[PD] [vcf~] with resonance?

William Huston williamahuston at gmail.com
Wed Apr 25 20:03:32 CEST 2018


Thanks Christof.

The help file for [svf~] shows a control-rate number box feeding the cutoff.
But I tested with [sig~] and it works. Thanks.

However I see no mention in the help of [scf~] for how to set Q.
Cutoff and Resonance only.

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On Wed, Apr 25, 2018 at 1:38 PM, Christof Ressi <christof.ressi at gmx.at>
wrote:

> Hi William,
>
> both [vcf~] and [svf~] let you set the cutoff frequency at audio rate and
> both have adjustable Q... what are you missing exactly?
>
> the vcf_* filters in iemlib are all suited for audio rate modulations.
>
> for moog style lowpass filters have a look at [bob~] and [ggee/moog~].
>
> Christof
>
>
>
> Gesendet: Mittwoch, 25. April 2018 um 18:46 Uhr
> Von: "William Huston" <williamahuston at gmail.com>
> An: "Julian Brooks" <jbeezez at gmail.com>
> Cc: "pd-list at lists.iem.at" <pd-list at lists.iem.at>
> Betreff: Re: [PD] [vcf~] with resonance?
>
> 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 <jbeezez at gmail.com[mailto:
> jbeezez at gmail.com]> 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 <pd-list at lists.iem.at
> [mailto: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[http://sharktracks.co.uk]
>
>
>
>
> On Tuesday, 24 April 2018, 14:40:20 GMT+1, Ed Kelly via Pd-list <
> pd-list at lists.iem.at[mailto: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[http://sharktracks.co.uk]
>
>
>
> On Tuesday, 24 April 2018, 14:33:13 GMT+1, Ed Kelly <
> morph_2016 at yahoo.co.uk[mailto: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[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[
> 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[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[http://sharktracks.co.uk]
>
>
>
> On Tuesday, 24 April 2018, 06:44:19 GMT+1, William Huston <
> williamahuston at gmail.com[mailto: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[mailto:WilliamAHuston at gmail.com]
> Binghamton NY
>
> Public Service Mapping / Videography / Research
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