0 Hz is an infinite amount of octaves below 20 Hz (10 Hz, 5 Hz, 2.5 Hz, 1.25 Hz, …).
A low pass filter “normally” has a slope of a certain amount dB per octave. However, a filter with a frequency of 0 Hz doesn’t have a slope with a certain amount of dB/oct because an octave up or down from 0 Hz (by doubling or halving the frequency) is also 0 Hz. The filter simply can’t move ;-)
Best!
Edwin
On 12 Apr 2024, at 09:49, Christof Ressi <i...@christofressi.com> wrote:
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 < c...@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
<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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