Help me sort out USB noise and a ground loop in a rather convoluted setup

[Eirikur]

Indeed. I am baffled too. Just to be clear - what your graph shows is the audio as received by the S/PDIF input, with input data that is totally silent (or even with no input signal)?

Yup, just that. Of course I only get to see the end of the pipeline, i.e. what Audacity receives from the Windows APIs, but I've been careful to match the SPDIF input settings and the Audacity project settings to avoid any sample rate conversions.

 

[Julf]

Yup, just that. Of course I only get to see the end of the pipeline, i.e. what Audacity receives from the Windows APIs, but I've been careful to match the SPDIF input settings and the Audacity project settings to avoid any sample rate conversions.

Those graphs could indeed be showing the results of windows processing the audio (basically rounding error).

 

[graz_lag]

So yet another confirmation it was a wise choice the one I did 15 years go getting rid of Windows and moving everything of my audio collection within Linux ...