# Linear vs Minimum Phase filters in REW for miniDSP



## AraiYuichi

Hi, Recently miniDSP announced the miniDSP2x4HD. This contains FIR and IIR filters. Which means that it makes minimum phase and linear phase happen. 
On the other hand the EQ generation facility of REW assumes minimum phase only. Does this idea correct? Or REW provides (or will provide) linear phase EQ too? like Dirac's mixed phase.
Yuichi Arai


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## AudiocRaver

REW will provide only minimum phase filter settings. An impulse can be generated for IIR filter application, but it will only contain minimum phase filter info.


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## AraiYuichi

Thanks, Yuichi


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## AudiocRaver

There are multiple tools available, some freeware, some for $$, some simple, some very complex, for generating FIR impulse that can be used with the 2x4 HD

miniDSP provides explanations and links to many of them on their FIR FILTER TOOLS PAGE.


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## AraiYuichi

Thanks again. I will take a look the URL and find out easier one. Yuichi


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## AudiocRaver

Look at rePhase or Align2 for ease of use.


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## AraiYuichi

Thanks Wayne start to look rePhase. Yuichi


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## AraiYuichi

AraiYuichi said:


> Thanks Wayne start to look rePhase. Yuichi


One question about REW and rePhase relations.
Assumption is the EQ generation of speaker sound property.

In REW, minimum phase EQ is generated after compensation of time delay (sound travel time). 
To generate FIR filter EQ (Linear phase) , measured data has to be exported from REW to rePhase. Exported REW data has time delay, therefore, phase graph is out of scale in rePhase screen. So, is there any way to generate minimum phase in rePhase or any other way to generate linear phase EQ on rePhase? 
rePhase works good for very small delay(phase) contained in the data. Does this mean that it is not a good way to use this combination to generate for the speaker sound EQ? 

Yuichi


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## AudiocRaver

If I am not mistaken, the time delay is to be removed before filter generation. jtalden is by far the expert around here in that area.


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## AraiYuichi

I think so. Therefore, IIR filter is properly generated by REW. But to generate FIR filter for miniDSP2x4 HD, it is recommended to use rePhase. Where, measured data which is obtained by REW is exported to rePhase as original. Which means that phase information contains delay time. This means that the phase curve on rePhase screen changes so sharply. So, I can not adjust phase to linear on rePhase. Does this mean that this method (REW->rePhase) is not applicable speaker measurement and EQ generation because of large delay (sound travel from the speaker to the mic)?

Or simply saying, how to generate FIR (Linear phase) filter for speaker sound?

Yuichi


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## jtalden

Yuichi,
There are lots of options regarding how to approach EQ. I am not aware of any consensus on the best scheme. There are good guidelines here in HTS for IIR EQ. FIR methods tend to be more complicated and have less coverage here. There are many other sources that can be easily found for more complete coverage. 

The general overall scheme I have been using is one that fits my equipment and level of understanding. 
> Use IIR minimum phase filters for SPL EQ. There are lots of different ways to approach the EQ effort. FIR linear phase filters are not normally recommended for basic SPL correction.
> Use FIR phase filters to remove the phase rotation of the direct sound. Just filter out or ignore the impact of the room when designing the filter.

I use REW to help establish the IIR EQ scheme. I think it is best to first find a house curve and EQ scheme that works well for you. It may take some significant effort to find settings you really like. I typically use averages in the listening area for the full range EQ effort, but have also had good results using single LP measurements. I am currently using EQ settings based on close mic measurements for the high frequency portion of the range and LP measurements for the range under 600 Hz. That has worked well also. First set the final speaker/SW/LP positions and room treatments and then spend the effort to find a house curve and EQ settings that work well for you. This is a challenging part of the effort, but necessary for good results.

The final REW measurement can then be appropriately filtered and input into rePhase to create an FIR filter to remove the phase rotation. We are then left with a linear phase system. This makes the measurement look more ideal in terms of impulse, group delay, and step responses. With my setup and my substandard hearing, I cannot hear any difference using the FIR phase correction filter, but many others people state that they can hear clear differences. Most all the studies I am have heard referenced apparently indicate, no or very little, difference is to be expected.

There other approaches that can work well. RePhase and other utilities can create an FIR filter that contains both the IIR EQ correction and the FIR phase correction all in the same FIR filter if that fits your setup better. I have used the DRC program for efforts into mixed FIR solutions. I didn't find anything that sounded or measured any better than my current setup. Some of the more automated commercial programs may be easier to use and provide better results than my more manual efforts.

Sorry for the these rambling comments.

There is no 'delay' that is a problem for input of measurements into rePhase There is however the need to filter or ignore the impact of the room. We want to create a filter to correct the direct sound phase. Many just correct for the calculated excess phase of the SPL response so that is an option. If you can provide an example file and a more detailed description of your concern, I'm sure I can help you clarify that particular issue.


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## AraiYuichi

Hello jtalden

Thank you for your detailed explanation for my question. I have understood most of your parts. 

The only thing I could not understand is the methodology how to export the REW measurement data into rePhase.
Or, how to remove phase rotation caused by group delay (sound travel time to the mic) on the rePhase. 

In the listening point, the mic is located approx. 2m far from the diaphragm. So, there are significant delay (sound travel time from speaker to the mic). My question is how to and where to remove this time delay. Otherwise, I can not adjust phase on the rePhase screen because of very quick phase rotation.

Yuichi


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## jtalden

For FIR Phase correction we use rePhase to create one filter that we apply to both channels of a stereo setup. Since the same filter is used in both channels the filter delay will be the same in both channels. The small delay is equal in both channels and thus has no impact to music.


REW automatically removed the time-of-flight delay if we did not activate 'loopback timing' or 'acoustic timing' when the measurement was taken. If we did have that active, or have any concern, we can we can activate 'Estimate IR delay' in the REW 'Graph Controls' panel on the Impulse graph. REW will then shift the IR to near 0 ms. If needed, we can also manually shift it to the exact ideal location using the manual offset features in that same 'Graph Controls' panel. It is not unusual to need to fine tune the IR location such that the phase tail at 20 kHz is falling at smoothly to a 0° phase angle. This fine tuning can also be done instead within rePhase using a control on its interface. Either method works fine.


If you need more detail please provide an .mdat file of your measurement. I will use your data it to post charts showing the steps needed. If you prefer, I can use my data. It is better to work from yours because IR appearance and the adjustments needed will be different thus possibly causing more confusion. Either way is fine with me.


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## jtalden

When the REW file is ready to export we just use the REW export function and choose the 'Measurement as text' option.


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## AraiYuichi

Hi jtalden

So, I can remove the IR delay first, then export it to rePhase.
I have reproduced what you said. Thank you so much. 

Yuichi Arai


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## jtalden

Yes, you can remove the delay in REW and fine tune it in REW or rePhase to fall smoothly to 0° at 20 kHz.

It helps to remove octave smoothing (set 1/48 octave) and then set a FDW of about 5 cycles in REW before you export it. That way the phase rotation of the direct sound will be obvious. Post a graph or better yet a file if you have any issues with the phase trace.


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## AraiYuichi

jtalden
Thanks so much, You helped a lot.
Yuichi


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## AraiYuichi

Hi jtalden

I did my experiment as you suggested. It works fine. And can be exported to rePhase. And I could generate the linear phase filters on the platform.

One thing I like ask you about the difference between the [Generate Minimum Phase] function and [Estimate IR Delay and correction] function.
I compared both way on the same several source data. The corrected phase properties by both way are almost identical, but there are some small differences especially in the higher band such as higher than 10kHz. 
But Impulse response wave forms look different in between. The Minimum phase IR is much front aligned. Does this mean that the Minimum phase generation process corrects not only sound travel time delay but also other phase shifts created by diaphragms and others? 

Yuichi


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## jtalden

AraiYuichi said:


> Hi jtalden
> 
> I did my experiment as you suggested. It works fine. And can be exported to rePhase. And I could generate the linear phase filters on the platform.
> 
> One thing I like ask you about the difference between the [Generate Minimum Phase] function and [Estimate IR Delay and correction] function.
> I compared both way on the same several source data. The corrected phase properties by both way are almost identical, but there are some small differences especially in the higher band such as higher than 10kHz.
> But Impulse response wave forms look different in between. The Minimum phase IR is much front aligned. Does this mean that the Minimum phase generation process corrects not only sound travel time delay but also other phase shifts created by diaphragms and others?
> 
> Yuichi


Yes (if I correctly follow your question).

The IR is the calculated transfer function of the entire system as measured at the mic position. It include the phase impact of the room and XO. The IR contains all the data needed for the various more friendly charts in REW. 

The minimum phase IR is created using only the SPL data from the captured mic data. The phase impact of the room and XO is discarded. It is the transfer function that represents the smallest phase rotation possible that can explain the measured SPL response. The minimum phase IR looks cleaner because the complex captured phase info is dropped and replaced with a simpler phase response. So one is calculated based on the captured real data and other instead is calculated to find the simplest phase possible for that SPL response.

'Estimate IR delay' just shifts the measured IR to near 0ms. It removes the Time-of-Flight and any other delays in the measuring system. It does not impact the phase so the measured IR shape is not changed. The phase difference you found above 10 kHz is most likely due to a difference in the settings in REW. There will normally be little or no difference there for the measured IR Vs the minimum phase IR when the reference time is properly adjusted .

rePhase only will allow the import of a text file containing the SPL and Phase response in table form. REW will not export the minimum phase response in that form. Thus, we need to first be sure that the phase response is windowed properly so we are not confused by the impact of the room. The actual sound phase response is the one we want to use in rePhase, not the minimum phase, nor the phase chaos due to the room impact.

Some info on your speakers and setup and an example REW data file would help me understand the situation and thus answer future questions more effectively.


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## AraiYuichi

Hi jtalden

I understood quite well. Thanks

What I am doing is to measure the Headphone transfer function and then correct the SPL and phase by using rePhase. Then, import generated filers to miniDSP 2x4 HD through its plug-in. It goes well.

Attached is the original REW data (before time shift) for your ref. The headPhone type is AKG Q701.

Thank you very much, anyway.

Yuichi


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## jtalden

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.


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## AraiYuichi

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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## jtalden

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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## AraiYuichi

Thank you very much. I learned a lot. Yuichi


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## AraiYuichi

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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## jtalden

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. 









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.









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









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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## AraiYuichi

Thanks, I will try to reproduce what you said first. Yuichi


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## AraiYuichi

Thank you. I reproduced the phase curve and understood the mechanism. Yuichi


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## AraiYuichi

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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## jtalden

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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## AraiYuichi

Thanks, very clear.

Yuichi


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## AraiYuichi

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


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## jtalden

AraiYuichi said:


> 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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## AraiYuichi

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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## AraiYuichi

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


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## jtalden

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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## AraiYuichi

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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## AraiYuichi

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


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## AraiYuichi

Subj:Travel 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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## jtalden

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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## AraiYuichi

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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## jtalden

> 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?]



AraiYuichi said:


> 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.



> 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.



> 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.



> 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.



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


Yes, about 3 ms. 



> 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?



> 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.



> 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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## AraiYuichi

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


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## jtalden

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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## AraiYuichi

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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