Tracking 1v/Octave on a Filter Not Designed for It

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witchbutter
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Tracking 1v/Octave on a Filter Not Designed for It

Post by witchbutter » Mon Jun 26, 2017 1:21 pm

Say you have a low pass or bandpass filter that has great resonance and sounds nice and warm, but you want the cutoff to follow the pitch, and it does not have a 1/v octave input. How can you patch this?

I understand that if you have CV control over cutoff frequency that that expects a continuous voltage that is just the amplitude of the dial, and not related to pitch necessarily. Is that correct?

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Dr. Sketch-n-Etch
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Post by Dr. Sketch-n-Etch » Mon Jun 26, 2017 2:05 pm

If the filter has an FM CV input, then plug your 1V/octave source into that and crank the attenuator all the way up. Dollars to donuts you'll get 1V/octave (or close to it). If not, then adjust the attenuator until your filter resonates more or less in tune.

I have designed many filters and they all have 1V/octave capability, either as a separate input or as the setting at the top of the FM attenuator. I can't imagine not doing that. It would be like selling a car without headlights -- it's a critical feature.
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witchbutter
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Post by witchbutter » Mon Jun 26, 2017 11:18 pm

Ok well example Erica Synths Fusion VCF. No FM input. No 1v/Octave Input. Is there a way to patch that?

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Dave Peck
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Post by Dave Peck » Mon Jun 26, 2017 11:22 pm

witchbutter wrote:Ok well example Erica Synths Fusion VCF. No FM input. No 1v/Octave Input. Is there a way to patch that?
It DOES have a CV input, it just isn't calibrated to be exactly 1 volt per octave. So as Dr. E says, plug your 1V/oct signal into that CV input and turn up the CV level knob. Now play octaves on the keyboard and adjust the CV amount knob until the keyboard is causing the filter to track the octaves correctly.

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Post by Paul Perry » Mon Jun 26, 2017 11:41 pm

I don't have an Erica VCF, but I believe it is based around a T section filter. These filters are usually vactrol based (varying the resistors across the top of the T) or by varying the resistance of the leg to ground.
I would be astonished if this filter could be made to track accurately over say 4 octaves - but I am always delighted to be proven wrong, because that means I have learned something.
The main question is, exactly how accurate an oct/V response is needed by the original poster.

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Shledge
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Post by Shledge » Wed Jul 12, 2017 7:50 am

I've had success with this using a disting in lin/exp convert mode. Was able to get decent tracking that way with any filter.

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Post by captjrab » Sun Jul 16, 2017 4:27 pm

If you are using an unquantized sequencer, just tune it up. It may not be 1v/oct in, but it doesnt really matter (in this case). Keyboards and quantized sources are a different story.

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nikmis
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Post by nikmis » Sun Jul 16, 2017 11:08 pm

This is another reason that silent way is so great, you can get more octaves out of your poorly calibrated oscillating filters.

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Post by mt3 » Mon Jul 17, 2017 12:53 am

Silent Way by Expert Sleepers allows you to save a configuration file for custom tracking.

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Post by addendum » Mon Jul 24, 2017 7:33 pm

Has someone tried that Silent Way feature with vactrol filters/ LPGs that are fed a pitch CV and an envelope at the same time? I would expect it to fail because the CV inputs on those are effectively "interactive", meaning you can't eliminate the final deviation from the 1V/oct or whatever custom curve that results from the vactrol's inertia still catching up with your envelope's decay slope while you're already playing another note... ?

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Post by Timmy » Mon Jul 31, 2017 12:51 am

Ornament & Crime has an "auto-tune" capability similar to Expert Sleepers Silent Way, but entirely in the module, which can be used to improve the 1V/octave tracking of VCOs and self-resonating filters. I use it to turn my Mutable Ripples VCFs, in self-resonating mode, into very accurate VCOs. The only caveat is that most filters don't incorporate temperature compensation, and so will drift as the temperature changes - thus you need to allow them to warm up in a stable temperature environment, and even then frequent re-tuning may still be required. However, re-auto-calibratiing them with O&C only takes a few seconds, and it is automatic.

Here's a video showing the process:

and here's the relevant part of the manual: http://ornament-and-cri.me/user-manual- ... references

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