Features, settings & updates

[guitarboy]

I've seen a few posts on the topic of TrueHD Atmos audio passthrough with the Ultra 4802 (2022).  From these posts I've surmised that the Ultra is not able to passthrough that format.  In my case I'm streaming from Plex so Plex transcodes the TrueHD 7.1 audio track to EAC3, and the Roku converts it back to Atmos before sending it to the soundbar.  There does not appear to be any way to stop Plex from doing this transcoding, and from other forums that also appears to be the case on other server platforms (Emby, JellyFin).

I thought it would be helpful to share my experience with the relevant settings on both Plex and the Roku, plus I still have a question about what is happening to the original TrueHD Atmos stream.

Source file

• 1080p (HEVC Main 10) (current TV can only handle 1080p)
• TrueHD 7.1 Atmos

Plex App on Roku

• Video
  • Local Quality: Original
  • Remote Quality: Original
  • Direct Play: Auto
  • Allow Direct Stream: On
  • Maximum H.264 Level: Auto
• Audio
  • Allow DTS-HD: On [Haven't tested this]

Roku Playback Settings

• Playback Options
  • Quality: Original (5.9 Mbps 1080p)
  • Direct Play: Auto
  • Allow Direct Stream: On
• Streams [assume this is what Plex is detecting in the source]
  • Audio: TrueHD Atmos 7.1
  • Video: 1080p (HEVC Main 10)
• Stream Info [what Plex is sending to the Roku]
  • Playback: Partial Transcode (throttled) [not sure what this means]
  • Format: mkv
  • Video: 1080p (HEVC Main 10) * copy [assume that "copy" means direct play]
  • Audio: EAC3 7.1 * convert [audio track transcoded by the Plex server]

Roku Audio

• Digital output format: Auto [see note below]
• Preferred streaming format: Auto

Plex Media Server

• Automatically adjust quality: On
• Transcode quality: Recommended

Further note: when the Digital Audio Format on the Roku is set to Passthrough, the soundbar reports that it is playing Dolby Digital Plus.  When set to Auto, the soundbar reports Dolby Atmos. So I'm thinking that Plex is actually downmixing the audio from TrueHD Atmos to DD+ 7.1, and the Roku is converting it back to Dolby Atmos.  Am I right about this?

Am I getting "true" Atmos playback with this setup? Should I be looking for sources that are "DD+ 5.1 Atmos" or "DD+ 7.1 Atmos" instead of TrueHD Atmos?

[guitarboy]

After doing some more testing, including comparisons between TrueHD and DTS-HD sources, I think I can summarize as follows:

• TrueHD is better than DTS, even if it is transcoded to DD+ on playback. The sound still has a spatial dimension that is missing with DTS. 
•  DTS-HD is stripped down to core DTS by the Ultra. Only the DTS core is passed to the soundbar.

I’ll probably swap out the Ultra for an Nvidia Shield Pro at some point. The Shield appears to be the only streamer that can passthrough lossless audio, both TrueHD and DTS-HD. Some smart TV apps can also do lossless passthrough but I’m not sure how well Plex works on those platforms. 

[guitarboy]

Not getting any response here, so I've moved over to AVS Forum where there's an active Roku Ultra thread.

I posted results from DTS testing there.

[guitarboy]

So to answer my own question, I'm pretty sure that Plex is stripping the TrueHD Atmos source down to Dolby Digital Plus 7.1 with no Atmos.  The Roku is then converting it back to DD+ Atmos when Digital Output is set to Auto, and passing it through as DD+ when Digital Output is set to Passthrough. So there's a lot of lossy conversion going on.