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

[Miller Puckette]

Hi Larry,

I don't know of any general technique by which you can change a filter's
characteristics instantaneously without causing clicks in the output.  I
think the only surefire way to proceed is as in the 35.sampler.poly example
in which the voice mutes itself for a very short interval in order to
be able to make discontinuous changes.  THis causes a short delay in the
output, usually 5 milliseconds the way I like to set it.

cheers
Miller

On Wed, Nov 28, 2001 at 10:07:08PM -0500, Larry Troxler wrote:
> 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