Hi all,
I wrote up a series of guides on how to put together a zero delay feedback filter:
http://radiofreerobotron.net/blog/2015/09/04/how-to-zero-delay-state-variable-filter/
http://radiofreerobotron.net/blog/2016/01/01/how-to-zero-delay-state-variable-filter-part-2/
http://radiofreerobotron.net/blog/2016/07/06/how-to-zero-delay-state-variable-filter-part-3/
Maybe someone can hook this up with an Axoloti object?
So called ZDF - as opposed to Chamberlin SVF - consists in using the trapezoidal aka "bilinear" transform.
Put aside the "analog modelling" consideration, this transform allows to preserve stability and resonance in the high frequencies (which is an interesting feature for dynamic filters).
I think that the Biquad filters implemented by Johannes are based on Robert Bristow-Johnson formulas.
And RBJ used the bilinear transform to derive his formulas from their analog equivalents.