Problems when Exporting Impulse Responses

Fredrik

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Thread Starter
Joined
Jun 25, 2017
Messages
4
Hi

I have a project where I am working with Binaural measurements and wants to capture the impulse responses from the left speaker to the left and right ear including room acoustics, and from the right speaker to the left and right ear (two stereo impulse responses, i.e four transfer functions). To do this I am using the fantastic program REW with the convenient function "export impulse response to wav". I am measuring the four impulse responses separately.

I am using the settings “Use loopback as timing reference” and this works very well when verifying the timing between the left and right (stereo) impulse responses in REW, but when exporting the impulse responses, this loopback timing seems to disappear.

To not lose too much resolution, I am trying to use the setting “Normalise samples to peak value” when exporting the stereo impulse response (left and right), but the normalising is applied on the two channels separately. This removes the important information about the amplitude differences between the two channels.

I hope that my findings are possible to solve, to make a really great program even better!

Regards
Fredrik
 

John Mulcahy

REW Author
Joined
Apr 3, 2017
Messages
7,212
For the amplitudes the original information can be retrieved from the Info panel for each measurement, the peak amplitudes are shown there and could be used to work out how to scale back to the original relative values. There isn't a way to have REW apply the same scaling factor to both the impulses exported in a stereo WAV, but I could add an option to do that, I'll add it to the todo list.

The timing is a little awkward. A WAV file is just a series of samples, there is no concept of absolute time within the file. If the option to apply windows is not selected REW exports the IR data directly, in that case the Info panel for the measurement will show the time corresponding to the first sample of the data in each case (labelled Time Axis Start). You can, however, use the IR windows to preserve timing alignment. If the option to apply windows is selected in the export dialog the exported data starts where the left window starts. To preserve relative timing between impulses define windows for them that have the same window ref time and the same left window width - use rectangular windows if you don't want a window function to be applied. The exported IR for each impulse will then start at the start of the left window, which will be at the same absolute time for each, so their relative timing will be preserved.
 
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