Yes, thanks for the detailed response, Ed! I was hoping for some guidance for a bolt-on method for adding resonance to vcf~, but maybe it's not so easy? or the wrong approach?
My error was thinking resonance was implemented with feedback. >From this video, it looks like resonance is caused by a bump in the filter shape at the cutoff frequency: https://youtu.be/XA_WnyA7D6k This looks like a shape that naturally occurs in analogue circuits. Someone in the FB group suggested [svf~] from Cyclone. It's really great, as it gives you HiP, LoP, BP, and Band Reject(!!). I would be in Filter Heaven if that had adjustable Q... PS: Can someone remind me of the filters where you can set parameters like cutoff freq at *audio rate*? Thanks! BH On Wed, Apr 25, 2018, 11:09 AM Julian Brooks <[email protected]> wrote: > Well, regardless of the correct number of inversions, some informative and > well linked posts there - cheers for that Ed :) > > On 24 April 2018 at 20:44, Ed Kelly via Pd-list <[email protected]> > wrote: > >> The 4-pole model in the ENS-85 paper does not have inversion at the >> input, but the Thomas Henry 2-pole design does. So I guess the 4-pole is >> more similar to vcf~ >> >> go figure. >> Signing off >> Ed >> >> >> _-_-_-_-_-_-_-^-_-_-_-_-_-_-_ >> >> For *Lone Shark *releases, *Pure Data *software and published *Research*, >> go to http://sharktracks.co.uk >> >> >> On Tuesday, 24 April 2018, 14:40:20 GMT+1, Ed Kelly via Pd-list < >> [email protected]> wrote: >> >> >> Ach! NO! inversion does not happen at the input! >> I guess you can look at the topologies and discover for yourself. >> >> I'm having a bad week. >> x >> Ed >> >> >> _-_-_-_-_-_-_-^-_-_-_-_-_-_-_ >> >> For *Lone Shark *releases, *Pure Data *software and published *Research*, >> go to http://sharktracks.co.uk >> >> >> On Tuesday, 24 April 2018, 14:33:13 GMT+1, Ed Kelly < >> [email protected]> wrote: >> >> >> Correct me if I'm wrong (and I'm sure someone will) but going back to >> analog electronics... >> >> If you examine the topology of a standard analog filter design, such as >> the Thomas Henry VCF1 (http://www.birthofasynth.com/ >> Thomas_Henry/pdf/VCF-1/Sheet_0002.pdf) you will see that the signal path >> from input to output goes through 5 inversion stages before being fed back >> into the input to generate resonance. Therefore it is inverted. >> >> Since most (resonant) filters consist of an even number of stages (and >> therefore an even number of poles and/or zeroes) you should probably invert >> the feedback. With an odd number you should probably not invert feedback. >> My guess is that vcf~ implements an even-number (probably 4-pole) resonant >> filter, a la Moog. You can also try mvcf~ from my library (ekext - >> http://sharktracks.co.uk/puredata/ekext-0.1.8.tar.gz) which is another >> attempt at emulating this topology, or there are lots of others I'm sure >> you know about. >> >> This might seem counter-intuitive, but the image below shows how this >> works with regards to vcf~ - the output of vcf~ is NOT inverted, whereas >> the output of an analog VCF should be. >> >> Odd-number staged filters are very rare in the analog world, and most >> (i.e. 18dB per octave) were created to implement the (mythical 3-pole >> topology) filter of the Roland TB303. However, according to the schematics >> of the TB303 it is a four-pole diode-ladder VCF using transistors as >> diodes, with extra resonance artifacts revealed by Tim Stinchcombe on his >> excellent website. Tim Stinchcombe - TB-303 Diode Ladder Filter model >> <http://www.timstinchcombe.co.uk/index.php?pge=diode2> >> >> Tim Stinchcombe - TB-303 Diode Ladder Filter model >> >> <http://www.timstinchcombe.co.uk/index.php?pge=diode2> >> >> >> >> Meanwhile, if you want to go deeper with regards to 4-pole filter >> topologies (and lowpass/highpass configurations) it is worth checking out a >> paper published in 1985 - http://electronotes.netfirms.com/EN85VCF.PDF >> >> The easiest way to think about it, from this paper, is that you think >> about each stage as an inversion, and the input itself is one of the >> stages, so for a 4-pole filter there are 4+1 inversion stages. But in the >> digital world I guess you'll have to test input and output at a relatively >> low frequency to the cutoff to determine inversion principles for the >> enhancement of resonance. >> >> I hope this helps. It gave me a break from mixing! >> Ed >> _-_-_-_-_-_-_-^-_-_-_-_-_-_-_ >> >> For *Lone Shark *releases, *Pure Data *software and published *Research*, >> go to http://sharktracks.co.uk >> >> >> On Tuesday, 24 April 2018, 06:44:19 GMT+1, William Huston < >> [email protected]> wrote: >> >> >> What's the best or "correct" way to add resonance to a [vcf~]? >> >> - Should the feedback be delayed? >> - Should the feedback be inverted? >> - Should the feedback be from the outlet I'm using? >> - Or should the feedback always come from the BP or HiPass side? >> >> Thanks! >> BH >> >> -- >> William Huston: [email protected] >> Binghamton NY >> >> *Public Service Mapping / Videography / Research* >> Blog <http://WilliamAHuston.blogspot.com> -- Facebook >> <http://facebook.com/billhuston> -- Twitter >> <http://twitter.com/WilliamAHuston>-- Youtube >> <https://www.youtube.com/channel/UCGijK1amWOLglT3YeTyEBNQ?sub_congfirmation=1> >> Document collections: VirtualPipelines >> <http://TinyURL.com/VirtualPipelines> -- BHDCSDimockArchive >> <http://bit.ly/BHDCSDimockArchive> >> Please support my work: TinyURL.com/DonateToBillHuston >> >> >> _______________________________________________ >> [email protected] mailing list >> UNSUBSCRIBE and account-management -> https://lists.puredata.info/li >> stinfo/pd-list >> _______________________________________________ >> [email protected] mailing list >> UNSUBSCRIBE and account-management -> https://lists.puredata.info/li >> stinfo/pd-list >> >> _______________________________________________ >> [email protected] mailing list >> UNSUBSCRIBE and account-management -> https://lists.puredata.info/li >> stinfo/pd-list >> >> >
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