Sorry for the delay. Too much going on.
So.. I've received excellent coaching from John via email. It really seems the minimum phase EQ's can and do change the rate of decay for the boosts caused by standing waves.
Have been discussing EQ'ing and room correction with various folks over the last year. None in the "EQ camp" have been able to provide any sort of evidence like that. My hat is of to John for finally making some sense out of one aspect of room EQ'ing! Namely: to tame low frequency peaks caused by standing waves.
The price: punching a hole in the direct sound.
It may be a worthwhile trade off in some circumstances.
There's also some other issues. More on that below.
JohnM wrote on Tue, 12 October 2010 17:06 |
This is incorrect. To be pedantic, the time response of a typical IIR filter is, well, infinite but for a real world example simply make a measurement of a very narrow EQ boost and look at how long its response rings.
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This was one of the biggest surprises in this exercise. Have of course looked at EQ impulse responses before, but I tend to use way lower Q's. The extremely long responses of the high Q low F filters explains a lot.
Geoff Emerick de Fake wrote on Tue, 28 September 2010 14:32 |
Most of the "reverse impulse response techniques" end up doing just about the same as their purely EQ counterparts. Those techniques that are supposed to work at MF are plagued with a too-narrow sweet spot.
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Right on. Basically, it boils down to the EQ giving a sort of impulse cancellation across a rather long stretch of time.
Which makes me wonder.. Cancelling stuff in the computer is all fine and cancelling at the point of an omni microphone is fine. But what about the hearing system? The ability to hear direction at low frequency isn't good, but it's not totally gone before one hits the lowest octave or so. This may be an issue, or it may not be an issue.
In any case, the situation is one where positive pressure is cancelled with negative pressure, and vice versa. How does it feel compared to simply having less pressure fluctuations in the first place? Listener fatigue comes to mind.
JohnM wrote on Tue, 12 October 2010 17:06 |
Quote: | Cutting in the direct signal to counteract standing waves will leave a hole in the direct signal
| This is absolutely correct, but we need to consider the time scale of the frequencies being discussed and our ability to detect changes on timescales that are fractions of a cycle. In a typical domestic sized room, for example, the reflections of a 50Hz signal are arriving at the listening position before even a single cycle of the direct signal has completed, so the room's contributions are already arriving well within the time in which the auditory systems starts to process low frequency signals.
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I think this deserves more discussion. Integrating 20ms (1/50Hz) is pretty common. Integrating several hundreds of milliseconds, as in a typical standing wave boom, seems a bit far fetched to me. Haven't read any particular tests on this or done much study on my own. From what I've gleamed, it seems that standing waves are heard more as a separate boom than an aural entity that fuse with direct sound like early reflections do.
Anyone?
JohnM wrote on Tue, 12 October 2010 17:10 |
AndreasN wrote on Tue, 12 October 2010 15:11 |
Quote: | Now we have something we can work with. Anywhere the excess group delay plot is flat is a minimum phase region of the response.
| I've also looked at this aspect in several different room measurement. The only way I can find any part of the excess group delay to "be flat" is by not looking hard enough. As I zoom in, the response observed is never flat in any region of any room responses.
| "Flat" is a relative term, if you use an electron microscope is anything flat?
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This situation is different. If looking at a pure minimum phase response, the excess phase plot is absolutely flat no matter how far one zooms in.
"Being sorta minimum phase" is obviously not the same as being minimum phase. At what point is the excess phase plot flat enough? 10 millisecond deviation? 1 millisec? 0.1 ms? It doesn't seem to hold up to scrutinous analysis.
Another strange thing is that all the excess phase plots I've looked at, from real rooms, tends to be flatter in the high end than the low end. If one is to take this literally, EQ'ing high end should be better than EQ'ing low end.. We know it doesn't work that way.
Geoff Emerick de Fake wrote on Wed, 13 October 2010 00:44 |
In your original post, you mentioned a membrane flapping: it is obvious that no EQ can correct this because it is both non-linear and time-variant, but reflections are enough linear and time-invariant, when they are significantly shorter than the period of the signal. Then, if the response has the shape and phase of a biquad, it is a biquad as far as processing is concerned.
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Hmm.. Shorter than the period of the signal? Impulses doesn't have periods.