I do, I love a good vactrol jobbie. I have a few filters with them, too.
The trick is emulating that ping. You can do that with the slide~ object, among others. Set the slide_down parameter to match the discharge of the vactrol - it should still be very fast on the slide_up.
I hear that Aalto can do some vactrol-esque sounds. I'd be curious to see how they do it.
I bet you could get similar plucky, long-tail sounds with physical modeling but I don't know enough to really be of help.
You can fairly easily model the CV response of a vactrol as mentioned above but given the complexity of analog filters, I think there is room to explore a "vactrol-like" sound from a holistic perspective too, indirectly modeling that sound.
Thanks for getting back, everyone. Wetterberg, you are talking about... lowpass filtering (via slide~), e.g., an adsr~ and using that to modulate.... a filter cut off? Could you please tell me or show me what you mean when you say "that ping"?
Peter et al, I should have been more specific; I meant physical analog filters that use vactrols. Low Pass Gates, as they're called. I have some diy units and a MakeNoise QMMG, if you want to look them up.
And yes, it could be modelled holistically, but honestly, apart from doing resonance-squeals, the main attraction and difference between normal and optocoupler-controlled filters is the ping-sensation, which is just rubbery goodness all day long.
What I mean by the ping is that the cv control of a lowpass gate is done through the vactrol sections, and those have a slew to them; so if you feed it a quick pulse you get a fast-attack, slow release thing coming out, and that is what controls the filter. It is reminiscent of the things you hear in phys.modelling, but on traditional synthesis sounds it's really a thing to behold.
On top of that a true lowpass gate has both vca and vcf properties, so there's some tricky stuff in modelling that, too.
I personally believe that it could be modelled sufficiently with a control-addon to an already existing max/gen~ algorithm, but I'd have to do some testing - it's certainly an interesting area, that I'd love to help out with! A joint forum project, perhaps?
I think slide~ is a great place to start if you want that vactrol sound. I've a lot of different hardware vactrols here and it's worth remembering that even two vacs from the same batch all have different rise and fall times so you there is no real definitive model. Don't get to lost in trying to model something to the nth degree and make some sounds that you like using the vactrol concept.
Here is a patch I made a couple of years back when I was hankering after a 303, this patch scratched the itch and it uses vacotrol style envelopes for Filter and Volume.
DON'T TRY THIS IF YOU ARE ALLERGIC TO RUBBERY ACID LINES! You have been warned.....
Coming to this a bit late, but that's my paper linked above by Manresa. Glad somebody noticed it :)
Wetterberg - The circuit modelled in the paper is closest to Thomas White's DIY version of the LPG, which is in turn almost identical to the original Buchla LPG circuits with the addition of an amplifier to control the resonance level in 'lowpass' mode. The original LPG has resonance too in 'lowpass' mode, but it's just at a fixed value. The amplifier simplifies down to just a scaling in the model, so it didn't make much sense to not include it.
The vactrol model in the paper is actually not so different to using e.g. slide~. The difference is that there is some extra feedback that gives it a more exponential shape. I've done a little bit of work on a lower-level physical model of vactrols in general, which would be able to capture all the behaviour including response to light leakage, long-term memory effects etc. Hopefully I'll get a chance to complete that in the future.
Hi Bacchus, I actually downloaded your code once I bumped into your article, quite by accident. I was looking into ways of getting more organic-sounding ( to me at least) envelopes since I've been thinking about getting away from traditional ADSR.
I would definitely welcome you sharing your implementations with the community. :-)