Drift VCO

biminiroad · Screen Shot 2017-10-30 at 11.32.46 PM.png

This is a VCO that implements a method of VCO drift I read about on the KVR forum.

When the oscillator frequency is below 1000Hz, an adjustable amount of FM feedback is sent to the oscillator's input. The amount of feedback is also scaled by the pitch - lower frequencies have more feedback, and thus more detune effect.

Above 1000Hz, a slow sine LFO modulates the pitch.

The reason for the crossover is that this FM will introduce noise near/above that frequency.

It's not the most special VCO, but it does have some charm to it. A nice way to add some character to your otherwise static VCO.

The demo patch is set up to have a duophonic sound. With the sine wave on, it sounds reminiscent of plucked acoustic/electric guitar and bass.

@Rudiger here's another "microtonal"-ish patch.

Rudiger

Very interesting! I’ve been taking a look at this and wondering how best to introduce a reset when there’s a pause in activity.

From the Richard D. James interview with Tatsuya Takahashi:

RDJ: …the drift on the monologue sounds REALLY nice. It seems to move, but then never go out… Sounds to me like it gets reset/synced at some point…

TT: That's bang on! So same thing in the minilogue and the volcas too: the oscillators are re-tuned when they're not being used.

Rudiger

OK, here’s a tryout version of the Drift VCO with a reset as described above – the oscillator is set back to its starting point when no tones are being generated. For frequencies above 1000Hz, the LFO is reset, and for frequencies below that point the feedback loop is stopped and fades back in when tones are generated again. Different fade-in times could be experimented with.

Since it depends on the envelope for the reset it’s been simplest to include an input for the VCO amplitude, but I suppose and always-on version with a reset input could also be made.

It’s fun to hold a perfect fifth (e.g. with middle C as the upper note) and listen to the beating increase and eventually reach a perfectly tuned interval after about half a minute, and then slowly start beating again.

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