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Old 27th September 2011, 03:21 PM   #551
dlr is offline dlr  United States
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Originally Posted by IG81 View Post
Yes, but I'm not trying to find relative distance, but absolute, within reasonable accuracy (only as good as I can take physical measurement and system latency I suppose). This is mostly for educational purpose, not for system design.

So is the start time marker the first numbered marker or a point on the dashed line?

IG
The window is the whole dashed line. You can use the Measurement Options window to manually set the marker (not sure how it actually relates to the dashed lines) after each measurement to the same sample point. Set the window/sample point initially, then re-set it to this same point each time.

I also don't know exactly how you would relate this to the absolute center, partly because that can't be determined precisely and because I'm less familiar with the windowing used in HOLM. The systems with which I'm most familiar use half-windows, not available in HOLM.

You can get an idea of this from the screen capture I posted some time ago, linked here.

Use the Impulse Time-Window setting and experiment a bit. You'll have to unlock it each time to manually set it.

Dave
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Old 27th September 2011, 03:29 PM   #552
IG81 is offline IG81  Canada
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Originally Posted by dlr View Post
The window is the whole dashed line. You can use the Measurement Options window to manually set the marker (not sure how it actually relates to the dashed lines) after each measurement to the same sample point. Set the window/sample point initially, then re-set it to this same point each time.

I also don't know exactly how you would relate this to the absolute center, partly because that can't be determined precisely and because I'm less familiar with the windowing used in HOLM. The systems with which I'm most familiar use half-windows, not available in HOLM.

You can get an idea of this from the screen capture I posted some time ago, linked here.

Use the Impulse Time-Window setting and experiment a bit. You'll have to unlock it each time to manually set it.

Dave
Allright, thanks. I'll need to sit down and play around with this some more.

IG
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Old 27th September 2011, 10:36 PM   #553
IG81 is offline IG81  Canada
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I can now see how you can display the measurement signal, which is spot on time zero naturally, followed by the IR of my speaker, but it places it at a ridiculously small distance compared to the actual distance my mic is located.

Example: my microphone is roughly 50cm away from speaker, the measurement signal is on sample zero, speaker IR 1st +peak follows at 0.37 sample. For 44.1kHz, this is roughly 8.39s. This is ~2.88mm distance.

IG

Last edited by IG81; 27th September 2011 at 11:06 PM.
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Old 28th September 2011, 01:57 AM   #554
IG81 is offline IG81  Canada
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Also look at the number of samples the impulse is offset. That includes some latency in the system, tho.
This is always a large number, say ~30k samples, which at 44.1kHz means my distance would be insanely large, or is there something I don't look at right?

I've pretty much nailed the relative distance procedure, but the measurement signal peak VS the 1st peak of the IR is always extremely close, I don't know why.

IG
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Old 28th September 2011, 02:21 AM   #555
Pano is offline Pano  United States
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Yeah, the offset in samples isn't easy to translate into distance. There is a delay even measuring a preamp.

Not sure I follow on the peaks, can you explain?
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Old 28th September 2011, 02:33 AM   #556
dlr is offline dlr  United States
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Originally Posted by IG81 View Post
I can now see how you can display the measurement signal, which is spot on time zero naturally, followed by the IR of my speaker, but it places it at a ridiculously small distance compared to the actual distance my mic is located.

Example: my microphone is roughly 50cm away from speaker, the measurement signal is on sample zero, speaker IR 1st +peak follows at 0.37 sample. For 44.1kHz, this is roughly 8.39s. This is ~2.88mm distance.

IG
HOLMAcoustics is a single-channel measurement system. You cannot get any absolute time reference with it. Even a dual-channel system that uses a feedback probe from the speaker input terminals cannot accurately determine an absolute acoustic center. I don't know why I had not thought to point this out earlier. You'd be better off putting your efforts into other areas.

Dave
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Old 28th September 2011, 02:37 AM   #557
IG81 is offline IG81  Canada
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Originally Posted by Pano View Post
Yeah, the offset in samples isn't easy to translate into distance. There is a delay even measuring a preamp.

Not sure I follow on the peaks, can you explain?
I'm talking about the "grey" curve, which is the measurement signal impulse centered at time zero. The impulse response of the speaker follows this, but at a distance/time that is ridiculously small. If anything, hardware/software delays should make that longer.

I'm able to get correct relative distances/times when locking time-zero after a first measurement, but this take away the possibility of seeing any "absolute" values.

IG
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Old 28th September 2011, 02:42 AM   #558
IG81 is offline IG81  Canada
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Originally Posted by dlr View Post
HOLMAcoustics is a single-channel measurement system. You cannot get any absolute time reference with it. Even a dual-channel system that uses a feedback probe from the speaker input terminals cannot accurately determine an absolute acoustic center. I don't know why I had not thought to point this out earlier. You'd be better off putting your efforts into other areas.

Dave
I thought it still had to know travel time from source for the purpose of relative measurements, but I guess that locking zero-time takes away all unknown hardware/software delay from the equation. I guess I can see what you mean and it makes sense that what I want to do might not be possible then.

IG
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Old 28th September 2011, 02:42 AM   #559
Pano is offline Pano  United States
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This is something I have not tried, because it doesn't matter much to me.
Hmmmm... if you get consistent offsets with the stream always on, can you subtract a known distance and arrive at a constant offset? Will it hold if you stop and start the stream?

If it does, you might then know the latency of the measurement circuit. Offset-known distance = device latency.
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Old 28th September 2011, 02:46 AM   #560
IG81 is offline IG81  Canada
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This is something I have not tried, because it doesn't matter much to me.
Hmmmm... if you get consistent offsets with the stream always on, can you subtract a known distance and arrive at a constant offset? Will it hold if you stop and start the stream?

If it does, you might then know the latency of the measurement circuit. Offset-known distance = device latency.
If anything can be kludged to arrive at a rough absolute measure, it could still be worth trying. I have not worked enough with HolmImpulse or similar software to know how to do this. Might need somewhat of a step-by-step for dummies.

IG
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