VCO for Guitar?

[drdn0]

Howdy.

Straight off the bat, I know the absolute bare minimum about LFOs, VCOs, envelope detectors, etc - I know what they do, I've messed around with them in VCV Rack, and I understand what they're used for.

My question is this - has anybody used an (analogue) VCO driven by a envelope detector as part of a guitar-specific effect, or is it more relegated to the synth world?

Long story short, I had an idiotic idea for a pedal - a sidechain comprising of an envelope detector that drives filters (ala autowah) while also driving a VCO, which itself is part of a tremolo (so the harder you play, the slower the tremolo effect happens). I've seen something similar done in the Pigtronix Tremvelope, but I couldn't find any specifics on the guts of it.

[Mark Hammer]

Check out Freppo's many clever devices at the Parasit Studio site:  https://www.parasitstudio.se/
He makes good use of the CD4046 phase-locked loop's ability to track guitar pitch and generate a waveform.

[ElectricDruid]

4046 PLL is one way. You need *something* extra, because you haven't got a Pitch CV, and without a control voltage, a voltage-controlled oscillator is just...well, an oscillator!

I also remember some noise-fuzz thing that was basically an oscillator, with the guitar signal used to reset the waveform (what would be "hard sync" on a synth). That means the pitch of the signal is determined by the guitar, but the waveshape is down to the oscillator's pitch. Craig Anderton, maybe? Can't remember.

Using an envelope to control an LFO's frequency (either up or down) is a good trick. You can do either filter or tremolo effects like that. It's a low frequency VCO, aka VCLFO. My PIC chips are one simple way to do that, since they use a 0-5V CV to set the frequency. Just feed it an envelope!

[Mark Hammer]

Robert Penfold had a construction project for a pitch-to-voltage converter for guitar in the October 1987 issue of Practical Electronics ( https://www.worldradiohistory.com/UK/Practical-Electronics/80s/Practical-Electronics-1987-10.pdf )  That will let you drive as many VCOs as you want.  I've built two of them, but like so many projects sitting around, they are etched and stuffed, but not completely wired up and functioning.

[drdn0]

4046 PLL is one way. You need *something* extra, because you haven't got a Pitch CV, and without a control voltage, a voltage-controlled oscillator is just...well, an oscillator!

I also remember some noise-fuzz thing that was basically an oscillator, with the guitar signal used to reset the waveform (what would be "hard sync" on a synth). That means the pitch of the signal is determined by the guitar, but the waveshape is down to the oscillator's pitch. Craig Anderton, maybe? Can't remember.

Using an envelope to control an LFO's frequency (either up or down) is a good trick. You can do either filter or tremolo effects like that. It's a low frequency VCO, aka VCLFO. My PIC chips are one simple way to do that, since they use a 0-5V CV to set the frequency. Just feed it an envelope!

If I only want it to respond to playing dynamics rather than track pitch, I don't need any sort of pitch CV right?

I basically just want an autowah to do its thing, and a trem to respond to how hard I play. I am thinking now that wanting a VCO to slow rather than speed up with picking dynamics might be easier said than done though

Obviously this isn't going to be full featured and never perfect, but I thought I could do it quick and dirty with something like a precision rectifier + smoothing, then just feed it into a basic VCO. I'd be happy with a square wave out as I'm more interested in audio weirdness than a perfect vintage-flavoured tremolo

[ElectricDruid]

If I only want it to respond to playing dynamics rather than track pitch, I don't need any sort of pitch CV right?

Right.

I basically just want an autowah to do its thing, and a trem to respond to how hard I play.

Ok. You're only really looking for LFO-territory frequencies too, then. So it's more VCLFO than VCO.

I am thinking now that wanting a VCO to slow rather than speed up with picking dynamics might be easier said than done though

No, it's no harder. If you've got a oscillator that responds to CV one way, you can just flip the CV with an inverting op-amp to make it respond the other way. We mostly think of envelope followers as following the *positive* peaks (have you ever noticed that?). But it's just as easy to design one that follows negative peaks, so you can get an inverted envelope out just as easily as a non-inverted one, if that's what you want.

Obviously this isn't going to be full featured and never perfect, but I thought I could do it quick and dirty with something like a precision rectifier + smoothing, then just feed it into a basic VCO.

Seems like a reasonable plan...

I'd be happy with a square wave out as I'm more interested in audio weirdness than a perfect vintage-flavoured tremolo

Ok, so what's the simplest oscillator with a control voltage input? (apart from mine?!?) 4046, maybe? I'm avoiding saying 7555 because any 555 variant seems to bring more problems than it solves.
There's a few designs in the LM13700 datasheet that would do too:

[PRR]

what's the simplest oscillator with a control voltage input?

Simple, ACCURATE, and fashionably odd may be the Bob Pease V/F converters:
https://www.electronicdesign.com/technologies/analog/article/21799040/whats-all-this-v-f-converter-stuff-anyhow-part-1
https://www.electronicdesign.com/technologies/analog/article/21799350/whats-all-this-v-f-converter-stuff-anyhow-part-ii
huhhhhhhh--- once again the magazine website has LOST the diagrams.... argh.

[soggybag]

Thinking outside the box, build an ugly face. It makes a great square wave and it's easy to build, parts are common. It doesn't do a voltage to frequency conversion. It does a sort hard sync on the guitar signal. It gates the signal or will run free when the threshold is turned up.

[StephenGiles]

Hmmmmmmm - What could be simpler than the front end of EH8000 Guitar Synth???