Hi,
a basic question would be if you want a filter for clearly defined tasks / properties or if you want to experiment with the filter concept to search for new stuff: the filter as a main source of sound synthesis / sound transformation (clearly my preference, as, at least for my purposes, the basic “filter jobs” are well done by the main SC implementations).
You can use the Fb1 class from miSCellaneous_lib for both, implementations of special well-researched filter types (a) or more experimental ones (b).
As an example for (a) see a TPT filter implementation:
As an example for (b) see the Dobson-Ffitch filter:
In my recent Berlin workshop I gave a gui example of Dobson-Ffitch, it’s Ex. 2c in the file
UOS_04_single_sample_feedback.scd
contained in this zip:
https://daniel-mayer.at/materials/Berlin_2020/UOS_2020_TUBerlin.zip
You could also explore the concept of “filtered maps” as described by Risto Holopainen:
https://www.researchgate.net/publication/263905411_LOGISTIC_MAP_WITH_A_FIRST_ORDER_FILTER
and do it with any non-linear map
Clearly such weird filters are, in general, highly unstable, so when experimenting please note the possibility of blowups and consider the option of using sigmoid functions in the feedback loop (as described in the Dobson-Ffitch example).