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Old 1st January 2013, 02:51 PM   #61
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Quote:
Originally Posted by Pano View Post
I don't remember which is which, I'll have to go back and check. One file has phase shift, two are the original.
No spoilers, I haven't had a chance to play them yet...
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Old 1st January 2013, 03:01 PM   #62
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You've already got a good clue that gives you a 1 in 3 chance.
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Old 1st January 2013, 03:05 PM   #63
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or may it's a trap !
3 files are identicals...

check with real time analyser,a nice dip would appear in the crossover region.(but no more delay in the lows )

Last edited by thierry38efd; 1st January 2013 at 03:10 PM.
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Old 1st January 2013, 04:10 PM   #64
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Originally Posted by Pano View Post
But can you hear it? Few seem interested to find out. (Markus, neither can I hear it)
There are all sorts of excuses (not just in this thread) but very little actual testing.
Two reasons I suspect.

1) Many people have long ago made up their minds whether phase shift or group delay are audible or not, probably without any proper, double blind controlled testing, (or any testing at all) and whether conciously or subconciously they are not willing to perform the experiment and thus disprove their personally held beliefs, whichever side of the debate they may fall on. (There is little motivation to go out of your way to disprove your own beliefs)

Or if they have performed the test and it doesn't give their expected result they're not willing to post "looks like I was wrong" in a public forum, especially if they have been publically vocal on their belief on the forum in the past.

I suspect many people with an engineering perspective or background find it particularly hard to reconcile the fact that phase shift (which clearly makes a waveform look different) may in fact be inaudible in most circumstances, and continue to take it as an matter of faith that phase shift is bad without empirically proving it. Too bitter a pill to swallow perhaps...

2) Nobody wants to be seen as having "cloth ears" in a public forum. If anyone admits they can't hear a difference and then other people come along and say that they can hear a difference, now you're seen as the person that obviously has substandard hearing or speakers...(eg its a matter of pride)

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Originally Posted by Pano View Post
You've already got a good clue that gives you a 1 in 3 chance.
I'm not actually too concerned about which is the original and which is modified, only whether the difference is audible or not. The original recording would have had group delay in it too, on each different component before mixing. For example most vocal mic's roll off below 100Hz (or the same is done during mixing) and that would obviously introduce group delay. No recording is free of group delay.

So really we're just comparing two recordings with different amounts of group delay, not group delay versus no group delay.

I did have a quick listen to the recordings on in-ear earphones just manually switching between them and I couldn't notice a difference. I will give it a try on speakers as well though later.

One thing I've noticed that has been indiscriminately mixed together in this thread is group delay versus excess group delay. The part of the group delay that comes from the minimum phase amplitude response is inseperable from the amplitude response - change one and the other changes.

So all the talk about the different group delay of a sealed box versus a bass reflex etc is irrelevant to the discussion of crossovers and vica versa, since the amplitude response is also different, (both sealed and bass reflex boxes are still minimum phase) so you cannot prove that its the group delay that you're hearing the difference of rather than the amplitude response. We're very sensitive to amplitude response variations, it's nowhere near proven that we're sensitive to excess phase/group delay variations to nearly the same degree.

The only way to tell if group delay by itself is audible or not is to look only at excess group delay - eg group delay which is due to excess phase where no change in amplitude response is present - such as your test, and such is the case in a properly summed crossover with a flat amplitude response.

In this case its a lot harder to prove that we can hear a difference, certainly if the excess phase change is smooth and gradual. These results can't be extrapolated to the minimum phase group delay of different bass alignments though.

Personally, if it turns out that smooth excess phase eg excess group delay without large peaks is not audible or only under exceptional circumstances then that's a huge relief, as the only ways of avoiding such excess phase in the summed response is simple 1st order crossovers with all their ills or complex dsp linear phase systems which inevitably introduce pre-ringing and potentially worse off axis ringing.

It's one less thing to worry about that then allows much greater optimisation of other parameters.
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Old 1st January 2013, 04:12 PM   #65
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Originally Posted by thierry38efd View Post
or may it's a trap !
3 files are identicals...

check with real time analyser,a nice dip would appear in the crossover region.(but no more delay in the lows )
If the filter that was applied was a perfect all pass filter there would be no dip in the amplitude response in the crossover region...that's the whole point of an all pass filter. Only the phase would be different, so you would need to compare the phase of the two recordings to find which had excess phase. (Something that could be done with the right software)
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Old 1st January 2013, 04:22 PM   #66
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The questions "is group delay audible" and "what's the audibility threshold at low frequencies" are quite different from the question "is it an audible problem in a real room with real speakers"?

Like some have pointed out, in a typical listening room the group delay from speakers may not be a problem because the room dominates the sound field and the perception of low frequency sound.

Of course, rooms and speakers vary greatly. Nevertheless it might be a good idea to test the audibility or threshold with decent headphones/earphones, and then do a separate test to see if there still is an audible difference with speakers.
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Old 1st January 2013, 04:32 PM   #67
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Yes, ditto the above. The room typically dominates. But what if the sound at least comes out of the speakers with little or no phase shift? Will that be an improvement?

Yes, Simon, I believe you're right. No one likes to look the fool. Since I'm already quite foolish, I tend to jump into these sort of tests with both muddy feet.
Also right about the dip. As stated before, I spread the crossover frequencies by 25% to reduce the 3dB bump that Butterworth gives at the x-over frequency. There is a tiny bit of ripple, but I can't imagine it would be audible.

Interestingly, if I mix the 2 files together, you can really hear it - like an echo effect. But alone? I don't notice it. I'm working now to correct phase only in my speakers. The phase now isn't awful, but it's far from perfect. With the convolving tools I should be able to make it near perfect. What will that sound like? I'm not sure. I would hope that flat phase will help imaging, but that remains to be heard.

I may post some new files with that have been high passed like a speaker box, but with flattened phase.
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Old 1st January 2013, 04:33 PM   #68
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Originally Posted by DBMandrake View Post
If the filter that was applied was a perfect all pass filter there would be no dip in the amplitude response in the crossover region...that's the whole point of an all pass filter. Only the phase would be different, so you would need to compare the phase of the two recordings to find which had excess phase. (Something that could be done with the right software)
i've understood that Pano only shifts the lows.

i'm using rePhase as inverse all-pass/all pass in a multi convolution engine.
it allows to manage time independently of frequency response.

rephase

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Old 1st January 2013, 04:45 PM   #69
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Yes, Simon, I believe you're right. No one likes to look the fool. Since I'm already quite foolish, I tend to jump into these sort of tests with both muddy feet.
I've made a fool of myself enough times now that I'm not worried about it either. If you go back through the last 2 years of my posts you'll see that I've changed position on a number of my "beliefs" in that time largely due to the discussions that have made me rethink my position. There's nothing like having to defend your position in public to make you question and rethink it - if you're open minded enough to accept that you could be wrong...
Quote:
Also right about the dip. As stated before, I spread the crossover frequencies by 25% to reduce the 3dB bump that Butterworth gives at the x-over frequency. There is a tiny bit of ripple, but I can't imagine it would be audible.
I'm curious why you didn't just use a summed L/R crossover of the same order ? Perfectly flat summed amplitude response with no messing around with spreading the crossover points... and more representative of actual crossovers in use. (Who nowadays would use an even order butterworth instead of a L/R in a speaker ?)
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I may post some new files with that have been high passed like a speaker box, but with flattened phase.
So, a 12dB/oct butterworth vs a 24db/oct with post filter phase linearisation applied to both ? Now that would actually be quite interesting...
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Old 1st January 2013, 05:03 PM   #70
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I used Butterworth because that's what my editing software does. That's all I got.

Yeah, I could do the 2nd order vs 4th order/with phase correction. Want to pick a high pass frequency? Remember, it's supposed to be a box.
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