[PD] iem butterworth filter clicks / dc-blocker for PD?

[Larry Troxler]

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