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Old 6th January 2013, 09:24 PM   #101
Pano is offline Pano  United States
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Thanks Simon and Thierry.

If we really do hear something, it might be the off-axis response faults, more than the filter function.GD itself. What do you think?
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Old 6th January 2013, 09:34 PM   #102
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Quote:
Originally Posted by Pano View Post
Thanks Simon and Thierry.

If we really do hear something, it might be the off-axis response faults, more than the filter function.GD itself. What do you think?
Not at 100Hz. For some reason when I wrote my reply it went completely out of my mind that your test was centred around 100Hz, so of course off axis response due to beaming, reverberant field etc that I was talking about is all meaningless at those low frequencies. It's really much higher frequencies that I was thinking about like midrange to tweeter crossovers.

I think its just that we simply aren't as sensitive to group delay at low frequencies as we are at high frequencies, with sensitivity peaking in the upper midrange. This much is well proven from the various literature. From your quick test people are struggling to detect 5ms at bass frequencies, yet thresholds at upper midrange are more like 1ms..

I really don't think modest amounts of group delay at bass frequencies is the big bogey man that many people think, we just don't have that much temporal resolution in our perception at those low frequencies...(with most of it coming from the harmonics of the bass notes)
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Old 6th January 2013, 09:44 PM   #103
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maybe a nearfield measurement at 60 vertical off axis could shows a little null.(but ears...).
as said before group delay in listening room reach 30-60 ms or more.
lenght wave at this FR are omnidiectionnals.

but why i do not feel any difference with a headphone ???
next try,a recombined file at 1kHz ?

Last edited by thierry38efd; 6th January 2013 at 09:49 PM.
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Old 6th January 2013, 10:01 PM   #104
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BTW,

to choose a file,i'm saying this is the second file that has been modified.

can we have the answer ?
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Old 6th January 2013, 10:03 PM   #105
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Quote:
Originally Posted by DBMandrake View Post
Not at 100Hz. For some reason when I wrote my reply it went completely out of my mind that your test was centred around 100Hz, so of course off axis response due to beaming, reverberant field etc that I was talking about is all meaningless at those low frequencies. It's really much higher frequencies that I was thinking about like midrange to tweeter crossovers.

I think its just that we simply aren't as sensitive to group delay at low frequencies as we are at high frequencies, with sensitivity peaking in the upper midrange. This much is well proven from the various literature. From your quick test people are struggling to detect 5ms at bass frequencies, yet thresholds at upper midrange are more like 1ms..

I really don't think modest amounts of group delay at bass frequencies is the big bogey man that many people think, we just don't have that much temporal resolution in our perception at those low frequencies...(with most of it coming from the harmonics of the bass notes)
With respect, I disagree wrt GD in low frequencies. It took a certain amount of listening before I could define a difference, but once my ears were trained it was relatively simple to discern which file was which.

The first few harmonics of a percussion instrument are the clearest test to me of phase rotation at a given frequency: kick drums for 100 Hz, snares for 250 Hz, although I haven't yet tested other rotation frequencies with things such as cowbells, claves, or triangles.

Steadier-state instruments such as organ or voice don't seem to be as obvious. I have a theory as to why that might be the case, and it also predicts we are not likely to hear phase differences above 1 kHz so long as the reverberant field is the same.
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Old 7th January 2013, 01:45 AM   #106
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Originally Posted by DBMandrake View Post
Not true at all, unless you have a different definition of minimum phase to everyone else. Two minimum phase filters can and usually do sum to a non-minimum phase result.

Take a 2nd order L/R low pass and high pass. Individually they are minimum phase. Sum their outputs together and the amplitude response is perfectly flat however the sum is not minimum phase because there is a 180 degree total phase rotation from low frequencies to high frequencies. It forms an all-pass filter.

The excess phase / excess group delay without any change of amplitude response was the whole point of pano's test.

Take Linkwitz-Riley 4th order 24dB/oct 1kHz crossover pair, high pass and low pass have identical phase. All pass sum has identical phase. 360 degrees. It is completely minimum phase system with flat frequency response:

Wrapped display of phase:

lr4 1khz cross and sum phase wrapped.png

Unwrapped phase:

lr4 1khz cross and sum phase unwrapped.png

Each order in filter places 90 degrees of phase shift from DC to Nyquist. Typical Cartesian phase plot show +/-180 degrees with vertical broken dashed line showing continuation of plot; this is not a 180 degree phase inversion.
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Old 7th January 2013, 02:38 AM   #107
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I tried to hear difference with all-pass (LPF+HPF) 24DB/oct @1Khz
loaded in convolver,bypass on the fly,with a headphone.
...nothing to notice.Impossible to hear the difference.

delay added below 1 Khz is less than 0.5ms.
need to try with 2 or 3 ms

Click the image to open in full size.
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Old 7th January 2013, 05:23 AM   #108
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Quote:
Originally Posted by Barleywater View Post
Take Linkwitz-Riley 4th order 24dB/oct 1kHz crossover pair, high pass and low pass have identical phase. All pass sum has identical phase. 360 degrees. It is completely minimum phase system with flat frequency response:

Wrapped display of phase:

Attachment 322061

Unwrapped phase:

Attachment 322062

Each order in filter places 90 degrees of phase shift from DC to Nyquist. Typical Cartesian phase plot show +/-180 degrees with vertical broken dashed line showing continuation of plot; this is not a 180 degree phase inversion.
Sorry, but you don't seem to be listening to what I'm saying. The individual high pass and low pass sections are minimum phase by themselves. The summed response of a 4th order L/R crossover (or indeed any order 2nd or above) is not minimum phase. I don't know how I can be any clearer than that.

Please sum the response of the low and high pass sections and show me how it can be minimum phase because it is not. This is basic stuff.
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Old 7th January 2013, 12:16 PM   #109
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Originally Posted by DBMandrake View Post
Sorry, but you don't seem to be listening to what I'm saying. The individual high pass and low pass sections are minimum phase by themselves. The summed response of a 4th order L/R crossover (or indeed any order 2nd or above) is not minimum phase. I don't know how I can be any clearer than that.

Please sum the response of the low and high pass sections and show me how it can be minimum phase because it is not. This is basic stuff.
Pictures in previous post are very clear; showing overlays of phase for low pass, high pass, and all pass as sum. Being all the same phase, all starting at the same time

Here are IR responses of Linkwitz-Riley 4th order 24dB/oct 1kHz crossover pair:

high pass:

lr4 hp ir.png

low pass:

lr4 lp ir.png

all pass as sum:

lr4 all pass ir.png

above in overlay view:

ir lr4 1k cross and sum.png

What about all pass IR pictured above isn't minimum phase?
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Old 7th January 2013, 01:52 PM   #110
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"what about all pass IR pictured above isn't minimum phase?"

The summed response of LP+HP ( LR4) is perfectly flat in amplitude but phase turns, giving a all-pass. Amplitude and phase in the sum are no longer linked by Hilbert's transform as were, separetely, LP and HP.

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