DTS-MA 7.1 and Direct Play on Roku TV - No audio unless entire video is transcoded



Setup details:
- Roku TV (TCL 55p) with new Sony receiver via ARC
- Roku audio settings: Audio mode (Dolby D+, DTS), S/PDIF and ARC (Dolby D+, DTS)
- Direct Play = Force
- Local Quality = Original
- Allow Direct Stream = No

I have a number of Blu Ray rips and after extensive testing, I've noticed the following behavior.
1.) DTS-MA 5.1 tracks will play perfectly fine and the DTS core audio is passed through to the receiver
2.) DTS-MA 7.1 will never be played with Direct Play
3.) When only DTS-MA 7.1 is present in the mkv, no audio
4.) Regardless of the audio track I've selected, if there is another audio track (i.e, AC3 w/ commentary), it will be played instead of the main DTS-MA track. This is a little bit annoying, but I guess that's a separate issue.

Here are the questions I have. Any answers from a Plex developer would be appreciated.
- Why can't the DTS core be sent to the receiver with DTS-MA 7.1 tracks the same way as 5.1 tracks? I'm aware of the blatent disregard for quality audio that Roku seems to have (no HD codec support, DTS sync bugs, etc), but in this instance, what's stopping Plex from allowing DTS-core with 7.1 tracks?
- I can get audio from these mkv files IF I set Direct Play to "auto" and allow transcoding. BUT this is a big deal, since it will transcode the video as well, causing me to lose HDR, as well as pegging my media server's CPU and causing playback to be less responsive and poorer quality. Why can't Plex only transcode the audio tracks? I would easily put up with an AC3 transcode of DTS-MA or TrueHD if I could keep the original video stream.


One correction: I just realized that I can get audio-only transcoding with Direct Stream enabled. However, that’s still less than ideal for DTS tracks, since the 1509kbps DTS core is far preferable to transcoded 640kbit AC3.

I’ll revise my question: why can’t DTS-core be sent when possible instead of being automatically transcoded to AC3, especially when the Roku audio settings are configured to allow DTS?


I can’t 100% officially answer your question as I’m not an employee, but I figure my experience here is 80% accurate towards an answer you’ll likely never get from a Plex Developer.
Roku (not Plex) has always preferred AC3
At one time, DTS 5.1 core was only allowed in MKV for Roku. And even that was buggy from Roku firmware to firmware.

Several other discussions re: 7.1 found through-out plex forums, and Roku forums

I always fall back on what Roku says is USB direct play compatible, and I’m reasonably certain Plex profiles for Roku are configured to do the same, along with perhaps, the tendency to prefer AC3 due to previously buggy Roku firmware.
Reference - https://support.roku.com/article/208754908-how-to-use-roku-media-player-to-play-your-videos-music-and-photos

Support for 5.1 DTS (Dedicated To Sound)

Roku devices will only pass-through DTS audio. To decode multi-channel surround sound, you must connect your Roku device via HDMI or S/PDIF (optical) to a TV or A/V receiver capable of decoding DTS.

Note: DTS-HD and lossless DTS are not supported.
^^^ See that note- it’s the key deal breaker for why something would be transcoded.

There’s also the Roku SDK, which only states it understands DTS passthrough.