[PD] filter stability

[José de Abreu]

sorry, but I'm very curious. Using a resonance filter implies phase
shifting right? (instead of using a non resonance linear phase filter) But
this means that the tuning of the KS will be affected only near the
resonance? i may not understand this fully, but I never thought about using
resonance inside KS

Em qua., 27 de abr. de 2022 08:51, Claude Heiland-Allen <claude at mathr.co.uk>
escreveu:

> Hi Alexandre,
>
> On 27/04/2022 06:01, Alexandre Torres Porres wrote:
> > hi list, I'm using a 2nd order lowpass resonant filter whose
> > coefficients I'm getting from the famous Eq-cookbook and using it
> > inside a feedback loop to implement karplus-strong.
> >
> > I also have a coded object for that (pluck~) and the 'q' parameter is
> > 0.5, which is a "safe" setting, i.e. the filter doesn't get unstable
> > and blows up.
>
> The filter in isolation should be stable for any positive 'q', but its
> gain might get bigger than 1 making the larger feedback loop explode.
>
> You can do some additional gain reduction if increasing the q factor
> increases the peak gain of the filter and makes the feedback loop explode.
>
> > I was now trying to find a higher 'q' coefficient but it's hard to
> > know where I can go "exactly" just under it could blow up.
>
> You want the total gain in the feedback loop for all frequencies to be
> less than 1, i.e. peak (over frequencies) gain less than 1.
>
> > Is there an easy way to know this other than trial and error?
> The filter gain probably depends on cut-off frequency as well as q, so
> the filter peak gain is a function of 2 parameters.  Maybe gathering
> numerical data and surface-fitting a mathematical function could work,
> if the maths to do it analytically is too hard.
>
> If you modulate the filter parameters, it could still explode (the
> filter theory as per eq cookbook is only valid for fixed parameters,
> afaik).
>
> If you implement with insufficient accuracy inside the filter feedback
> (e.g. single precision floating point for 'y' in a biquad
> implementation), rounding errors can accumulate and can affect the
> actual gain (vs the theoretical gain you'd get from exact maths).
>
>
> Claude
> --
> https://mathr.co.uk
>
>
>
>
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