Frequency measurements with different inputs

[f1eng]

Is there any chance the microphone was in a slightly different location or an adjacent piece of furniture was moved between measurements?

I can see no reason why differences between the various digital connections would cause what you are showing, but moving the measuring microphone, or a reflecting surface near it, a few cm would do exactly what your measurements show (except the optical result ???).

All your plots look like the errors are down to room reflections, as if the room is a very sparsely furnished modern style room (which are hopeless for good sound quality, unfortunately) or reflections from a hard surface near the measuring microphone.

Good questions!

All the measurements above were taken within a 5 minutes period, with absolutely no changes in microphone position (or anything else for that matter), and even I myself was standing in the same position, well outside of the "magic triangle" of both loudspeakers and the microphone.

I did not have to move since I was controlling the computer with a wireless mouse-like device.

No appliance was switched on or off, and only the refrigerator was on during the whole period. The only lights on were "old style" lightbulbs - the noisier led or fluorescent lighting was switched off for the test. No air conditioning (we're doing the test in Oslo!)

Immediately after performing the Toslink test I repeated the wifi and Ethernet again (due to my surprise) and the results were equivalent to the initial ones.

Wow! maybe that is why my Astell & Kern portable into the optical input sounds so good!
The comb filter treble is still typical of interference patterns from reflections.
It is conventional to average room measurements at a number of microphone locations around the listening position to cancel these out, which is a pita.
Does the sound change noticeably if you move about?