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Old 1st January 2013, 09:41 PM   #81
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Quote:
Originally Posted by Bob Richards View Post
My intended point is that I don't know how you can introduce a significant GD anomaly without simultaneously creating a delay effect. How do you separate the two? How do you say the effect is from GD and not TD or phase shift?
But group delay is a time delay. A delay at certain frequencies relative to other frequencies, and is caused by a difference in the slope of the phase at different frequencies. It's all intertwined and inseparable.

Say you had a 1Khz 24dB/oct L/R crossover that was electronically summed to a flat amplitude response. (no speakers involved) Over most of the audio range you'd have essentially zero group delay, but around 1Khz the delay would peak up to something like 0.5ms (approximate off the top of my head figure) with a nice bell shaped curve. Over that narrow range of frequencies there is a time delay in arrival of the signal relative to other frequencies.

In an acoustic environment you also have a fixed propogation delay through the air from drivers to mic/ear due to distance which is constant for all frequencies, (assuming time aligned drivers) so in taking a group delay measurement you usually choose the frequencies with the shortest delay as your arbitary zero reference, with additional delay being a positive figure.
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Old 1st January 2013, 09:43 PM   #82
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try Xotimes (Jean luc Ohl)
Xotimes (standalone or vst)

check yours ears and your abilities to discern a perfect transient system !
you can listen your library,headphone or loudspeaker.

worhwhile to try and read feedback here.
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Old 1st January 2013, 10:27 PM   #83
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Originally Posted by Pano View Post
I can try some other stuff, can anyone suggest a music sample to alter? The Steely Dan had pretty good energy below 100Hz, but not all the way down to 20. Not many do.
I find that bass and kettle drums in classical music have a much better sound when the woofs are flat to 20HZ, rather than only 40HZ. They might make a good GD test. Perhaps it's the way the harmonics mix that's going to be noticeably different (?).
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Old 1st January 2013, 10:31 PM   #84
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Originally Posted by DBMandrake View Post
But group delay is a time delay. A delay at certain frequencies relative to other frequencies, and is caused by a difference in the slope of the phase at different frequencies. It's all intertwined and inseparable.
If I understand correctly, GD is not the delay itself, but the rate of change of that delay or phase shift (?)
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Old 1st January 2013, 10:32 PM   #85
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This is from Is Linear Phase Worthwhile?

I pontificate on many of the points discussed here and also show 'real' delay in various xover circuits and other stuff.

The important point about Minimum Phase transfer functions is that there will exist another 'real life' Min. Phase function that will exactly correct it WITHOUT ANY RESULTANT DELAY.

I also make the point that the concept of Group Delay only makes sense if either
  • the amplitude response is constant over the frequency range of interest
  • or the group delay is constant over the frequency range of interest
References, maths etc in the paper.

Also Listening Tests.
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File Type: gif PhaseResp.GIF (52.2 KB, 203 views)
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Old 1st January 2013, 10:56 PM   #86
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Originally Posted by Bob Richards View Post
If I understand correctly, GD is not the delay itself, but the rate of change of that delay or phase shift (?)
Not quite.

Group delay is delay plotted versus frequency. It's the rate of change of phase. (or the slope of the phase graph) As I said before, its the first derivative of phase.

Imagine you measure the response of a driver at a certain distance and our imaginary driver has a nice flat amplitude response.

The further the distance from the speaker to mic, the steeper the phase curve will be, because a constant time delay will cause more phase shift the higher in frequency you go. If there is no group delay the phase shift will be a straight line on a linear frequency axis.

That's why the steepness of the phase curve is equivalent to time delay.

Now imagine what happens when we cross over between two drivers, one of which is closer to the mic than the other. Well into the passband of the closer driver the steepness of the phase curve will be a certain slope, however when we fully cross over to the further away driver the phase curve must be steeper because of the additional distance (delay) causing more phase shift per Hz. This causes a transition from less to more group delay as we transition from one driver to the other, therefore a shelf in group delay due to the physical offset.

It's not quite that simple though because the all pass response of the summed high pass and low pass also causes a temporary increase in phase slope around the crossover due to the way the phase of the two sources sum, even if the drivers were time aligned. The delay is purely electrical but just as real.

So what you end up with time misaligned drivers crossing over to each other is a shelf between the two frequency ranges but also with a peak in group delay near the transition region that goes above the higher side of the shelf.

See below for an example of a 3rd order crossover at 4Khz with the tweeter about 20mm closer than the midbass. The measurement is only accurate down to about 500Hz due to the gating time but you can clearly see the peak in GD as well as the shelf on either side.

The reason why the curve is so smooth despite being an acoustic measurement is that its actually excess group delay, which factors out the group delay related to amplitude response changes.

PS I'd forgotten just how small the group delay is in typical crossover arrangements, its far below the 0.5ms guess I suggested earlier, with the group delay from the 3rd order crossover being less than that caused by the 20mm error in driver alignment. In this example about 0.1ms from the acoustic alignment error and a further 0.05ms from the crossover itself.
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Old 1st January 2013, 10:59 PM   #87
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Originally Posted by DBMandrake View Post
In an acoustic environment you also have a fixed propogation delay through the air from drivers to mic/ear due to distance which is constant for all frequencies, (assuming time aligned drivers) so in taking a group delay measurement you usually choose the frequencies with the shortest delay as your arbitary zero reference, with additional delay being a positive figure.
not only fixed delay due to mic distance,also reflexions come later to the listener,it depends of the wall absorption,some region can be delayed more than other.(reverberation time ).
but it's a quite complex for me.acoustic treatement,improvement needs in-depth knowledge (and wiseness ).

that's why i think,we need to correct loudspeaker before (spl and phase (word "delay" is most suitable)).
and in a second time,correct (or trying to ) the room,DRC...etc..

Last edited by thierry38efd; 1st January 2013 at 11:03 PM.
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Old 2nd January 2013, 06:37 PM   #88
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Quote:
Originally Posted by kgrlee View Post
This is from Is Linear Phase Worthwhile?

I pontificate on many of the points discussed here and also show 'real' delay in various xover circuits and other stuff.

The important point about Minimum Phase transfer functions is that there will exist another 'real life' Min. Phase function that will exactly correct it WITHOUT ANY RESULTANT DELAY.

I also make the point that the concept of Group Delay only makes sense if either
  • the amplitude response is constant over the frequency range of interest
  • or the group delay is constant over the frequency range of interest
References, maths etc in the paper.

Also Listening Tests.
Convolution of any two minimum phase functions with group delay, which they all have, results in minimum phase function with group delay. That first sample in each impulse is peak means propagation time for maximum output sample is minimum. A class of minimum phase filters exist for which convolution of their amplitude conjugates returns perfect IR: parametric EQ, which is monotonic to a single frequency. The higher the Q, the longer they ring. These filters may be summed aligned to filter sample peaks (1st sample every time), but now we define minimum phase because each frequency is at same phase at 1st sample. The lower the frequency, the longer the time before amplitude peak in minimum phase filter. Summed series of parametric filters may be used to flatten frequency response, with smoothing of phase/group delay, but this leaves fundamental high pass phase behavior of drivers to deal with, and an information system that won't pass a square wave without waveform distortion.
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Old 6th January 2013, 03:46 AM   #89
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Originally Posted by Pano View Post
If you want to test this by listening, I've made some files for you. You'll recognize the song. It has good content below 100Hz
The audio file was split into 2 parts, one was 4th order low pass filtered at 100Hz, the other high pass filtered at 125Hz. The spread is because the filter is butterworth and combines flatter with the slight spread. The file was then recombined. Phase is rotated 180 degs at 110Hz (because of the spread) and continues down to 360 degs at DC. Or vice-versa, depending on how you look at it.

The files are here, you can download them. About 1 min long, each 2.2meg in 320KBS mp3 format. Can you tell which was manipulated, and which was not? Or at least if there is a difference or not between the 3?
My MP3 file player (Audition) also shows the waveform, so I couldn't do the ABX test because the differences were obvious to the eye. Given that, I tried listening to the snare drum -- a previous phase rotation test at 250 Hz was more obvious on that particular instrument over others, but not this time. My guesses were no better than chance.

On the other hand, listening to the kick drum during the first few seconds, and its separation from the bass guitar, did show a subtle difference. The kick in one file was not as "slammy" and also less distinct from the bass. I could rapidly control-tab for a few seconds between the two files (a crude randomiser), and upon listening carefully still be able to tell which was which with my eyes closed.

I'll PM Pano to tell him which one I thought was processed and which one was clean.

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Old 6th January 2013, 07:22 AM   #90
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Yes, percussion on track is only sound with hard transient. Pano's Butterworth filtering for this is form of shelving filter, and from group delay perspective looks something like this:

shelving allpass 100 125.png

Very mild compared to temporal aberration with speaker in living room measured at listening position:

sub gd raw solo.png

Above is raw cardioid woofer setup with previously posted DSP correction results in #36
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