I have a slight problem in conjunction with using the Butterworth filters in iemlib (specifically, the lp2~ extern). It seems that if the cutoff frequency is changed for a filter object that has now slew-rate argument (for instantaneous cutoff frequency change), that a click is introduced. I assume that this is an artifact of changing that state variables of the filter.
In my case, this problem surfaced in the use of a patch to implement one voice of a polyphonic instrument - when the voice gets re-used, and the new cutoff frequency is different than the one for the last note that the voice got used for, then I get a click at note onset.
Assuming that this is not a bug in the iem filter code, this leads me to the question of is there a dc-block extern available for PD? I guess for now I could try chaining another low pass filter to try to reduce the click; but from my experiences with Csound and Perry Cook's STK, there is a special type of filter (one pole, one zero I think - but could be wrong) that is used to get rid of DC offset.
Larry