Hey Oden,
here's my attempt..
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Originally Posted by
Oden
Okay, so I don't understand what causes EQ's to pre and post ring. I have read that this is related to the gibbs phenomenon, but I don't really see the connection.
ok.
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Originally Posted by
Oden
I understand the difference between linear and minimmun phase EQ's though (at least in practice).
Mhhh... probably not.
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Originally Posted by
Oden
I also understand that EQs delay the existing signal, phase distort it and then mix it back with the original, is this phase delay the cause of ringing?
generally, yes. But I think we have to go back to a handful basics. The term "ringing" is easy to confuse with other things.
First of all, when engineers use the term ringing in relation to filters, they mean a
visual effect appearing in the impulse response representation. More clearly, they do not mean "frequency magnitude ripples", "filter resonance" or anything related to the ringing of a bell.
Look at these two impulse response plots below. On the left, you see the frequency response alternating with the group delay response (= phase shift expressed in time). To the right, you see the according impulse response:
The first filter is of the linear phase type. In this case, the frequency vs delay plot draws a flat line. Accordingly, the impulse response is symmetrical. The impulse rings "in" and "out" after the event.
The second filter is of the minimum phase type. That is, it uses the smallest amount of delay possible to achieve its task. The task, in this case, is the low pass filter behaviour you see in the frequency magnitude plot. Note that both filters have practically the same frequency magnitude. The second filter however, distorts the original phase relationship, it clearly delays some frequencies by stronger amounts than others. Looking at its impulse response, you can see that the filter rings "in" almost instantly, and visually, only rings "out", although to a much stronger degree.
Just for reference, a pure wire's impulse response looks like an infinitely small spike. That is, it doesn't change the original impulse at all.
The stuff appearing before and after the original impulse is what we call ringing. The steeper a filter is, the stronger the impulse response ringing has to be. Note that pre and post ringing doesn't always have to look wavy (see: MA or Gaussian impulse).
Many misunderstandings arise from the fact that most music producers/musicians and even audio engineers never bother understanding mathematical definitions.
Point is, in order to understand the impulse response of a filter, it is absolutely crucial to understand what the dirac impulse represents in itself! The dirac impulse is an infinitely (or in bandlimited systems: a maximally) small spike at unity gain. This little spike contains all (representable) frequencies of the system. All possible frequencies from 0Hz to Nyquist. As such, it really is infinitely wide! The blank areas before and after the spike are essential parts of the dirac impulse. They really are "filled", but all waves stack up in such a manner that they cancel out to silence right before and after the visual spike. Imagine a single 1Hz event, it's 1 sec long, yet it's in that spike. Fascinating, eh?
With this in mind, it's much easier to understand ringing. When the filter does his work, it will trouble this perfect stack and reveal previously hidden content (either after and/or before the visual spike).
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Originally Posted by
Oden
Is EQ ringing a necessary part of any EQing whether done in physical world by objects like walls in or any EQ devices? I think so, but doesn't this mean that walls and different parts of instruments and such also change the phase of a signal and cause this ringing?
Yes, they do. Have you ever seen mechanical engineers hitting their their machines with a hammer to find cracks in the structure? That's the same principle. They send an impulse through the system and listen!
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Originally Posted by
Oden
If that's the case, wouldn't there be a way to "correctly" EQ a signal so that there is none of this ringing / phase change. But what if say a string sounds DULL, is there a way to make it sound brighter without EQing it in some way? Is the EQ ringing going to be a necessity with dull sounding oscillator, if you want to make it brighter?
In theory, yes. Via re-synthesis and/or additive synthesis. Problem is, nobody has found a way to re-synth a sound without using conventional filter and their ringing.
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Originally Posted by
Oden
If I create two sine waves in a DAW at 200 and 800hz, I can boost the other by 4db without any phase change or ringing, thus EQing without any artifacts. If I do same thing with an EQ I get ringing and phase change. Why can't any EQ boost harmonics in similar way to what I just did in my DAW, without creating ringing or phase change? Simply by looking at what's already there and boosting it! I could layer simple in-phase sine waves on tracks to boost certain frequencies without ringing and phase shift, correct?
Yes correct. In the real world, ringing only appears with short, wideband signal (such as drums). Keep in mind my explanation of the dirac impulse. The filter doesn't add anything, it only uncovers what's been there in the dirac impulse since the beginning.