Any SVFs able to process CV?

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arthurgrimley
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Any SVFs able to process CV?

Post by arthurgrimley » Sun Aug 11, 2019 7:45 am

Hi there, apologies for the bizarrely specific question, but does anyone know of a State Variable Filter in Euro that's able to process CV as well as audio (e.g. similar to the Serge VCFQ)? Doepfer offers the A-121, but that only goes down to ~15Hz.

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hlprmnky
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Post by hlprmnky » Sun Aug 11, 2019 9:14 am

I don’t know of one, no.
I’m asking this question out of honest curiosity and a desire to learn - what purpose would it serve to process CV with a filter? I’m trying to think about what that would look like on a scope, and what I can come up with is a varying-rate LFO that gets attenuated as it exceeds (in the case of an LPF) the cutoff frequency. Is that the desired outcome, or is there some other effect that I’m not thinking of?
Thanks for reading, and if you choose to help educate a novice with a reply, thanks for that as well.

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flo
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Post by flo » Sun Aug 11, 2019 9:43 am

A lowpass would create a slew effect, so just use a slew instead. A highpass would simply filter out the DC (below the cutoff frequency, obviously), which can be useful for certain things but is probably not what you wanted. So yeah, what do you want to patch?

Cheers :guinness:

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lohacker
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Post by lohacker » Sun Aug 11, 2019 10:07 am

Animodule SOB

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Graham Hinton
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Post by Graham Hinton » Sun Aug 11, 2019 10:38 am

If the signal path is DC coupled and the frequency goes down low enough (and that's two big IFs) then the high resonances possible with SVFs will allow pinging at sub-audio frequencies.

arthurgrimley
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Post by arthurgrimley » Sun Aug 11, 2019 12:35 pm

Hi flo,

I'm recreating a patch from an old analog computer textbook, and part of the patch uses a SVF to model the behaviour of a pendulum - the different outputs of the SVF representing pendulum acceleration/velocity/position.

@hlprmnky - more generally, as flo suggests, a low-pass response can be useful for performing time integration on the incoming signal, i.e. slewing the incoming CV, while a high-pass response performs time differentiation on the incoming signal, i.e. removing removing any DC below the cutoff frequency.

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gentle_attack
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Post by gentle_attack » Sun Aug 11, 2019 10:20 pm

Mutable KINKS should be able to do the HPF part.

Just run a dummy cable into one of the logic ins (aka '0'), and your CV into the other, and MAX will keep only the positive part of your CV. The rectifier part of the upper section would give you a different take on "only CV >= 0" but I think the logic will approximate the hpf behavior.




Now that I think about it though, Maths (even half of Maths) can probably do both duties, easily. Can do the Slewing, and can do the ">= x" where x = some CV level, using the logic OR.

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Jason Brock
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Post by Jason Brock » Mon Aug 12, 2019 8:22 am

Serge VCFQ - the normal input is DC coupled. The AGC input is AC coupled.

But as others have said a filter is just going to act as a slew, and since filters are designed to have an extremely wide frequency range you'll probably have a difficult time dialing in any useful results. It would be easier to just use a voltage-controlled slew module for your waveshaping needs.

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BugBrand
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Post by BugBrand » Mon Aug 12, 2019 8:53 am

Not in Euro, but one side of my Dual Filter (twin State-Vari) allows you to switch in larger caps, thus shifting the response down to sub-audio ranges (the module is DC coupled of course). The idea was, indeed, for slewing etc - frankly adding resonance makes for weird wobbles I've not tended to use.

So you might well be able to modify an existing design by adding extra capacitors.

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