# loopback calibration



## omega6666 (Jan 4, 2014)

Hi!

I've bought myself a Behringer UCA 202 USB soundcard (takes 5 weeks to be shipped here :sad. I'm planning to use this with a Behringer FBQ1000 (100% compatible with DSP1124) and a Galaxy CM140.

In advance I've been reading this;
http://www.hometheatershack.com/for...ards/10001-rew-cabling-connection-basics.html (I'm not allowed to use HTML, right?)

I needed the soundcard, since my laptop doesn't have a line-in (only mic-in). But I would like to use my optical out from my laptop to my receiver, and only the line-in of the USB soundcard. Is this possible? Can I do the loopback calibration thing with the output of my receiver (LFE) connected to the USB soundcard?

Thanks,
Tom


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## JohnM (Apr 11, 2006)

You could do that, but you won't be able to use the LFE output for a loopback as it is frequency limited, use the left or right output.


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## omega6666 (Jan 4, 2014)

Thank you John. Surprisingly the soundcard was already waiting for me when I got home, and apparently it had an optical out itself.

For the first measurement I wanted to connect only the subwoofer, without any crossover, so I did the initial soundcard loopback calibration with an RCA cable. Results were pretty nice: 20 Hz .. 20 kHz flatness: +0,1, -0,7 dB.

I had already done the right settings for the SPL meter;









I then connected the output to the sub, and the input to the CM140, and noticed that the dB readings on the LCD display of the SPL meter immediately got pretty high, just by connecting. It went from 40 dB to 68 dB (range 32-80, C-curve, slow setting).
When I wanted to do my first measurement, it asked for the initial calibration, where you need to enter the dB reading from the LCD display into REW, with the use of the REW subwoofer cal signal. Had to put the volumes within the soundcard settings much lower, to prevent clipping and found the 75 dB setting.

Now it was time for my first measurement, which showed an odd peak at 50Hz. Then I checked RTA, and was a bit shocked at the noise;










I get the same results when connecting the SPL with a mono jack to RCA, and a composite jack to RCA, so I'm reeeeally hoping it's because I don't have a stereo jack to RCA cable yet.

Is this the reason for this noise? If not, what's the culprit here? This is not normal, right?

Thanks in advance,
Tom


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## omega6666 (Jan 4, 2014)

This is the noise after disconnecting the SPL meter;


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## omega6666 (Jan 4, 2014)

In this post someone has two issues, but he managed to get the 50Hz hum removed by swapping the SPL meter with a Shure mic. Like him, I run the laptop on batteries, so no AC interference would be likely;

before









after









Could still be because of a faulty jack plug, of course...


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