I bought a Xonar DGX to hook it up to my Sony 5.1 surround system with SPDIF. Windows 10 says the SPDIF port only supports stereo and when I start the sound test it plays only sounds on FL and FR. In the settings of the SPDIF pass through device I can start the Dolby and DTS test, which plays sounds on all 6 channels. When I play AAC or DTS with VLC, only stereo will be played. When I enable "use SPDIF when possible" in VLC settings, it will play DTS in 5.1 and AAC in 2.0. Even video games like Witcher 3 or Dark Souls 3 output only 2 channels.
What do I have to do to always output 5.1 over SPDIF?
Best Answer
If your sound card manufacturer hasn't paid for the Dolby licensing when they bought the chips then you can't really.
Pass-through from movies works because the audio has been "baked in": it has already been encoded.
For games and real-time audio, 5.1 over 2-channel needs to be matrix encoded to pass along the two wires on a coax (or fiber), and your sound card usually must do this.
It may be possible to get a matrix driver wrapper in the middle that pretends to be a 5.1 system; encodes on the fly; and then send the audio over the SPDIF on the actual soundcard, but this will cause latency and use CPU. In the past, I never even got that method to work.
Note that "Dolby Headphone for 5.1 headphone experience" is marketing speak for pretending 2-channel audio has a wide field. ( https://en.wikipedia.org/wiki/Dolby_Headphone ) It requires a compatible headphone that can decode, and the non-virtual surround sound still must be encoded (or passed through) on the sound card.