To all who are in this subject discussion.
All along I had the impression that the xiaomi cable was using optic to convey the digital signal. Unfortunately, those pics available didn't have enough resolution or was not at the right angle for me to be sure. That was the reason why I asked
i1Magic to check the tips of the 2 plugs.
Thanks to the link provided by
k8118k, more info was available. Specifically, the cable was described as "...stranded center conductor...". This means that center conductor is metal, not optical fibre. Mystery solved. So the cable can be used for both analogue and digital, and the single port on the xiamo tv2 supports both format.
Devilsire asked what's the difference between 3.5mm and SPDIF. In a normal 3.5mm jack, there are 3 terminals - for left, right and ground respectively (see pic below) for stereophonic sound. In the case of xiaomi, there are 4. I don't know exactly how they are used to convey the surround sound.
In the case of SPDIF terminations, it can either be coaxial (same dimensions as RCA), or Toslink (which is the optical equivalent) to convey the surround sound.
If the xiaomi special cable is not used for connection to the speakers and the latter is bluetooth capable, the signal is probably connected by bluetooth.
Learning new things everyday