[PD] [vcf~] with resonance?

Julian Brooks jbeezez at gmail.com
Wed Apr 25 17:09:12 CEST 2018


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 at 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
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>
> On Tuesday, 24 April 2018, 14:40:20 GMT+1, Ed Kelly via Pd-list <
> pd-list at 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_2016 at 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 <
> williamahuston at 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:  WilliamAHuston at gmail.com
> Binghamton NY
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