Therefore, it may not be possible to connect some S/PDIF signals (OPT/COAX) of old or cheap products.
I posted here a few weeks ago when I was considering a D30Pro, to use with a CD transport SPDIF. Based on advice here, (thank you all) I decided against the D30Pro for that use case, but bought one for the system in my office, which is USB only.
So out of interest I tested the D30Pro with my Cambridge CXC CD transport - and I can report that they are
NOT compatible. Repeatable tests, "dropouts" at the same points in the music.
Same issue on Coax and optical.
Now, I don't know if folks here would consider the Cambridge CXC as "old or cheap" - but I would suggest that the issue is not only with "old or cheap" sources.