[PD] vcf~ producing output without input for 0Hz cutoff?
Thomas Mayer
thomas at residuum.org
Sat Apr 13 14:11:29 CEST 2024
Hi,
On 13.04.24 10:57, Peter P. wrote:
> Thank you Christof,
>
> your equation helped me a lot, and now I see why some DC component will
> be circulating the filter forever.
>
> Thank you Edwin,
>
> explaining that "0 Hz is an infinite amount of octaves below 20 Hz" made
> me understand that a cutoff at 0Hz will not reduce DC by -3dB.
>
> 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 reckon that I make a highpass filter by subtracting the
> lowpass-filtered signal from the same unfiltered signal?
Yes, but not as simple as that, because there is a phase shift at the
cutoff frequency, so that you will get artifacts.
https://dsp.stackexchange.com/questions/75064/what-is-phase-shift
>
> Thanks again to everyone,
> Peter
>
> * Christof Ressi <info at 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
>>>> <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
>
>
>
> _______________________________________________
> Pd-list at lists.iem.at mailing list
> UNSUBSCRIBE and account-management -> https://lists.puredata.info/listinfo/pd-list
>
--
"When one's home has a really excellent computer capable of reaching
other computers anywhere in the Galaxy, one scarcely needs to budge."
(Janov Pelorat in: Isaac Asimov - Foundation's Edge)
http://www.residuum.org/
More information about the Pd-list
mailing list