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mystran
KVRAF
 
4461 posts since 11 Feb, 2006, from Helsinki, Finland

Postby mystran; Sun Mar 19, 2017 10:18 am Re: Why modulating Peak filter introduce signal/harmonics?

Nowhk wrote:The filters are the ones within Sytrus, made by "gol" (not a random DSP idiot).


In Sytrus the "res" setting for "peak" type is essentially inverted with respect to the other types, in the sense that low knob values give a narrow bandwidth (= high Q) where as for resonant types high values of "res" knob map to higher Q.

So essentially you have to compare a "peak" with low "res" knob position to "lowpass" with high "res" knob position to get an "apples to apples" comparison (in which case the low-pass has the same behavior). Similarly if you set a wide peak (=low Q), it will be "clean" just like a low-resonance low-pass filter.

This is probably what is happening in other plugins too, since peaking filters are typically parameterized for bandwidth rather than resonance, which then results in same actual "Q" values at the opposite ends of the knob if the same control is used for both.
Image <- plugins | forum
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aciddose
KVRAF
 
10962 posts since 7 Dec, 2004, from Vancouver, Canada

Postby aciddose; Sun Mar 19, 2017 1:03 pm Re: Why modulating Peak filter introduce signal/harmonics?

mystran wrote:
aciddose wrote:The control might as well have been named "pink flamingo multiplication rate".


There birds are red (rather than pink) though.


That's just it, the name of the control doesn't need to be even remotely accurate! :hihi:
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aciddose
KVRAF
 
10962 posts since 7 Dec, 2004, from Vancouver, Canada

Postby aciddose; Sun Mar 19, 2017 1:04 pm Re: Why modulating Peak filter introduce signal/harmonics?

Nowhk wrote:Also biquad by RBJ for example...


Are you certain these aren't all identical RBJ biquad implementations? 9/10 plug-ins use this copy-pasted code without modification.
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BlitBit
KVRist
 
120 posts since 28 Nov, 2013, from Germany

Postby BlitBit; Sun Mar 19, 2017 1:39 pm Re: Why modulating Peak filter introduce signal/harmonics?

mystran wrote:
aciddose wrote:The control might as well have been named "pink flamingo multiplication rate".


There birds are red (rather than pink) though.

Which only shows that the "pink flamingo multiplication rate" has been set to zero for this image. :hihi:
Nowhk
KVRian
 
513 posts since 2 Oct, 2013

Postby Nowhk; Tue Mar 21, 2017 11:05 am Re: Why modulating Peak filter introduce signal/harmonics?

mystran wrote:
Nowhk wrote:The filters are the ones within Sytrus, made by "gol" (not a random DSP idiot).


In Sytrus the "res" setting for "peak" type is essentially inverted with respect to the other types, in the sense that low knob values give a narrow bandwidth (= high Q) where as for resonant types high values of "res" knob map to higher Q.

So essentially you have to compare a "peak" with low "res" knob position to "lowpass" with high "res" knob position to get an "apples to apples" comparison (in which case the low-pass has the same behavior). Similarly if you set a wide peak (=low Q), it will be "clean" just like a low-resonance low-pass filter.

This is probably what is happening in other plugins too, since peaking filters are typically parameterized for bandwidth rather than resonance, which then results in same actual "Q" values at the opposite ends of the knob if the same control is used for both.

Yes, true :) They are inverted, I can hear the "tone" introduced by the low pass (during modulation) as well. Thanks for the help.

Now, two question (if you can help me):

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?

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).

Thanks again for the time you are spending to noob like me :wink:
matt42
KVRian
 
876 posts since 9 Jan, 2006

Postby matt42; Tue Mar 21, 2017 11:48 am Re: Why modulating Peak filter introduce signal/harmonics?

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?
Yes.

Nowhk wrote: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).
A filter will ring and normally the ringing will decay. The amount of ringing will increase with increased resonance. At self oscillation the resonance is high enough that the ringing no longer decays.
mystran
KVRAF
 
4461 posts since 11 Feb, 2006, from Helsinki, Finland

Postby mystran; Tue Mar 21, 2017 4:04 pm Re: Why modulating Peak filter introduce signal/harmonics?

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.
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JCJR
KVRAF
 
1846 posts since 17 Apr, 2005, from S.E. TN

Postby JCJR; Thu Mar 23, 2017 11:02 am Re: Why modulating Peak filter introduce signal/harmonics?

Hi Nowhk

Maybe it was already mentioned, but of course these causal IIR filters of any type, lp, hp, bp, ap etc-- They all have phase shift. The phase viewed on a graph generally descending from low to high frequencies. For instance a highpass might typically have positive phase shift at low frequencies approaching zero phase shift at high frequencies. A bandpass might have positive phase at low frequencies, zero phase at the center frequency, and negative phase at high frequencies. But all of them have that same-direction of phase shift, a slope descending from left to right.

If you modulate the cutoff or center frequency of any of the filter types, the phase shift curve shifts up and down tracking the cutoff frequency.

If you sweep the filter downward, it will "twitch" harmonics flat in pitch. Sweeping the filter up will twitch harmonics sharp in pitch. The more-drastic the phase shift, the stronger the transient pitch shift. High-Q filters bunch the phase shift near the filter center frequency, so if you compare filter sweeps of the same speed, high-Q filters will twitch harmonic pitches more drastically because the phase changes faster for each harmonic as the filter sweeps thru each harmonic.

Similarly, if you compare filters of the same Q, swept at different rates-- The slow sweeps will have small amounts of transient pitch shift and faster sweeps will have larger amounts of transient pitch shift.

If you compare a high-order filter with a lower order filter, each with the same nominal Q, perhaps butterworth, the high-order filter has more total phase shift and so it will modulate pitch stronger than the lower-order filter, if both are swept at the same rate.

Filter-sweeping a harmonic-rich source such as sawtooth wave, each of the harmonics gets "twitched" in series as the filter moves thru them. Sweeping down, first the high harmonics briefly go flat and return to normal pitch as the middle harmonics briefly go flat, and as the middle harmonics are returning to normal pitch the lower harmonics briefly go flat before returning to normal pitch. Sweeping up, the harmonics get twitched sharp, in sequence from low harmonics to high harmonics.

So during the sweep, even with a single-pitch single oscillator, some of the harmonics are out-of-tune with other harmonics in the tone. The ear can hear it, and a FFT display will show "frequency smear" even on pure tones.
matt42
KVRian
 
876 posts since 9 Jan, 2006

Postby matt42; Fri Mar 24, 2017 9:46 am Re: Why modulating Peak filter introduce signal/harmonics?

Nice post, JCJR. I don't think it explains much about the OPs observations, but I found it interesting - I'd never really considered the phase modulation effects of sweeping an IIR filter.
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