[PD] filter stability

[Alexandre Torres Porres]

I guess I can get the coefficients and derive an overall gain parameter. I
got objects in ELSE that do that [coeff2pz]. But if it also depends on the
frequency I should calculate this all of the time which doesn't seem
reasonable. Maybe just keeping a safe 0.5 q is fine...

You know, using something like lop~ is pretty stable, I am now wondering if
I should just use if for the sake of simplicity and efficiency as well. Do
I really need a 6db decay per octave instead of 3db? What do you people
think?

will make some tests...

thanks

Em qua., 27 de abr. de 2022 às 09:11, José de Abreu <abreubacelar at gmail.com>
escreveu:

> 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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