# How to measure absolute phase in REW?



## svenr (Aug 30, 2013)

Hi everyone,

how do I configure REW to measure the absolute phase correctly? At the moment, REW automatically shifts the time t=0 to the onset of the measured impulse, thus removing the allpass delay due to the physical distance between the source and the microphone. I'm therefore loosing all information needed to correctly sum measured impulse responses.

I'm using loopback measurements, no IR decimation and no automatic time shifting. Which other options do I have to change to get correct measurements?

Many thanks for your help!


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## HifiZine (Feb 7, 2013)

Hi, are you sure you have the loopback turned on? REW should put the impulse at the time delay, not at zero. I think...


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## JohnM (Apr 11, 2006)

Tick the "Use loopback as timing reference" box in the Analysis settings.


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## svenr (Aug 30, 2013)

Hi JohnM,

thanks for pointing this out, but my problem lies somewhere else. I measure an impulse response using Matlab or ARTA, and find the first peak at the expected arrival time larger than zero. I save the impulse response as a WAV, import it into REW, and all of a sudden the peak is set to t=0! So it's gotta be a processing setting in REW. I checked the Preferences / Analysis settings and the Impulse Controls, but no luck yet.

What other setting could cause this problem?

Many thanks for your help anyway.


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## JohnM (Apr 11, 2006)

WAV files do not contain any absolute time information, so REW arbitrarily sets t=0 to the impulse peak. If your exported WAV started at the t=0 time of your original measurement then use the t=0 offset adjustment controls on the Impulse graph to move t=0 to the start of the imported response. Or measure with REW


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## svenr (Aug 30, 2013)

Hi JohnM,

is there a way of deactivating this processing feature and have t=0 assigned to the first sample of an imported impulse response? Otherwise it's a rather laborious process... However, the import options or preferences should allow one to chose whether this processing technique is applied or not. If the impulse response was measured using any 2 channel technique, the response does contain all the relevant information.


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## JohnM (Apr 11, 2006)

The problem is that there is no standard way of exporting an impulse response to a WAV, so the first sample of the WAV file could be t=0 for the original measuring system or it could be anywhere else - some packages let users choose a sample or time range from the response for export, some centre the impulse response etc. I'll add an option to set t=0 to the first sample for imports though.


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## svenr (Aug 30, 2013)

Much appreciated!


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