Thanks to a comment by
, I have found some new codec support with my Ultra. Bear in mind that I have the 4640, which contains a Dolby Digital encoder that no other Roku player has.
Testing both Plex and Roku Media Player, I can now play Dolby TrueHD audio without transcoding. That's not to say that the lossless audio is being bitstreamed, as my AVR is indicated a DD+ audio stream. Somehow the Ultra is taking the Dolby Digital core audio and stripping out the TrueHD layer. I don't know what this does with an Atmos track, as I don't have my Ultra connected to a 4K set and Plex insisted on transcoding a 4K video, even though the Ultra should have been able to take the H.265 video without issue. I forgot to try it with RMP.
However, with DTS it's a different story. in the past, the Roku would stream the DTS audio core and strip out the Master Audio layer. But several updates ago that seems to have stopped working. If the audio is standard DTS, both Plex and RMP sent the audio through to my AVR. But Master Audio tracks I could see Plex was transcoding to PCM, and the Ultra turns it into DD+. RMP would refuse to play the tracks, stating that the file was unplayable. However, if I enabled audio transcoding on my Serviio console, the file plays fine, just with standard DD audio as transcoded by the server.
So, like MPEG-2 support, Roku has slipped some new codec support into the firmware without telling anyone. It's something we have to stumble across. Again, I have the 4640, so I can't say for sure it works with the 4660 or any other of the newest Roku players.