OK I'm back to this thread and I just can't let this go. Earlier we all (sort of) decided that Netflix, Prime and D+ must be using some "special" kind of DD 5.1 because we can't seem to get DD when using them on ROKU. Well keep reading.
I decided to continue my testing by adding these same disputed channels (Netflix, Prime & Dis+) to my PS4. Conclusion, my PS4 has no issue streaming DD 5.1 to the ARC. So in essence I get full DD 5.1 from the PS4 but not using ROKU (when setup correctly on PS4 of course, i.e., Bitstream, etc.).
Incidentally, I also was shocked to find that my standard TV remove can control my PS4! WAT??? Yes when Anynet+ is setup and some other PS4 setting (link devices I think), your standard Samsung TV remote can control your PS4, allowing you to pause, rewind, FF, etc. Blu-Ray, Netflix, Prime, D+, etc. I found it by accident but was shocked that all these years...who knew?
So anyway, I'm back to "what gives with the Roku version of DD 5.1 on these channels?!?"
NetFlix, Prime and Disney+ are all using DD+ (E-AC3), which most services are using now (and not DD (AC3)).
There's nothing particularly special about it, its been around for 18 years and standardized for 15.
Your PS4 includes Dolby decoding functionality, and part of Dolby decoding functionality (mandatory requirement from Dolby when licensed) includes converting/transcoding DD+ to DD for output, so your PS4 is merely converting/transcoding the DD+ from those apps' streams to DD for output. (FYI, XBox does the same thing...)
Most Roku player models dont have Dolby decoding capability (just passthrough), only two did/do: The 2016 Ultra (4640) and the 2020 Ultra (4800).
Roku and its apps arent using any special codec or special version of Dolby audio - just plain old standardized DD+ (E-AC3); however, unless you have one of those two models, you only get passthrough, which means another device in the chain needs to convert/transcode it to DD for connection to legacy DD-only (AC3) equipment.
I have the PCM problem also. My setup is a Roku ultra 4600x, Sony x900h, Sonos Arc. Music (Pandora) plays stereo PCM out of the Roku, Dolby digital plus 2.0 from Pandora on the Sony TV. These outputs reported by the Sonos app.
I can get Dolby Atmos on Disney Plus on my Roku ultra. But this is the only channel so far. All the others are coming out stereo PCM. My settings on the Roku are in auto. I did try settings, audio, digital output format, (Auto or custom, dolby digital Plus) with no change.
I also had an arcana HD fury splitter. Just sold it, I upgraded my TV, so I didn't need it anymore. I used it to get Dolby Atmos from my Roku ultra to my Sonos arc. But now, the Roku stopped sending Dolby sound from some apps, I get Dolby Atmos from Disney Plus, but not Netflix or Pandora.
Bless you Dan. I'd examined everything in my set up trying to figure out this issue and was just getting online to try to find some answers.
It would be nice to see the ROKU show the user a reminder every so often that night mode is enabled so one remembers to turn it off to get the full experience. No doubt some customers have it enabled for the life of the product completely unaware that their content is not being played back as intended.