[PD] [vcf~] with resonance?

William Huston williamahuston at gmail.com
Wed Apr 25 18:46:43 CEST 2018


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> 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>
> 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 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
>>
>> *Public Service Mapping / Videography / Research*
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