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Discussion Starter #1
Hello

I'm trying to calibrate my sound card before measuring my sub's FR in my room. I used the connection diagram for REW but it's not working.

I use 2 of the 3.5mm stereo to 2 RCA adapters. The RCA outputs are colored red and white but I'm not sure which is left and right. I plugged one of them to the L/R out of the card, and the other to the flexijack which has been set to line-in. I've tried all the combinations between the adapters but none of them produced any input, as you can see in the screenshot.

What am I doing wrong?

Thank you.

 

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First, uncheck the "use left as calibration reference", you don't need to use that.

Usually the right channel would be red. Can't see from the screenshot how the X-Fi output is set up, but it would be worth making sure you are getting some output by connecting it up to your system to make sure you can hear the test signal. Input setup looks OK.
 

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Discussion Starter #3
Thanks John.

Turns out I've been connecting to the wrong stereo output the whole time, lol :R

Now I've matched the in and out levels, which are at -12 ~ -9. But in the next step, about 75% into the sweep, both the in and out signals drop out, then I get a very low signal warning.

Any idea? The graph looks like this:

 

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The audio capture continues for a second or so after the sweep ends to get the decay in the room.

When the output was playing, which input channel was showing an input level, left or right? Was it the same channel that is selected as the input to REW? The plot looks like crosstalk picked up on an unconnected input.
 

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Discussion Starter #5
You're right. I selected right input but plugged in the left. :dizzy:

I got the card measured now. My questions are:
Why are the Soundcard graph and Corrected graph pretty much on top of each other?
And why is there so much rippling from 1khz out?
Should I expect to see a flat line like in the REW help file, after corrections?
Anything I can do to fix it?

 

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Why are the Soundcard graph and Corrected graph pretty much on top of each other?
The "Corrected" trace shows the predicted effect of any EQ filters that have been applied, with no filters it is the same as the measured trace

And why is there so much rippling from 1khz out?
That is most likely a characteristic of the card's output filter - bear in mind the scale of this graph, those ripples are much less than 0.1dB and nothing to be concerned about.

Should I expect to see a flat line like in the REW help file, after corrections?
Once you have made the measurement use the "Make Cal..." button to save that as a calibration file. Then close the settings, leave the loopback in place and make a normal measurement (by clicking on the image of the subwoofer and mic), it should be a flat line.
 

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Discussion Starter #7
Once you have made the measurement use the "Make Cal..." button to save that as a calibration file. Then close the settings, leave the loopback in place and make a normal measurement (by clicking on the image of the subwoofer and mic), it should be a flat line.
Okay I did that and this is what I got :sneeky: The green line is "Measured data" and the dotted black line is "Measured response of sound card". I'm guessing I only need to care about the dotted line, but what's the purpose of the green line?

 

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The green line would be flat, but you have "C weighted SPL meter" checked in the Mic/Meter settings so you are seeing an inverse C weighting shape - that is not applicable when making the loopback measurement as there is no meter :). If you uncheck "C weighted SPL meter" you will get a flat line. If you also calibrate the REW SPL meter reading you will get a line that is closer to 75dB.
 

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what's the purpose of the green line?
The green line is your actual measurement of the loopback cable. Notice how it's the exact mirror of your meter calibration file? You were suppose to clear the meter calibration file before measuring the loopback cable. That way, only the soundcard calibration file would be loaded and would compenasate for the soundcards response with the result of a flat line. Your resuly instead, has the meter calibration file added in. Redo the measurement of the loopback cable with the meter cal file cleared. (and also run the Calibrate SPL routine before the measurement and set the level to 75dB)...

brucek
 

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Discussion Starter #11
Thanks to both of you :hail: now the green line is as flat as it can be!

Now the SPL cal... I'm not sure what this process really does. Is it trying to match the level which REW thinks it's playing with the level from the meter? Then how should I set the receiver volume? At this stage my meter is plugged into the line-in, and the line-out goes to analog L/R on the receiver, cal files are also loaded.

I did the SPL cal routine anyway, with the receiver volume at my usual listening value. The meter reads 73db and I put that value into REW. That sounds about right? Should the sub be moved to the listening position?
 

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Is it trying to match the level which REW thinks it's playing with the level from the meter?
That's it exactly.

Then how should I set the receiver volume?
Run the Check Levels routine and set the receivers volume to 75dB at the listening position with the real SPL meter.
Then set the REW input level on the REW VU meter.
Then run the SPL Calibrate routine and set it to match the real meter (75dBSPL).
Measure.

brucek
 

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Discussion Starter #13
So I've been making a few graphs and I noticed something.

I run the level check in the measurement window just before measuring because it seems each sub location gives a different level. My SPL meter is the analog radioshack. I set the range to say 70db and turn the receiver volume until it reads 75db. Then I set the range to 80db and check level again, it reads 65... Am I doing something wrong or is this just the quality of the meter?
 

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I run the level check in the measurement window just before measuring because it seems each sub location gives a different level.
Correct.

Then I set the range to 80db
Leave the RS meter at 80 position for the whole procedure. No need to change range. An absolute 75dB is not important. Just get it close and then tell REW that it's 75dB when running the Calibrate SPL routine.

brucek
 
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