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 <pd-list@lists.iem.at> 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 < > pd-list@lists.iem.at> 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 < > morph_2...@yahoo.co.uk> 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 < > williamahus...@gmail.com> 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: williamahus...@gmail.com > 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 > > > _______________________________________________ > Pd-list@lists.iem.at mailing list > UNSUBSCRIBE and account-management -> https://lists.puredata.info/ > listinfo/pd-list > _______________________________________________ > Pd-list@lists.iem.at mailing list > UNSUBSCRIBE and account-management -> https://lists.puredata.info/ > listinfo/pd-list > > _______________________________________________ > Pd-list@lists.iem.at mailing list > UNSUBSCRIBE and account-management -> https://lists.puredata.info/ > listinfo/pd-list > >
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