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

[Christof Ressi]

Alex made a great point there!

It's that very reason why DAW plugins usually don't let you go down all 
the way to zero. (Typically, they stop at 20 Hz or something.) If your 
signal has components below that, you'd need a high pass filter.

On 12.04.2024 09:35, Alexandre Torres Porres wrote:
> 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
>     >
>     >
>     >
>     >         _______________________________________________
>     > Pd-list at lists.iem.at <mailto:Pd-list at lists.iem.at> mailing list
>     >         UNSUBSCRIBE and account-management ->
>     https://lists.puredata.info/listinfo/pd-list
>     <https://lists.puredata.info/listinfo/pd-list>
>     >
>     >     _______________________________________________
>     > Pd-list at lists.iem.at <mailto:Pd-list at lists.iem.at> mailing list
>     >     UNSUBSCRIBE and account-management ->
>     https://lists.puredata.info/listinfo/pd-list
>     <https://lists.puredata.info/listinfo/pd-list>
>     >
>     >
>     > _______________________________________________
>     > Pd-list at lists.iem.at mailing list
>     > UNSUBSCRIBE and account-management ->
>     https://lists.puredata.info/listinfo/pd-list
>
>
>
>     _______________________________________________
>     Pd-list at lists.iem.at mailing list
>     UNSUBSCRIBE and account-management ->
>     https://lists.puredata.info/listinfo/pd-list
>
>
> _______________________________________________
> Pd-list at lists.iem.at  mailing list
> UNSUBSCRIBE and account-management ->https://lists.puredata.info/listinfo/pd-list
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.puredata.info/pipermail/pd-list/attachments/20240412/990efa7a/attachment-0001.htm>