Confirmation:
set as output, the coaxial SPDIF port provides main out - and it doesn't even hog a track. Nice.
For the curious, getting there required a few detours. First forgetting to set the input to 44.1 kHz, of course (SPDIF sampling frequency mismatch noise is easy diagnosis). But then also taking much more time to understand that neither Open Broadcast Studio nor Audacity deal with multichannel audio interfaces such as my Komplete Audio 6 Mk2. And let's not mention installing development libraries for Pulsaudio and OBS, to compile the
obs-pulse-multichannel-input that ended up not helping at all.
Good that I had
Friture (a very nice realtime graphical audio analyzer - perfect to insert in the corner of a live play video) to show me that sound was getting across... That puny program let me select one of my audio interface's inputs just fine - while the two heavyweights did not.
I solved my problem by installing Ardour (a DAW), which has all the fancy Linux audio routing facilities... Add an audio track, select the audio interface's ports through the Alsa driver (puzzling as the system runs wireplumber's pipewire emulation - gainfully used by Friture), arm, record - and voilà: a proper recording - noise free, thanks to the fully digital path from MPC to recording through the MX-1 (in this family we don't do lo-fi !)
Since I just need stereo SPDIF in, I'll probably get one of those
cheap SPDIF to USB adapters that will let me use Audacity and OBS... And the now unemployed (but otherwise excellent) Komplete Audio 6 Mk2 might go on sale.
