Linear vs Minimum Phase filters in REW for miniDSP - Page 4 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #31 of 45 Old 11-02-16, 06:52 PM Thread Starter
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Thanks, very clear.

Yuichi
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post #32 of 45 Old 12-17-16, 10:41 PM Thread Starter
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Re: Linear vs Minimum Phase filters in REW for miniDSP

I like to ask about the impulse response waveform.
Attached is the IR data taken by REW. DUT is a 12cm full range speaker mounted on a enclosure.
The purpose is to adjust accurate dual speakers positioning. So, as the first step, I try to capture accurate impulse waveform.
Attached F120AbyREW is taken by REW. I wonder why I have the first negative spike in front of main positive wave on time zero. Is this negative spike true?
As a reference, attached F120AbyDirac taken by Dirac Live measurement facility. This wave is before EQ. The DUT is identical including miniDSP UMIK-1 mic and other signal chain. for both measurements. This has no negative spike in this case.
Any one knows, why? or any specific settings on REW for this purpose?

Yuichi Arai
Attached Thumbnails
Linear vs Minimum Phase filters in REW for miniDSP-f120abyrew.png  

Linear vs Minimum Phase filters in REW for miniDSP-f120abydirac.png  

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post #33 of 45 Old 12-18-16, 08:05 AM
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Quote:
AraiYuichi wrote: View Post
any specific settings on REW for this purpose?
Yes, see REW 'preference/soundcard'. Under 'input option' there is an 'invert' check box. This box in normally 'clear'. If it 'checked' the IR of all measurements will be inverted. This is there to correct for some low cost soundcards that invert the signal as a result of its design.

Also REW provides the option to invert individual measurements (handy for some types of analysis). This control is only seen when the 'Impulse' graph panel is selected. It is found when 'show graph controls' is open. The box is labeled 'Invert Impulse'. The box inverts the IR of the active measurement if it is 'checked'.
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post #34 of 45 Old 12-18-16, 06:39 PM Thread Starter
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Thank you for your comments.

I did not set the 'Invert Impluse' check both in the 'Sound Card' tab and the 'Analysis tab' in the Preferences settings. Those are both clear.
Is there any effects generated by digital calculation? Or speaker diaphragm behavior?
It looks to me that the large positive pulse looks the first arrival sound like the Dirac one?
How do you think?

Yuichi
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post #35 of 45 Old 12-18-16, 08:20 PM Thread Starter
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Re: Linear vs Minimum Phase filters in REW for miniDSP

On top of Impulse waveform question, I have following one.

First, settings are as pic 'Analysis', SoundCard and Measurement for all through following measurements.
I have some times the 'NotGoodSPL' curves. In this case, impulse is like 'NotGood Impulse'.
When it works fine, the curves are like 'GoodSPL' and 'GoodImpulse'
The provability is ..let's say 50% to 50%.
It looks like that the Sweep does not reach to the high or the sweep does not synchronize??
If you have any suggestion, please let me have it.

Thank you in advance. Yuichi
Attached Thumbnails
Linear vs Minimum Phase filters in REW for miniDSP-analysis.png  

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

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

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

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

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

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

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post #36 of 45 Old 12-18-16, 09:54 PM
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Re: Linear vs Minimum Phase filters in REW for miniDSP

The negative going initial IR peak is just due to the extra phase rotation when the sweep extends well past the point where the SPL starts to roll off significantly. This is not surprising. We normally set the sweep so we are testing within the bandpass range of the speaker. There is no problem sweeping beyond that range, but then we may see the initial IR peak be negative as you found above. Again - no problem. If the purpose is to confirm the polarity using the IR graph we should chose a sweep that stops within the bandpass range of the speaker. The IR chart has some uses, but the other charts show polarity and almost everything else much more clearly. The charts just above show the phase to be at -720° from 300 Hz to 15 kHz. That is the same as 0° so the speaker is definitely wired with positive polarity. Ignore the IR chart in this case. The likely reason that the Dirac Live IR did not show the same initial peak is that its sweep did not extend past the bandpass range of the speaker.

Regarding the high frequency truncation of some of the sweeps:
The only idea I have is to try reducing the buffers from 32k to maybe 16k. It's possible that there is too much delay and the high frequency is not being captured. This is just a guess based on some recent comments in other threads. I think it was only happening on some Mac OS X computers. This may not help. If reducing the buffer sizes doesn't work, you could post an mdat containing the 2 sweeps and we can take a closer look. You can also look at the scope chart to see if the current measurement looks to be properly captured. Only the last sweep is available to view in the scope chart and it is not saved with the file.
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post #37 of 45 Old 12-18-16, 10:36 PM Thread Starter
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Thank you for your quick reply.

I use Mac OS X platform in this case. So, reduction of the buffer size to 16K gave me a good result. I will try it on Windows platform. Also will try the negative spike question, too.

Many thanks,

Yuichi
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post #38 of 45 Old 12-18-16, 11:53 PM Thread Starter
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Re: Linear vs Minimum Phase filters in REW for miniDSP

On Windows platform I obtained desired result. Attached are what I want. Also, I have not experienced the sweep problem on this platform.

Thanks for your great support. Yuichi
Attached Thumbnails
Linear vs Minimum Phase filters in REW for miniDSP-spl-phase.jpg  

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

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post #39 of 45 Old 04-14-17, 11:11 PM Thread Starter
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Subjravel time measurement for miniDSP delay setting

I am using REW V5.19-Beta4.
I have tried to measure the sound fly time from the diaphragm to the mic. Purpose is to find out the acoustical difference between two diaphragms, so the accuracy of absolute value is not needed.

Impulse response Calculation setting...I did was..
(1): [No timing reference] and [no check] on [Set t=0 at IR peak] -> Always Peak is set to 0.
(2): [Use loopback] the value does not depend on the distance
(3): [Use acoustic timing...] ->Measurement value is not reliable such as 1 meter shows 0.33 msec and 10 cm shows 0.31 msec something like these.

In the case(3), I measured 1 meter and approx. 10cm distance, the following numbers are shown.
1m :number on the measurement Panel is 0.33msec delay estimation on control is 0.361 msec
10cm : 0.31msec and 0.350 msec respectively.

Please advice correct settings for this purpose.

Yuichi Arai
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post #40 of 45 Old 04-16-17, 11:43 AM
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Re: Linear vs Minimum Phase filters in REW for miniDSP

You want to measuring the difference in delay between 2 channels? Why would it be different unless the mic is slightly off center?

I am not understanding your issue with #3. It is the correct method. I do not look at the REW reported delay number so I don't know how accurate and repeatable it is. You can evaluate that yourself.

The basic method I suggest is:
> Set your music player/AVR/DAC to stereo mode.
> confirm stereo connections from the PC soundcard through your system to the L & R speakers
> Set REW acoustic timing on.
> Set Left channel as the REW reference channel.
> Set Left channel as the REW measurement channel.
> Measure [a chirp will occur in the left channel before the full sweep occurs in the left channel. The right channel will be silent.]
> Set Right channel as the REW measurement channel.
> Measure [a chirp will in occur the left channel before the full sweep occurs in the right channel.]
> Open the REW overlay impulse chart and find the 2 impulses. [You can measure any offset accurately by zooming in then using 'ctrl-right mouse button' top drag between 2 identical locations on the 2 IRs. The distance between these points should agree with REW reported difference in the impulse locations.]

Note:
Take a few repeated measurements with this process to assure good repeatability. If the result changes significantly there is a problem that needs to be corrected.
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