[PD] vcf~ producing output without input for 0Hz cutoff?

[Peter P.]

* Antoine Rousseau <antoine at metalu.net> [2024-04-11 13:40]:
> That doesn't seem incorrect to me; after all, a lowpass filter at 0Hz
> implies that its output is constant (any change would involve frequencies >
> 0Hz).

Thanks Antoine,

Why does a lowpass filter, that has a cutoff frequency of 0Hz imply that
it's output is constant?

I will describe the problem again hoping that I will understand it
better myseld:
I have an oscillating input signal that has some DC offset (unipolar
sawtooth from phasor~). I fade this signal's amplitude to -inf dB using
[line~].

I also fade down the filter cutoff (defined as the -3dB point of the
filter curve) from 400Hz to 0Hz. The filter will then continue to produce an
non-decaying output.

If I fade down the filter cutoff down to only 1Hz, it's output will decay (somehow
counterintuitively to me). This is the part I don't get.

I understand that vcf~ is a resonant filter, and it can have a gain
greater 1 around the cutoff frequency, especially for high Q values. The
above behavior can also be observed for Q=1.

Thanks for all hints!
Peter