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Discussion Starter #1
John, I don't know if you'll see this but I wondered if in the future you'll have a way of dealing with this?

As REW adjusts t=0 to correspond with the impulse peak when using this mic, presumably that means that all of my phase measurements for different drivers is also off? As is it obviously not possible to have a loopback with a USB system users will need a way of being able to rely on phase measurements and viewing delay for different drivers in a speaker as this type of mic becomes more popular.

Maybe in the future it would be possible for the user to set t= ? If we know the delay for a signal to propagate through the mic and into the USB interface we could set it to that manually, or if miniDSP let you know what the delay is you could incorporate it into the auto detection routine setup for the UMIK-1?

Alternatively could we just assume that this propagation time is zero and set the t=0 at the first sample, This may not be entirely accurate but would at least be consistent across measurements and allow us to view the difference in delay between measurements.

Maybe I am missing something?

Many regards

Stefan
 

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A reference is needed to allow comparison between measurements because the operating system latencies for the audio replay and capture paths can be very different between measurements. If there are no (or only known delays) in the path after the audio output then the REW "Estimate IR Delay" feature can be used to remove the measurement delay, any known delays (e.g. time delays programmed into digital crossovers) could be added back manually using the t=0 offset adjustments in the Impulse graph controls.

The long term solution for USB mics involves generating a reference signal ahead of the measurement sweep and using that for alignment. I have a scheme in mind for that but I'm not sure when I'll get to implementing it. A workaround is to place a tweeter very close to the mic and drive it with the same measurement sweep that goes to the unit under test, the impulse response will then have a peak from the tweeter and a later peak from the unit being measured (delayed by the additional time of flight). The tweeter peak in separate measurements can act as a reference to align them.
 

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A workaround is to place a tweeter very close to the mic and drive it with the same measurement sweep that goes to the unit under test, the impulse response will then have a peak from the tweeter and a later peak from the unit being measured (delayed by the additional time of flight). The tweeter peak in separate measurements can act as a reference to align them.
Now I get it...

Thanks, John!

Charlie

PS. Eagerly waiting for your "long term" solution, though : )
 

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Discussion Starter #4 (Edited)
A reference is needed to allow comparison between measurements because the operating system latencies for the audio replay and capture paths can be very different between measurements. If there are no (or only known delays) in the path after the audio output then the REW "Estimate IR Delay" feature can be used to remove the measurement delay, any known delays (e.g. time delays programmed into digital crossovers) could be added back manually using the t=0 offset adjustments in the Impulse graph controls.

The long term solution for USB mics involves generating a reference signal ahead of the measurement sweep and using that for alignment. I have a scheme in mind for that but I'm not sure when I'll get to implementing it. A workaround is to place a tweeter very close to the mic and drive it with the same measurement sweep that goes to the unit under test, the impulse response will then have a peak from the tweeter and a later peak from the unit being measured (delayed by the additional time of flight). The tweeter peak in separate measurements can act as a reference to align them.
Hi John, I hadn't realised that there was so much discrepancy because of operating system latency, I can see how that would mess everything up!! I like the idea you posted in the other thread about 'keep audio path open' Hopefully you'll get to that quite soon, I'm sure the growing community of active speaker builders using your excellent software with a UMIK-1 would applaud that, there seems to be quite a lot asking very similar questions. I'd certainly be happy to make a donation to help as would others I'm sure. The tweeter thing isn't really doable for driver measurement as that tends to be done at least in part in near field and squeezing in another tweeter closer that the one I am measuring seems impossible!!

This part of your answer confused me a little, could you expand, is this something that could help me?

If there are no (or only known delays) in the path after the audio output then the REW "Estimate IR Delay" feature can be used to remove the measurement delay, any known delays (e.g. time delays programmed into digital crossovers) could be added back manually using the t=0 offset adjustments in the Impulse graph controls.

Many regards and I look forward to hearing back from you.

Stefan
 

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'Estimate IR Delay' attempts to remove pure time delays from measurements, so using it on a set of driver measurements should give results that can be cross-compared. If there were deliberate delays in the crossover (e.g. if the crossover included a time alignment delay) then they could be added back by applying an equivalent adjustment to the t=0 position for the corresponding driver's measurement.

Whether that would work in practise is hard to know for sure though.
 

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Discussion Starter #6
Hi John, I wondered if you'd had any mare thoughts on implementing the scheme you had in mind for timing references forbusb mics?

Stefan
 

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Sorry Stefan, haven't got any further with that yet. It is near the top of my todo list though.
 

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Discussion Starter #8
Xmas present maybe? Or further into the future than that?

I'm very pleased that it's in a list at all, that's more solid than just a thought. :smiley:

Stefan
 

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Discussion Starter #9
Sorry Stefan, haven't got any further with that yet. It is near the top of my todo list though.

Hi John,

Just in the throws of speaker building AGAIN!! Just wondered if you were any closer to having this functionality sorted out?

Many regards

Stefan
 

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I have issues with the time lock moving from measurement to measurement in Holm, but my pc based system is quite complex. The only solution for me is a regular mic and preamp in REW.
 

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Active xo/eq done in software on the same pc I do measurements. Depending on how I do it the audio is either grabbed from the Windows mixer or its sent through an ASIO driver.
 

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Discussion Starter #16
Hi John, I just wondered if you'd got to this yet? Or if it might be on your list of things to do soon?
 

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Discussion Starter #18
That's great John. I know how long my 'soon' can often stretch to though. Are we talking this quarter or maybe he spring???
 

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Discussion Starter #20
You star, I'll buy you a pint if you are ever near me. Doing a speaker measurement tutorial in March at an audio show and was hoping to be able to demonstrate that one. Brilliant.
 
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