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Old 21st January 2012, 09:06 PM   #1
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Default ARTA Question - Getting some problematic results

Okay so, I'm recording impulse response in ARTA. I'm using a log sweep signal, and I have the signal window set to hanning. What setting should I have for "IR Wnd" (gate tapering)? As you can see, the "uniform" setting is drastically different than the various hannings, and the wide ripple in the "uniform" plot being subtracted as I go up in hanning %.

As I understand it (which is not well), going from uniform to, say, hanning 25%, should be similar to just making the gate time smaller, right? That would seem to make sense with what I see, except I have NO idea why the level drops 9dB when I go up to hanning 50%. Can anybody explain this to me? Thanks.

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File Type: png IR wnd comparison.PNG (33.6 KB, 177 views)

Last edited by dumptruck; 23rd January 2012 at 10:01 PM.
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Old 21st January 2012, 10:43 PM   #2
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The IR Wnd setting sets the shape of end of the gated period, eg whether the end marker represents a hard cut-off, or has a curved taper to it. A hard cut-off will better eliminate reflections which are present closely after it, allowing you to maximize window time however if the impulse has not settled by that point there will be a DC error which causes errors at low frequencies, typically appearing as ripple at low frequencies.

A tapered window can help to minimize these low frequency errors, but may allow some reflections close to the end marker to creep in, which will result in ripple at all frequencies.

If by changing the IR Wnd setting you see large changes in the response and large variations in the amount of frequency response ripple, this means that your windowed part of the impulse response is not free of reflections - eg there are reflections present before the end marker. This is going to give you errors no matter what window taper you use.

As for the level dropping 9dB when you choose hanning 50%, that doesn't happen for me, again I would suspect that you've not set your start and end markers appropriately. Can you post a copy of the impulse screen to show where you're setting the markers ?

I've included an example measurement where the response with all IRD Wnd settings is very close - high frequencies exactly overlay, and the only difference is small variations in low frequency ripple due to residual DC offset in the impulse at the end marker.

A little tip - when setting the end marker increase the gain (amplitude) setting on the impulse screen so that the initial impulse is about double the actual screen height, so you can see where reflections begin more clearly.

I always use the default settings which are IRD Wnd Uniform and Sig Wnd Hanning, and have seen little need to change these.
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File Type: jpg Impulse.jpg (134.5 KB, 163 views)
File Type: jpg IRD Window.jpg (287.5 KB, 165 views)
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Old 21st January 2012, 11:21 PM   #3
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Thanks for the detailed response! I think your suspicion is correct - it sounds like these settings work just as I thought, and my real problem is with the gating / start point. See, I have been forced to set the start marker before the impulse, because my desired gate (around 2ms) results in ARTA gives me "Error: Gated length < 128 samples!". Shortly after I posted the question, I realized that the level drop is proportional to how far back the start marker is set (which makes sense with what was seen in the original question here, too).

Strangely, the FR results from setting the marker way before the impulse are pretty much in agreement with what I get in HOLMimpulse (letting it do these things automatically). I am doing all this stuff for the first time (obviously, maybe).

I have attached an impulse example. As you can see, this gate is only 1.33ms, 64 samples. This is in a temporary position to measure some off-axis HF things, but in the position I want to measure, I can still only get a little over 2ms before the first reflection, which is okay for the frequencies I'm trying to measure right now, but that is still be less than 128 samples. I must be missing something, right?
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Old 21st January 2012, 11:35 PM   #4
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In the impulse you just posted your start marker is actually set after the beginning of the impulse, which will always cause problems - this could be the reason for the drop in amplitude.

The dip in the impulse before it rises is a bit unusual - if its a multi-way speaker I'd suspect that the woofer was acoustically closer to the microphone than the tweeter ? (And is out of phase) In any case its a part of the impulse and can't be left out.

It's ok to set the start marker somewhat before the impulse (you always want it at least a little bit before it) provided that its a dead flat line before it - some sound cards (mine included) can introduce a little noise spike about 10ms before the start of the real impulse - try to avoid putting the start marker before that if you can see one with the gain turned up.

Another thing I'd be checking for is that your sound card is not looping input to output - this can cause all kinds of funny results in measurement programs like this, and is the default on many sound cards and sometimes difficult to switch off - try tapping the microphone while you're set up for a measurement and if you hear it through the speakers you've got a problem. Your impulse response looks a bit "weird", and makes me suspect you may have a pass-through issue.

On a few sound cards - like my Audigy 2 ZS, it's only possible to disable line "pass-through" mode when an app is actually recording - my sound card fails the tap test between measurements when ARTA is not actually recording.

64 samples is a bit problematic. I sometimes cheat and set the start marker a bit earlier to go under 128 effective samples, but I don't generally go under about 110, I think you'd be having serious measurement errors by the time you get to 64 samples. (There's a reason why ARTA tries to enforce 128 as the minimum after all)

If you're trying to measure treble with very short gate times the only option is to use a higher sample rate - assuming your sound card actually supports it. (Many say they do in the drivers, but they really just resample 48Khz to a higher rate and feed it into the application) My card supports 96Khz so I use that for all measurements which means I can get half the gate time with the same number of samples.
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Last edited by DBMandrake; 21st January 2012 at 11:41 PM.
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Old 21st January 2012, 11:55 PM   #5
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Quote:
Originally Posted by DBMandrake View Post
In the impulse you just posted your start marker is actually set after the beginning of the impulse, which will always cause problems - this could be the reason for the drop in amplitude.
Oh, the marker was not carefully set above - I was just showing the approximate time of the impulse for you. It won't actually let me set it anywhere near the beginning of the impulse (because of the min. sample requirement).

Quote:
Originally Posted by DBMandrake View Post
It's ok to set the start marker somewhat before the impulse (you always want it at least a little bit before it) provided that its a dead flat line before it.
Okay, so I guess I'm stuck doing that unless I can manage to get set up for a longer time to first reflection then, huh? Like I said, the greater the time before the impulse I set the marker, the more I get the differences shown in the first post above. I don't understand what it is doing to the math behind the scenes when setting the marker back farther, which makes me nervous...

Quote:
Originally Posted by DBMandrake View Post
The dip in the impulse before it rises is a bit unusual - if its a multi-way speaker I'd suspect that the woofer was acoustically closer to the microphone than the tweeter ? (And is out of phase) In any case its a part of the impulse and can't be left out.
I have wondered about that. That was just a quick and dirty measurement with a tiny fft so I could post a picture, but I am getting the little dip every time. This is just a woofer measurement.

Quote:
Originally Posted by DBMandrake View Post
Another thing I'd be checking for is that your sound card is not looping input to output - this can cause all kinds of funny results in measurement programs like this, and is the default on many sound cards and sometimes difficult to switch off - try tapping the microphone while you're set up for a measurement and if you hear it through the speakers you've got a problem. Your impulse response looks a bit "weird", and makes me suspect you may have a pass-through issue.
Hmmm, I think it's doing that all the time, in a way. There is a pot to mix between monitoring the output and directly monitoring the input. Got any other ideas for testing whether that's a problem? I certainly can't hear the input with the monitor knob turned all the way to output, although maybe if I turned it up enough, I will check in a minute.

Quote:
Originally Posted by DBMandrake View Post
If you're trying to measure treble with very short gate times the only option is to use a higher sample rate - assuming your sound card actually supports it. (Many say they do in the drivers, but they really just resample 48Khz to a higher rate and feed it into the application) My card supports 96Khz so I use that for all measurements which means I can get half the gate time with the same number of samples.
I am using a Lexicon Lambda interface, which can't do over 48Khz. Thanks again for helping out here.
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Old 22nd January 2012, 12:10 AM   #6
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P.S. Hmm, it does fail your tap test. I can't hear it with the output volume set as I had it for testing, but I can just barely feel the cone respond when I tap on the mic. If I turn the volume, way up I can hear it.
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Old 22nd January 2012, 12:22 AM   #7
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Apparently there are 3 pulses above the noise floor in the measurement... see below.
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Old 23rd January 2012, 10:00 PM   #8
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I will be back to investigating this more tonight, if anyone would like me to try something in particular. It seems to me like the leaky pass-through thing discussed and probably shown above shouldn't cause any problems with a log sweep since it's so much earlier in the time domain? I just had a thought that the initial dip in the impulse response looks a little like a filter artifact...
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Old 24th January 2012, 04:05 PM   #9
EssB is offline EssB  United Kingdom
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Quote:
Originally Posted by DBMandrake View Post
...
Another thing I'd be checking for is that your sound card is not looping input to output - this can cause all kinds of funny results in measurement programs like this, and is the default on many sound cards and sometimes difficult to switch off - try tapping the microphone while you're set up for a measurement and if you hear it through the speakers you've got a problem. Your impulse response looks a bit "weird", and makes me suspect you may have a pass-through issue.

On a few sound cards - like my Audigy 2 ZS, it's only possible to disable line "pass-through" mode when an app is actually recording - my sound card fails the tap test between measurements when ARTA is not actually recording.
....
Hi DBMandrake,
You can permanently fix that Audigy loopthrough - read this :http://www.trueaudio.com/downloads/audigy-setup.pdf

Or 2nd Q down from here:
http://www.trueaudio.com/rta_faq1.ht...Card Questions

Worked fine on my original Audigy.

Last edited by EssB; 24th January 2012 at 04:09 PM. Reason: URL corrected
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Old 26th January 2012, 06:54 PM   #10
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Since my initial question is answered and my current issues are pretty much unrelated, I have started a new thread: Mysterious Impulse Measurement Problem
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