- Jan 12, 2022
Aaah, i see the root of one difference in our understandings.
I don't consider a constant delay to be group delay, or a part of group delay.
Acoustic time or flight, constant processing latency, FIR time, etc....any fixed-time delay....needs to removed from a transfer function to make meaningful sense of phase, imo.
And yes, if phase is linear across the spectrum, the group delay is constant... but it's zero in my book ! (for reason just stated)
I see what you mean, but it's not right. First of all, group delay is group delay regardless if it is constant or not.
Secondly, you cannot get rid of it in online processing applications. If the the group delay is part of the filter and you cannot "skip ahead" (such as in offline processing applications like a music player) then you cannot just remove it. It's an integral part of the transfer function.
Constant group delay also doesn't make a transfer function any less meaningful.
Phase response plots of filters with huge phase shifts become awkward to read, sure, but that's why we also have phase delay plots (which I think you also mentioned).
So it's not that simple to just say that flat delays need to be removed, but I understand what you mean.
If, for example, you look at the transfer function at the listening position and there's a constant extra group delay that is shared among speakers/drivers then you can obviously remove to simplify things when we're talking about music playback, but that's not necessarily true in other applications.
That's not correct. The linear phase filter has phase shift as well, and much more at that.The minimum phase low pass is the only one that has phase shift !
Phase shift is frequency dependent. Just because it's linear vs. frequency does not nullify that phase shift.
I made the comparisons to clarify this common misunderstanding and to show you that my statements are absolutely correct.Until the idea is set aside, that constant delay is group delay.... the further comparisons that follow, simply make no sense imho.
The huge group delay (and therefore phase shift) is part of the linear phase filter.
Summing all this up, I think the term you're looking for is "zero phase" filters, but that's only applicable in offline processing like I mentioned before for obvious reasons.
Yes, except I don't understand the "too perfectly" remark. Obviously you cannot use a min phase filter to perfectly invert a filter or transfer function with all-pass components.I agree with the exception of complementary linear phase xovers. There's no reason for pre-ring with them if well implemented.
Yep, that's often the problem with such inversions. Too perfectly.
A pure inversion like that inverts everything, the good, bad, and ugly Lol.
Yes, and is why simple inversion doesn't work for an entire speaker that is not a full-ranger (one driver, no xovers)
Xover regions, being non-min phase with their all-pass phase rotations, are not really suitable for such inversions (or phase corrections)
That's one of the reasons I've so far stayed away from room correction products...wondering how they actually separate min phase single-driver-only regions from xover-two-driver all-pass phase regions from room reflections from yada.....
On room correction: a highly interesting topic but that's for another thread I think.