M Audio MobilePre Issues

[Nat Ward]

Hi guys, newbie here to REW and after two days I'm still having issues setting everything up. I'm using a M-Audio MobilePre soundcard and ECM8000 mic for measurements.

Basics:
ECM8000 to MobilePre via XLR cable into channel 2 mic input
MobilePre to processor via 1/4" to RCA adaptor from channel 2 output

I've been able to set the measurement level, and set the input volume level, my problem is that I need to turn the channel level up on the MobilePre almost the whole way to get a -18dB input level.

I can then calibrate the REW SPL meter without issues. What I am having a problem with is calibrating my soundcard. I can run the calibrate function over and over and never get the same response. I am using the 1/4" jack channel 1 output into the 1/4" jack channel 2 input. Here is an example:

It looks like it's clipping, but the only way to resolve that is to turn down the input channel level knob on the MobilePre down, which then makes the input level below -30dB. And even then I can run the soundcard calibrate function and still get a funky graph. I had similar issues the first day I tried this but they went away after I loaded the soundcard's drivers. I uninstalled and reinstalled them again today without luck. I have "direct monitor" turned off in the soundcard menu.

Help! Is my soundcard hosed? I don't like the idea of dropping >$200 on a setup that doesn't work. Does anyone else run the ECM8000 with the MobilePre have the same issue with the channel level needing to be cranked up?

Thanks
-Dissapointed Nat

[JohnM]

Looks like a feedback path in the soundcard mixer, for example the input signal is not muted in the output mixer. REW won't be doing that for you because you haven't selected an input and an output, so you'll need to do it manually. The only signal which should not be muted in the playback part of the mixer is WAVE, mute everything else (if the MobilePre mixer is not too clear on that try looking at the Windows mixer settings to see what they show).

[Nat Ward]

Thanks for the response John. If I mute all outputs except WAVE I get "Unable to detect feedback loop....." so I left things alone (only thing muted is direct monitor)

I was able to play around with things again and turned the input knob down to about 1/4 volume, this gave me a good looking soundcard calibration result that I was able to repeat (and matched my results from 2days ago). See below:

I then went back and ran "set input volume" to see where the responses was, and as you can see it's off the charts low. I only get peaks in the mid -30dBs.

I left things alone and then proceeded to set the SPL meter. I can adjust the level on my processor and REW follows accordingly, will I have problems if the input volume is low as noted above?

I ran an automatic measurement on channel left, as directed by the setup guide, and came upwith this. Any reason why the responses are at the levels they are (soundcard cal now at ~85dB and measured at ~95dB?)

It looks like I'm getting some sort of improvement here, but not sure - could be all screwed up! Thanks again.
Nat

[JohnM]

The soundcard cal looks fine, and it doesn't matter that you use a different volume setting to derive that. Set the input volume using your mic as you had it earlier (i.e. volume nearly full to get -18dB when playing the cal signal), re-calibrate the SPL reading and all should work OK.

[Nat Ward]

I was just able to make a sweep (as set up above), and to be honest things look good. The results pretty much match (peaks and nulls) a similar sweep I did by hand using the RS meter, of course with better clarity on REW. The only thing that happened was the window popped up saying that I had low signal level.

Should I be able to take measurements like this? Can you calibrate a soundcard at a lower input level and then turn up the input level to make REW happy with it's -18dB desire? I know I'd have to adjust the SPL meter, I just don't know if that affects the results.

Sorry for the million questions!
Nat

[Nat Ward]

You read my mind John, answered the question before I asked

I'll adjust things again and start playing, whew!