I should've made it clearer. I actually do the routing in audio interface, not in ADI-2, to monitor the analog and digital input at the same time, from PH3/4. Here is my connection:
Analog 1/2 -> AD (ADI-2) -> SPDIF in(audio interface) -> SPDIF out(audio interface, mixer) -> SPDIF in (ADI-2) -> DA (ADI-2) -> PH3/4
System(Windows) -> SPDIF out(audio interface, mixer) -> SPDIF in (ADI-2) -> DA (ADI-2) -> PH3/4
When I set the Dig. Out to Default, the PH3/4 can perfectly get the mixed results from analog1/2 and system(win), one is analog and the other digital.
But optical output cannot act like the PH3/4. If I set the Dig. Out to Main Output, then the AD results from Analog 1/2 cannot be mixed in.
Looking at the diagram, I think I kind of know why (as there's no mixing happening in side ADI-2 Pro).
Using the PH3/4, I'm only monitoring the mixed SPDIF from my audio interface. The reason that it can be mixed, is because Dig. Out is at default, so analog->ADC can then pass to coaxial and then to my computer, mixed with the system's sound then output to SPDIF, which is then picked by PH3/4.
But when using optical out, we have to flip Dig.Out to Main Out (in this case, there'll be signal from optical, or there'll be no signal at all). But then optical can only get digital out from SPDIF, not from analog->ADC path. So if we do the same mix, the final results from SPDIF will only be the original SPDIF, mixed with a copied SPDIF going one round, which results in a louder signal converted only from the digital side.
I'm not sure if I made myself clear. I think looking closely to the diagram answered my question. Just wondering if there'll be a solution to this problem:
1. monitor the analog and digital input at the same time, from optical output on ADI-2 Pro.
2. with the help of an audio interface (consider it something like RME UCX, and no extra digital out available so the only way out is from ADI-2 Pro)