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

[Alexandre Torres Porres]

oh sure, but that's more like a slew or glide and you don't go down to zero
anyway :)

and if you're filtering audio, you don't want to keep inaudible stuff. All
I'm saying is that if you are soing this to fade to slience you need a DC
filter

Em sex., 12 de abr. de 2024 às 04:12, cyrille henry <ch at chnry.net> escreveu:

> 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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