Linear vs Minimum Phase filters in REW for miniDSP - Page 5 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #41 of 45 Old 04-16-17, 09:09 PM Thread Starter
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Thank you for your timeto my question.

I like to make the thing simpler.
Purpose is to measure the TOF (Acoustical Distance between Diaphragm and Mic.)
Environment: V5.19-b4, Mic:miniDSP UMIk-1, Output Devise:USB-DAC, number of diaphragm is one.

Settings:1 [No timing ref., Not set t=0 at IR Peak] (note that set t=0 does not change the result)
Result
Mic. location:10cm from the diaphragm [Time is 0msec]
Mic. location:1m from the diaphragm [Time is 0msec]

Settings:2 [Acoustical timing]
Result
Mic. location:1m from the diaphragm : IR Estimation shows -0.31msec, IR window shows the IR peak at -310.usec
Mic. location:10cm from the diaphragm :IR Estimation shows -0.35msec, IR window shows the IR peak at -350.usec

Questions are

1) Why [Not set t=0] does not affect the result? Hove to be some number?
2) The difference of the mic location is about 90cm, so the timing deference have to be about 3msec. Why so small?
3)90cm apart should give + delay, why -?

Is there any incorrect setting to measure the IR delay?

Thanks Yuichi
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post #42 of 45 Old 04-17-17, 06:57 AM
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Re: Linear vs Minimum Phase filters in REW for miniDSP

> Thank you for explaining the test situation. That helps.
> You should be posting the mdat or at least an IR overlay chart so I can see the 2 IRs. Posting both is better.
> Did you repeat each measurement several times to assure repeatability as suggested? [This is a first critical test of the test method.]
> Was the timing chirp and the measurement sweep both within the bandpass range of the driver? [What is the actual frequency response of the driver and what was the sweep range?]

Quote:
AraiYuichi wrote: View Post

I like to make the thing simpler.
Purpose is to measure the TOF (Acoustical Distance between Diaphragm and Mic.)
Environment: V5.19-b4, Mic:miniDSP UMIk-1, Output Devise:USB-DAC, number of diaphragm is one.
So: variable measurement latency due to the usb mic thus acoustic timing is necessary.

Quote:
Settings:1 [No timing ref., Not set t=0 at IR Peak] (note that set t=0 does not change the result)
Result
Mic. location:10cm from the diaphragm [Time is 0msec]
Mic. location:1m from the diaphragm [Time is 0msec]
Okay.

Quote:
Settings:2 [Acoustical timing]
Result
Mic. location:1m from the diaphragm : IR Estimation shows -0.31msec, IR window shows the IR peak at -310.usec
Mic. location:10cm from the diaphragm :IR Estimation shows -0.35msec, IR window shows the IR peak at -350.usec
Okay, measured distance predicts the time difference at ~3 ms and the measured time difference is 0.04 ms so something is wrong.

Quote:
Questions are

1) Why [Not set t=0] does not affect the result? Hove to be some number?
'Set t=0' selected: REW places 0 ms at the largest peak of the IR. Some of the charts are best interpreted with that setting. It is a convenience for the user. It works well in most cases.
'Set t=0' not selected: REW places 0 ms at the intitial rise of the Impulse. The first peak of the IR then is slightly to the right of 0 ms. It is a another, optional convenience for the user. It works better for some IR's particularly those where the largest peak is not the initial peak. It can be worse in other cases.
I'm guessing REW is reporting 0 ms in both cases as it placed the impulse at the location you selected as the reference? There is no absolute correct position to place the impulse relative to 0 ms that works best for all types of analyses.

Quote:
2) The difference of the mic location is about 90cm, so the timing deference have to be about 3msec.
Yes, about 3 ms.

Quote:
Why so small?
The reason REW reported delay difference is so small is probably due to the test method used. I cannot comment effectively on the test method without knowing all the details of what was done or at least see the mdat. My initial guess was the 4th bullet at the top of the post (sweep range chosen), but it is based on very little information and is unlikely to be the problem. Strong reflections from a nearby floor, wall or desk may also be a problem?

Quote:
3)90cm apart should give + delay, why -?
It is most likely the standard convention for this engineering analysis - JohnM knows the material. It seems backward to me, but many things seem backward to me. It all depends on ones frame of reference. I rationalized this as; negative time = positive delay. If the event was expected to arrive at a specific time and it arrives later than that by 3 ms then I can say it arrived at -3 ms, or we could say it was delayed by 3 ms. This is just my guess/rational. I then set it aside and moved on to my main concerns.

Quote:
Is there any incorrect setting to measure the IR delay?
I assume there is an infinite number of wrong ways. It's very easy to get the test method wrong. Each test situation has to be carefully considered. A general test method that works well in one case may not work in others. I get the test method wrong frequently.
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post #43 of 45 Old 04-17-17, 08:23 PM Thread Starter
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Hello

Thank you for your description. I understood most of the mechanism.

I attach two measurements by using the [Acoustic measurement setting].
In this time, I used the USB-Codec instead of the USB-DAC as Output Device. DAC does complicated process. Codec is much simpler.
In the .mdat file attached, there are two measurements. One is mic. is located 10cm from tweeter and the other is 100cm apart. So, it must have reasonable difference, I guess.

One other thing I wonder in this measurements is that I can not see phase change. It must change so rapidly in higher freq-zone. I did not do IR Center=0 setting in the process.

Yuichi Arai
Attached Files TOF-Experi.mdat (10.18 MB) 
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post #44 of 45 Old 04-18-17, 08:57 AM
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Okay, Thanks.

> A quick look shows the SPL of the 10-22k Hz sweep was not captured. The only measurement is a narrow peak; 35 dB at 60 Hz. This is probably measuring system / room noise. My guess is that the DAC output has a significant delay and REW is finished with its capture before the sweep has started. There is an easy fix for this.
> Post-38 shows the SPL response of the speaker under test (bandpass from 60-20k Hz).

My Recommendation
Retain all your current settings except:
> Change the REW sweep to 4k-20k Hz for highest accuracy. [A typical TW range. Any sweep start setting setting will work fine for this purpose, but the bass range is not needed. Also, I cannot see if the speaker is capable to 22k so let's stop the sweep at 20k in case it rolls off rapidly above that.]
> Mic on axis with the driver.
> In the measurement pop-up window be sure to select the output channel where the speaker is connected. [The info panel indicates 'Both' channels were selected, but possibly that is the general setting and not what was properly selected here for the measurements.]
> In the measurement pop-up window select the REW option that indicates 'wait for timing reference' [REW will thus accommodate the DAC delay.]
> Make 8 measurements. 4 at 10 cm and then 4 at 100 cm.
> Zoom in on the impulses in overlay chart to confirm that the 4 measurements at each distance overlay exactly on top of each other, in each of the 2 groups. If they do, then you can delete all but 1 measurement at each distance. If not, we have another problem.
> Post the mdat of the 2 measurements here so we can see if this resolved the issue.
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post #45 of 45 Old 04-18-17, 11:00 PM Thread Starter
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Re: Linear vs Minimum Phase filters in REW for miniDSP

Thank you so much for your support.

Since I could not get accurate delay finder. But I learned a lot, and also found its work around process.
So, I am fine.

Thanks again.

Yuichi
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