to a stand alone unit, but they are no longer for sale, as the newer Pro-Solos already automatically convert MIDI data to either scale. I suspect it’s accomplished through software (?), so they may not have this discrete circuit built into the newer units.

I’m not sure what you’re exact purpose is for this type of circuit, but there are often ways to get around this scaling problem with different synths. For instance, with the MS20, I know of a modification that allows you to send Volt per octave CV directly to a certain point on the oscillator circuit, and then just add a seperate jack for this CV.

I’ve also been able to control the oscillator on the MF102 Ring Mod from my MS20 hertz scale (over a few octaves only) with just an attentuator on the CV signal.

Also, a CV/MIDI box with Hertz and Volt/Octave CV can be used to coordinate incompatible synths pretty easily.

You probably know all this already, but for anyone who is wondering about this same problem, this is all that I can say about it. If you do find a proper circuit for this conversion, I’d love to see it also. :slight_smile:

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<< Also, a CV/MIDI box with Hertz and Volt/Octave CV can be used to coordinate incompatible synths pretty easily. >>

The Future Retro Mobius is one such box. It’s actually a rack-mount sequencer with storage for 256 patterns, but can also be used as a MIDI/CV box either for v/oct or v/hz.

http://www.future-retro.com/

At $399, it’s a bit expensive to be purchased just for voltage conversion, but it seems to be a very flexible multi-purpose box that can be used with any synth.


G

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nothing from synthesizers.com will really convert a v/oct signal to a v/hz signal directly since all the standard modules can do is linearly scale one voltage range to another. same thing with the cp251.

well, this isn’t entirely true. one hack that i’ve tried is to use a synthesizers.com vca to do the trick. this has a switch for a linear or an exponential response. so you feed the signal you want converted into the control input, and feed a straight 5V dc voltage into the audio input (which you can get from the signal processor module with no input shifted positive). then the positive output is the scaled signal. but it seems kind of wasteful to use all these modules that could be doing other things for this. and the curve doesn’t map perfectly.

i’ve got a kenton prosolo that can output in v/hz, so another option would be to go from v/oct cv to midi to v/hz cv, but that seems kind of silly too, and i don’t actually know of anything that goes from cv to midi.

i will check out the mobius though.