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Old 27th August 2009, 11:42 PM   #391
tresch is offline tresch  United States
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Default Problem with stiching phase

I'd like to get people's opinions on this. I'm suprised noone's posted about it yet, I searched the thread and found nothing about stitching or phase problems.


I'm doing near field and gated far field measurements to get a complete picture of the driver's response in its enclosure. Standard operating procedure. After getting decent measurements in each configuration, I used the modify menu to stich the two together into a new complete response graph.

As the stitching tool mentions that it should both match amplitude AND phase before stitching, I'm assuming that I should get a nice resulting phase graph that blends from one measurement to the next, just as with the blending of the amplitude.

This seems not to be the case. Whenever I do a stich between two measurements, the phase in the resulting measurement EXPLODES IN A WRATHFUL BALL OF DOTTED LINES.

*ahem*

Let me simply demonstrate with a screenshot! (The built-in image capture feature is awesome btw)

The first image is the two measurements, with both amplitude and phase displayed, before the stitch

The second image is all three measurements; the near field, far field, and resulting stich; displayed. Obviously one can't make out much as the phase on the stitch is all over the place. The amplitude, btw, is a perfect blend between the two at 800hz with a 200hz overlap. In this instance I checked "match amplitude" and "match phase" before performing the stitch, but I have tried this with all combinations of different frequencies and different settings for the match, and everything behaves normally, but the phase always explodes as shown.
Attached Images
File Type: png a5-prestich.png (61.6 KB, 305 views)
File Type: png a5-postich.png (163.6 KB, 299 views)
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Old 28th August 2009, 12:20 AM   #392
tresch is offline tresch  United States
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Default More phase stuff

I'm building an MTM so my workflow for generating a response for my woofers is as follows:

1. Nearfield measurement of woofer 1
2. Nearfield measurement of woofer 2
3. Sum measurements of woofers 1 & 2
4. Farfield measurement of both woofers together
5. Stich the Summed measurement from step 3 and gated far-field from 4.

Ignoring phase, this is still fine and good from a frequency standpoint, until I get to the final step. It appears that the stitch function also refers to the phase calculation when blending the amplitude from one measurement to another, and creates a frequency null if the drivers are out of phase, as would happen in a crossover situation.

The problem is that when using the "Sum" function to add the two nearfield measurements, the resulting phase graph is all over the place, so when doing the final stitch, unless I use a stich width of "1" I get a huge dip in response, which I'm guessing comes from bizarre phase information generated from the Sum function.

Not sure if this is how it should behave or not. Either way, here's a screenshot of what I'm seeing. The first two graphs (blue and red) are nearfield measurements of both drivers in the box, which are appropriately similar in both amplitude and phase (they are not normalized in the measurement options). The third measurement (green) is the Summed measurement of the first two, and the resulting phase.
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File Type: png da175-sum-phase.png (71.4 KB, 290 views)
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Old 28th August 2009, 02:09 AM   #393
Pano is offline Pano  United States
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I have the same phase problem here when stitching. Just thought I was doing something wrong!
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Old 28th August 2009, 10:57 AM   #394
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Merging gated far field data with low frequency near field data isn't of much use if the near field data isn't corrected for baffle diffraction/baffle step effects.
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Old 28th August 2009, 11:35 AM   #395
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Default New version with Bug fixes 1.3.0.4

Quote:
Originally Posted by tresch View Post
This seems not to be the case. Whenever I do a stich between two measurements, the phase in the resulting measurement EXPLODES IN A WRATHFUL BALL OF DOTTED LINES.
It was a bug. - Stitching now works again

Version 1.3.0.4 (2009-08-28)

Features/Changes:
* Lowpass/Highpass variable length
* Old autosaved file (MeasAutosaved.hlm) is moved to AutosavedObsolete
* QBox fit - Automatic plot
* Options - Sample offset increment
* Logarithmic frequency axis - Grid improved
* Export - Overwrite confirm dialog (Removed from browse)

Bugfixes:
* Stitch Measurement C = A -> B
* Impulse domain: Auto Zoom
* Options: Clear measurement resets values
* Clear measurements and close does not overwrite autosaved measurements
* Memory leaks
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Old 28th August 2009, 05:57 PM   #396
tresch is offline tresch  United States
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Yay! It fixed the phase problem with the Sum function, too! THANKS MR. BOJESEN
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Old 28th August 2009, 07:09 PM   #397
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Quote:
Originally Posted by john k... View Post
Just browsing this thread. I don't know where the above quote was initiated but it is not entirely correct.
Thanks. The quote was made by Earl Geddes here.
http://www.diyaudio.com/forums/showp...&postcount=369
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Old 1st September 2009, 09:09 PM   #398
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Default New Release 1.3.0.6 - Import of DUT response

Quote:
Originally Posted by KSTR View Post
And with the complete import option as you plan it one could contercheck if the processing/DUT is really consistent with the linear superposition principle, that is if the chains
a) signal --> response --> user filter
b) signal --> user filter --> response
are identical in the basic response (but not in the S/N and in the distortion, that is obvoius). Which then yould be a nice check for the LTI-quality of the system under test, etc

- Klaus
Version 1.3.0.6 (2009-09-01)

Features/Changes:
* Import of DUT response Import > 'Signal & Recording'


------------------------------------------------------------

Attached picture shows the response of 30sec mp3 encoded music
Attached Images
File Type: png ScreenShot005.png (60.4 KB, 878 views)
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Last edited by askbojesen; 1st September 2009 at 09:10 PM. Reason: Attachment not shown
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Old 1st September 2009, 09:48 PM   #399
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Default Music as measurement signal

What I need next is to play music through my speakers and correlate with a recording.

Another attachment shows the difference between 128 kbs, 320 kbs mp3 -
Does anybody know where a new thread regarding frequency response of encoded files would belong? Or If it is old news - where can I read about it.
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File Type: png pic1.png (57.0 KB, 887 views)
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Old 1st September 2009, 10:07 PM   #400
Key is offline Key  United States
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Hmm I dunno exactly where to read about it. First thing you'll maybe want to do is find the er "best sounding" mp3 algorithm or at least the most popular ones (LAME for instance) and test them. But a friend and I (more him than me) have done a little experimentation. There could be a couple of commonly overlooked variables imo when using mp3s. I think some of them clip the signal if you don't drop the gain a little before encoding. Something to do with floating point - please feel free to tell him if I am wrong everybody because this is more what I have seen people report and I think I did notice once or twice back when I dealt more with lossy.

But a lot of it will have to do with perceptual encoding. I like testing every step of the chain in a recording but I have not been brave enough to do it with the common mastering techniques combined with lossy encoding. I pretty much predict it will be VERY messy.

Last edited by Key; 1st September 2009 at 10:10 PM.
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