Filter 2 Cutoff Frequency Weirdness

Well I was wondering about this Filter 2 frequency wierdness thing again and decided to bring it into conversation since I remember not having read about it, at least for a while.

The thing is that cutoff point seems to be modulated by db/octave setting, which is not the logical, wanted or right behavior as I understand. I have always thought that filter, low pass filter in Renoise f.ex., should work in such way that cutoff is rolled off from the point set in hz by steepness of set db/octave. But alas, this is only true in Renoise when using 48db/octave filter while 24 and 12db/octave filters lower the cutoff point so that for example when using 24db filter the cutoff frequency seems to be roughly 1/3 of the setting according to master spectrum meter. This can be easily seen by filtering with high Q and moving the cutoff point around. Normally filters don’t do this, do they? This has been like that for ages, still I remember no-one opening his/her mouth about it. So is there something wrong or is it just me?

We’re in the dark as much as your are on this one.

Just use your ears. If it sounds good use it.

Always have, always will.

The filters are and always were one big mess in Renoise. They are full of copatibility hacks and attempts to fix them, so I dare to touch them. Sooner or later we should throw them away and rebuild them from scratch (->Filter3).

For now I can only give you the tips already mentioned above: Use Mathlab to write your diploma. In Renoise, rely on your ears: If it sounds good, use it.

I have nevertheless checked the Filter2 responses again and fixed the displayed frequencies for the “LP 2x2 Pole”, “LP 2 Pole”, “HP 2x2 Pole” and “HP 2 Pole”. Those where totally wrong, the rest is roughly OK. Thats all I can do for this release.

Again, thanks for clearing that up!

More can be read here:

Ah, thanks for the link. I’ll check that out more. This already clarified some:

“The cutoff frequency of a passive low-pass filter does not define the frequency at which the filter starts to work; it is itself defined as that frequency at which the signal is already attenuated by 3dB. And, since an attenuation of 3dB is easily perceived by the human ear, this means that you are already significantly affecting the signal at the cutoff frequency.”

This seems not as simple as I thought it to be. But as you said, how it sounds is more important than how it works yechnically.