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

```Em sáb., 13 de abr. de 2024 às 06:30, Peter P. <peterpar...@fastmail.com>
escreveu:```
```
> Looking at [vcf~]'s help patch closer, I read that the filter's real and
> complex outputs may be combined to allow for other filtering possibilies
> than band-pass and low-pass. Does anyone have a quick example of such a
> combination?

I don't, and this information was there before I revised all help files. I
suspect Miller wrote this, but I can't see how this is helpful or
insightful information. This would not be something particular to [vcf~],
any filter can be cascaded or combined in parallel, and I guess this is
beter suited for the raw filters, that can be used and combined to more
properly design filters. I would vote for just suppressing this
information, unless someone convinces me there's a good point and example.

> I reckon that I make a highpass filter by subtracting the
> lowpass-filtered signal from the same unfiltered signal?
>

Well, this is not MAX, we do have [hip~] :)

>
> Thanks again to everyone,
> Peter
>
> * Christof Ressi <i...@christofressi.com> [2024-04-12 09:36]:
> > Just expand on Antoine's post, let's look at the formula of Pd's 1-pole
> > lowpass filter:
> >
> > k = freq * 2pi / sr
> >
> > y[i] = x[i] * k + y[i-1] * (1 - k)
> >
> > For freq=0 this becomes:
> >
> > y[i] = y[i-1]
> >
> > As you can see, this would just repeat the previous output infinitely,
> > ignoring the input altogether. There is no decay to zero!
> >
> > The same reasoning applies to bandpass filters such as [vcf~].
> >
> > Christof
> >
> > On 12.04.2024 09:10, cyrille henry wrote:
> > > 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
> > > > <anto...@metalu.net <mailto:anto...@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. <peterpar...@fastmail.com
> > > > <mailto:peterpar...@fastmail.com>> a écrit :
> > > >
> > > >         * Antoine Rousseau <anto...@metalu.net
> > > > <mailto:anto...@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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