SPDIF can only carry multichannel in compressed formats like some Dolby and DTS flavors, there needs to be support for such encoding in the OS/driver etc. to get multichannel over SPDIF and one will have to live with the additional latency from both encode and then decode processes in the receiving device. Analog multichannel is able to provide uncompressed output and no extra latency which perhaps makes it preferred at least as far as games are concerned.

Tiido wrote on 2023-04-11, 13:05:

SPDIF can only carry multichannel in compressed formats like some Dolby and DTS flavors, there needs to be support for such encoding in the OS/driver etc. to get multichannel over SPDIF and one will have to live with the additional latency from both encode and then decode processes in the receiving device. Analog multichannel is able to provide uncompressed output and no extra latency which perhaps makes it preferred at least as far as games are concerned.

Interesting. Given that the DTT 3500 dates back to 2000, the options for OS' back then would be 98SE and 2000, neither of which I presume have support for the Dolby encodings.
Therefore, I'm interested how the decoder outputs 5.1 channel for anything of that period that's connected to it, given that the only available inputs are front and rear 3.5mm jacks (4.1 maximum), mini-DIN (max 2.1?) and SPDIF.
It has a built-in Dolby Pro Logic encoder, but that's only producing a compressed mono channel for the rear as well as 2-channel stereo (sort of 3.1?).

It probably is for the software DVD players etc. that can send their compressed stream directly over SDPIF to approrpiate receiver device.