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Measure MOTU M2/4 ADC distortion while using its headphone amp

Multicore

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It's been reported that the MOTU M2 distorts the input signals when its headphone amp is turned up. I tried to verify this on my M4 and could see the effect.

I used my DAW (Reaper) to generate a 1kHz test tone on output 3, TRS cable from there to input 1, music output to the phones, adjusted phones knob to min, mid and max, and the spectrum analyzer on the recorded signal shows a dancing skirt around the test tone that's bigger with more headphone drive.

That's an observation, not a measurement.

Can you suggest how to quantify the effect? What software can I use (cheap or free)?
 
It's been reported that the MOTU M2 distorts the input signals when its headphone amp is turned up. I tried to verify this on my M4 and could see the effect.

I used my DAW (Reaper) to generate a 1kHz test tone on output 3, TRS cable from there to input 1, music output to the phones, adjusted phones knob to min, mid and max, and the spectrum analyzer on the recorded signal shows a dancing skirt around the test tone that's bigger with more headphone drive.

That's an observation, not a measurement.

Can you suggest how to quantify the effect? What software can I use (cheap or free)?
A loopback with REW or Multitone Analyzer (both free)

This is a quick and rough guide,read down the next couple of pages for more:

 
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