Linear vs Minimum Phase filters in REW for miniDSP - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #21 of 38 Old 08-08-16, 09:13 AM
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Re: Linear vs Minimum Phase filters in REW for miniDSP

This info helps me a lot. Thanks.
> The phase difference we see over 10 kHz is due to the Right Ear (red trace) being wrapped at 180 and the Left Ear (green trace) being unwrapped. They both measure the same. Below, in the second chart, I show the impact of unwrapping both the traces using a cursor position at 200 Hz. Any cursor position below the ~13 kHz phase divergence will achieve the same result.
> This is a case where the SPL response rolls off steeply at ~13 kHz and thus the HF Phase response rolls up steeply as a result. In this case you have placed the 2 IR's properly for export into rePhase. You will just not try to correct the phase above 13 kHz.
> This is a single driver with no XO or room impact so the measured phase response is the expected minimum phase response.

It all makes sense to me now.

Linear vs Minimum Phase filters in REW for miniDSP-phase1.png

Linear vs Minimum Phase filters in REW for miniDSP-phase2.png
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post #22 of 38 Old 08-08-16, 08:18 PM Thread Starter
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Thank you for your analysis.
I attached one chart which original data is the same one as I sent. The display on the curves are SPL, Phase(IR Delayed and minimum phase).
The difference between [IR Delay] and [mini.phase] curves is caused by your previous explanation, right?
Then, I can export the TF(.txt) file after [IR Delay] process to rePhase, and I can just ignore over 13KHz.
I got you. Thanks a lot.

Yuichi
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Linear vs Minimum Phase filters in REW for miniDSP-pic.jpg  

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post #23 of 38 Old 08-08-16, 11:28 PM
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Re: Linear vs Minimum Phase filters in REW for miniDSP

The SPL/phase measurement (Blue traces) are the correct traces to export. The IR is located well for export to rePhase. The phase falls to the 0 axis, i, e., to where the SPL rolls of at ~13 kHz. So yes this is a good measurement to export, and yes, it is correct to avoid any SPL/phase correction in rePhase above 13 kHz. Just now I am looking more closely at the SPL trace, the rapid SPL falloff is actually nearer 11 kHz so that would be a little better upper limit for any SPL/phase correction.

The minimum phase trace is of no practical interest for this purpose.
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post #24 of 38 Old 08-08-16, 11:56 PM Thread Starter
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Thank you very much. I learned a lot. Yuichi
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post #25 of 38 Old 08-22-16, 09:07 PM Thread Starter
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Hi
I have one other question regarding IR and phase display.

Attached is a 2 way speaker measurement. The tweeter location is miss aligned.
In this case, the phase curve at 2.74KHz and 3.17KHz slipped down sharply. I guess on these frequencies, SPL is very small, so it is difficult to compare the phase. If so, why so large phase difference exists before and after the 2.74 and 3.17KHz??
Minimum phase display shows reasonable curve.

In case of original SPL curve is reasonably flat (no sharp drop:the tweeter is reasonably located), IR phase curve looks reasonable and close to the minimum phase curve.

Yuichi Arai
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post #26 of 38 Old 08-23-16, 04:32 PM
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Assuming this is a typical speaker, this is normal for LP measurements in a room.
> The phase is 'correct', but includes the impact of the strong reflections in the room.
> We normally have little use for the plot in this format. We normally want to see the phase of the direct (first arriving) sound.

I loaded your file in REW 3 times and then changed the settings on the second 2 copies to illustrate the changes needed to put this measurement in a more usable form.

1) Below, we show what happens with the phase unwrapped (red) Vs what would be displayed with the phase wrapped (blue). REW unwrapped the phase based on your current cursor position, so the cursor must have been in the range of the green arrow around 15Hz. We know that because the 2 traces are aligned in that range. It is standard practice to set the highest frequencies measured near 0. REW normally places the IR location near 0 ms so that will happen. Had your cursor been maybe 10 kHz when it was unwrapped there would be the expected change in the relative offset. This is just a side comment to explain why the 2 traces are located where they are.

Linear vs Minimum Phase filters in REW for miniDSP-unwrapped-vs-wrapped.png

2) Below, we show how to eliminate the impact of the room so we can better see the phase rotation of the direct sound. We can use the REW Frequency Dependent Window (FDW) feature to mitigate the room impact. This feature allows us to look at the phase of the first few cycles at each frequency. In this case I used a 5 cycle setting as shown below. Larger settings still had at least one of the shifts. I often find that 5 cycles is a good setting for this purpose as it often removes the room influences and still does not distort the phase at the extreme frequencies. Some shift of the phase will occur at the extreme high an low frequencies with lower FDW settings. I zoomed in on the wrapped phase trace (blue) and made the needed setting changes on the FDW (green) trace. If we look closely we see that at about 2700, and 3100 Hz strong room reflections resulted in 360 sudden phase changes in the trace. These sudden 360 shifts are more easily seen the first chart where the unwrapped (red) trace drops 360 suddenly at these 3 frequencies.

Linear vs Minimum Phase filters in REW for miniDSP-no-fdw-vs-fdw.png

3) Below is just to show what the FDW trace looks like when it is properly unwrapped.

Linear vs Minimum Phase filters in REW for miniDSP-fwd-unrapped.png

Now for the big disclaimer. This looks like more than one driver was active in this measurement. If there is an XO near 3 kHz and these 2 driver are not on the same baffle then the large phase shift near 3k may be due to distance offsets of the drives rather than room influences. Also, If timing delays are being applied to align the drivers phase then the delays may be set wrong. It is impossible to be sure of the situation by just looking at a measurement. There is not enough information about the details of the setup.
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post #27 of 38 Old 08-23-16, 07:15 PM Thread Starter
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Thanks, I will try to reproduce what you said first. Yuichi
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post #28 of 38 Old 08-24-16, 01:36 AM Thread Starter
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Thank you. I reproduced the phase curve and understood the mechanism. Yuichi
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post #29 of 38 Old 11-01-16, 09:34 PM Thread Starter
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Hi

Is there any way to calcurate two measurement data such as "add" or "subtract" or "Invert".... I like to use this function to subtract the free space data from the headphone data.

Yuichi Arai
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post #30 of 38 Old 11-02-16, 07:45 AM
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Yes, REW refers to these IR functions as 'Trace Arithmetic'. They are located in 'Graph Controls' of the 'All SPL' graph panel. REW 'Help' has more info and screen shots of that panel if needed.
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