Inconsistent Impulse Measurements

[Rob Greer]

I'm using REW 5.20 Beta 41 and when I measure my 4 SVS PB16s individually I'm getting inconsistent impulse measurements. For example, if I run the same measurement 5x on the same sub with each measurement following the next, I'm lucky if 3 of 5 measurements match. Sometimes only 2 match. And now I can't get any to match.

Back in January I used my current rig (Macbook Bootcamp on Win 10 + 25' USB + 25' HDMI + + UMIK + MiniDSP 4x10HD) to successfully time align and calibrate the subs. But I changed my room and now I can't get past this impulse issue to time align the subs. Things I've tried:

Googled for answers
I've moved the HDMI from AUX 1 on the front of the 8805 to the back AUX 2.
I've rebooted my computer.
I've closed and opened REW.
I've uninstalled and reinstalled REW.
I've "deleted preferences and shut down" in REW (and set it up again)
I've unplugged and replugged the HDMI on both ends
I've unplugged and replugged the UMIK on both ends

Can you offer any other troubleshooting suggestions? Bad UMIK? Bad cables? Bad computer?

I've attached an MDAT of my last run and here's a screen shot of an impulse overlay from a different set of measurements.

 

[John Mulcahy]

Curious. The frequency responses all match well, as do the later parts of the IR. You aren't currently using a timing reference for the measurements, could be worth turning that on to rule out any capture alignment issues. Do the subs have any kind of auto-off/signal-dependent turn-on that might be active? Could also look at the captured waveforms (last graph) to see if anything looks odd there, like the capture starting at different times or being truncated.

 

[Rob Greer]

Thanks so much for taking a look.

I saw that I had the timing reference turned off right after I made my post, but enabling that again (and it’s always enabled except for that test) doesn’t change the behavior.

As for the sub auto on, before I run the tests I check the levels and that sends a signal to the sub which means it’s on and I can hear it. All of the measurements a run right after the other measurement so there would be no time for the sub to turn off.

I noticed that for some of the tests that the line was relatively flat before the first upward rise and then for others it was really wavy before it started to make a big rise. That seems suspicious but I don’t know what it means.

Any other ideas about where I can check?

 

[John Mulcahy]

Have a look at the capture waveforms I mentioned. They only show for the latest measurement, so grab some screenshots of them from successive measurements to see whether they are consistent. Could also try some shorter USB and/or HDMI cables temporarily to see if that has an influence, and make sure the sample rate configured for the audio interface in the mac's Audio Midi Setup app is the same as the REW sample rate.

 

[AustinJerry]

It might also be worth a try to use a Windows laptop if you have access to one.

 

[Rob Greer]

Have a look at the capture waveforms I mentioned. They only show for the latest measurement, so grab some screenshots of them from successive measurements to see whether they are consistent. Could also try some shorter USB and/or HDMI cables temporarily to see if that has an influence, and make sure the sample rate configured for the audio interface in the mac's Audio Midi Setup app is the same as the REW sample rate.

I looked at the waveforms for 3 successive measurements and the waveforms looked the same.

It might also be worth a try to use a Windows laptop if you have access to one.

I'm making the measurements from a Win 10 OS machine. And it's the same device I used to previously make successful measurements.

SOLVED: Removing the active USB cable between umik and computer fixed it I guess the cable was damaged sometime in the last 3 months.

 

[AustinJerry]

@John mulkahy: John, have there been previous reports of active USB cables affecting measurement timings, or does this look like just an isolated case of a bad cable?

 

[John Mulcahy]

An isolated case I think, I have seen posts from others using active USB cables without issues.