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

[Alexandre Torres Porres]

and you got a strong DC component over there :)

anyway, it also seems weird to have a lowpass or a bandpass going as low as
in the 20hz range. If you wanna do it just so it fades out to silence, you
need a DC filter, something like a [hip~ 5] object, so when the lowpass,
bandpass gets there, then you have nothing.

cheers

Em qui., 11 de abr. de 2024 às 15:40, Antoine Rousseau <antoine at metalu.net>
escreveu:

> Well, let's simplify a bit, forget all the filter complexity (Q, slope,
> definition of the cutoff frequency...).
>
> Let's just say that the output of a lowpass filter cannot move faster than
> the cutoff frequency: a 1Hz filter output cannot move faster than 1Hz (so
> it can't go back and forth in less than a second or so), a 1kHz can't go
> back and forth in less than about 1ms, etc. The output of a 0Hz filter
> can't move... at all. When you set the cutoff to 0Hz, the output freezes to
> its current value. It won't magically decay to 0.
>
> Hey, if you set the framerate of a movie to 0 frame/second, it will just
> stop, and will show the same image forever; it won't fade to black!
>
> Antoine
>
>
>
> Le jeu. 11 avr. 2024 à 14:08, Peter P. <peterparker at fastmail.com> a
> écrit :
>
>> * 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
>>
>>
>>
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