It would be great if someone could replicate the test. You need the following:
RightMark Audio Analyser:
http://audio.rightmark.org/index_new.shtmlA USB soundcard with line-in
An external USB DAC.
An analogue cable to connect your DAC to your soundcard's line-in.
First perform the loop-around test with your soundcard to set a reference. For this you need to connect the analog inputs of your soundcard to the analog outputs, and this way it measures the DA-AD path of your soundcard. It sends a test signal through the chain and analyses that. It can only analyse it's own test signals. Try to run this test first to get yourself familiarised with RMAA. It's performed by pressing the button with the loopback cable in the RMAA software. Do this at 24/96 so you can see the full dynamic range abilities of your soundcard.
Then, connect your DAC to your laptop so that audio is being transferred to the DAC (whilst recording remains with your soundcard). Connect the DAC's analogue out to the line-in on your soundcard, and perform the test again. This time, you are sending the test signal from your laptop to your DAC, which is outputting it in the analogue domain to your soundcard to be recorded.
You will now have two results: The reference loop-back test for your sound card, and the results of your DAC as fed by your laptop.
Finally, perform the same test but with your Pi playing the RMAA test signal. For this you need to generate the test signal in RMAA (click the generate wav button) and play it on your Pi at full volume with software volume mixer disabled. Of course, you need to connect your Pi to your DAC. In RMAA, click the Record Only button to record the incoming Test File as played by your Pi>DAC.
There is an option in RMAA to calibrate the incoming signal. You should notice that this doesn't need to be altered for the first two tests. It will alert you that the distortion is high on the third test. Do not alter the incoming signal level as it should not be necessary to do so if the Pi were operating properly.
