AndreasN wrote on Mon, 27 September 2010 06:35 |
bruno putzeys wrote on Sat, 25 September 2010 17:17 |
There's no "EQ camp" claiming this. There is no such thing as a "minimum phase phenomenon". What exists is a minimum phase transfer function (=magnitude & phase response rolled into one). When, in acoustics, someone says that a measured transfer function is minimum phase they most certainly mean it's a combination of a minimum phase transfer function PLUS a pure delay (which is NOT minimum phase).
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Thanks for the clarifications!
Sorry for the poor phrasing in the original post. What I was trying to address is the link between observation system and reality. Don't want to mention names here. There are however, in the room EQ industry, a typical assumption that parts of a room response is close enough to "being" minimum phase. Close enough that the resulting observed aberrations in frequency response can be corrected by minimum phase IIR filters where the poles and zeroes are set by virtue of the seemingly "minimum phase" behaviour in parts of the observed response. Typically done by finding areas where the excess phase calculation is flat. In other words, looking for "a pure delay + minimum phase" response in the room and then disregard the "pure delay" part.
It doesn't seem to me that it's a procedure that have any physical basis. How is a delay+minimum phase response supposed to be corrected by a pure minimum phase response?
(didn't intend to mention names, but here's an example to make it clearer)
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I'll try to shed a different light on this.
Note: I assume here for simplification that direct signal is time-normalised at the listening position, hence qualified of "undelayed".
Many people think that since many acoustic problems are caused by delays and combination of non-delayed and several differently delayed signals, the cure can only be using time-delay techniques to fix them.
Although this may be philosophically true if one wants to address the whole frequency spectrum, it turns out that, at lower frequencies, the effect of combining these signals is merely a modification of the frequency (and phase) response that can generally be addressed by rather conventional EQ techniques. It is clear that, if we consider the simplest comb-filtering effect, which shows complete phase-cancellation when two signals of equal amplitude are delayed by a half-wave, there is no way an EQ would fix this (the least difficult aspect being capable of producing infinite boost).
But in fact, it has largely been demonstrated that up to half the cancellation frequency, standard EQ can be applied, that will correct both amplitude and phase response.
In a real-world situation, there are an infinite number of reflections, but generally, there's not one of sufficient amplitude to introduce complete phase-cancellation, but rather a series of smaller reflections of more or less spread delay, which a good acoustic designer will "tame" in order to make them closer to minimum-phase behaviour, and as a consequence, more EQ-friendly.
Indeed, it is not possible to solve delay-related problems with EQ at MF and HF, but neither is it actually practically doable with delay techniques.
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.