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

[cyrille henry]

I don't think it's weird for a lowpass filter to go under 20Hz. They are not restricted to audio signals.
I use them a lot to smooth control signals, or to replace line~.
(I really hate line~ to control sound amplitude or preset transition, it's way too robotic)

cheers
c

Le 12/04/2024 à 08:01, Alexandre Torres Porres a écrit :
> 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 <mailto: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 <mailto:peterparker at fastmail.com>> a écrit :
> 
>         * Antoine Rousseau <antoine at metalu.net <mailto: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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