Nowhk wrote:1 - why this tone is introduced only if I modulate the filter? If I place it fixed and fc before/after the sine frequency, I dont hear/see any new "nearest" tone; only with modulation. Is background noise (added by the fc mod) excited and so raised by the res of the filter itself?
This is typically because when the cutoff crosses over a signal frequency, the filter "picks up" some energy from the signal (that it then carries around even if you keep it moving). It can also happen (to a lesser extent) in some filters if the modulation itself introduces noise that modulates the signal (as the filter sees it) effectively creating harmonics on the frequency that the filter is sensitive to.. but see the next answer, things will make more sense afterwards.
2 - what do you mean with "ringing" here? It seems to "ring" here even if the filter's res is not so higher to self-oscillate (which is what I generally call ringing).
As the filter "Q" increased the filter will produce "ringing" or decaying oscillations. Essentially the input on the resonant frequency will excite the filter (which then stores some "energy") and the filter will release this over time. These decaying oscillations are essentially what makes us hear a boost (or cut) in the frequency response (it's a little more complicated than that I guess, but that's the basic idea).
The "Q" is essentially a measure of how long it takes for these oscillations to decay (in fact classic definition of Q relates to the decay of the oscillations directly, but modern practice uses a related mathematical measure that also works for the very low Q cases where no actual cycles occur). This makes the frequency that the filter acts on more specific (and the other way: any narrow filter will create some ringing, it's just how the time-frequency duality works). Once the "Q" is increased to infinity the decay becomes infinite as well and the filter self-oscillates (ie. does not decay anymore) and the filter will act as a sine-wave oscillator, specific to just that one frequency alone (assuming that it's somehow amplitude-limited such that it doesn't simply blow up; "over infinite" Q will cause it to "grow" instead of decay and the "exactly keep it's amplitude" case is generally unstable).
However, the important thing to understand is that there is a continuum
from a very wide filter (no ringing at all) through minimal ringing that we don't really hear as such, to sinusoidal tails that decay over multiple seconds, all the way to where the filter keeps "ringing" from time to eternity. The basic rule of thumb though is that filters that are very specific to a narrow bandwidth (or very steep in their transitions) will create long ringing (even though they don't self oscillate unless you try to make them so narrow that they only cover a specific frequency), while "smooth" filters acting gradually over wide ranges will have very little ringing (ie. often so little that they don't even complete a single cycle).
Hopefully this helps.