This is one of the problems/limitations of using the Digital output format = Auto setting ("Automatically transcode the source audio to the highest detected Dolby level of the connected device") in the 4800/4802: you may/can get empty channels, incorrectly mapped channels, volume level issues, broken audio, stuttering audio, audio delay/sync issues, etc, all either preventing upmixing/surround processing in a TV/AVR/SB or making it problematic to do so.
Roku (or any other lower-end streaming device manufacturer) isn't going to provide many (or any) granular controls over how its Dolby decoder (and encoder) operates: in fact most devices with a Dolby decoder don't even allow the user to disable it (e.g. FireTV OS, TiVo Stream 4K), and otherwise only allow settings like volume leveling/dialogue enhancement (FireTV OS) or to restrict the Dolby codec level (e.g. FireTV OS - DD/DD+/Best Available) for output (the 4800/4802 has the same ability).
The fact that Roku allows the user to put the Dolby decoder into a Passthrough mode at all (though not completely disable it, and its not completely passthrough) is very unusual - even the Nvidia Shield 2019 only allows the user to either enable or disable their Dolby decoder and otherwise only has one or two indirect settings for it (e.g. matching Dolby & PCM volume, or not resampling for output), though its possible to manipulate the Dolby decoder's behavior through other audio-related device settings.
So yes, the Roku could (and does) identify PCM 2.0 source audio, and when using Auto mode then transcodes that to the highest detected/configured Dolby level for output.
If you don't want any transcoding (and most users are very rarely ever going to want this) you'll want to configure Passthrough mode and use your TV/AVR/SB's upmixing/surround processing modes.
Since you are aware of Roku's AAC support limitations and want/need AAC 5.1 you can either:
1) Wait indefinitely for Roku to reduce/eliminate them (e.g. Transcode 5.1 AAC to Dolby 5.1 or passthrough AAC including 5.1) <-- very unlikely to happen for any current or even future product
2) Re-encode your AAC-encoded media files to EAC3/AC3 (would have much better support in any/all streaming devices) <- a pain, but relatively easy
3) Use a different streaming device(s) with better AAC support (e.g. NS19) <-- a pain, but relatively easy
Thanks. I honestly didn't have much hope Roku would implement my suggestion, but figured it wouldn't hurt to put it out there in plain language. I fiddled with my PS4 and found its Media Player did the same thing as Roku on Auto, but was less capable (in fairness tho it's much older).
Could you give more detail on the "NS19"? I searched online but mostly found references to a Blue Origin flight.
I'll look into transcoding my videos to AC3. The trick there is to get the video stream to just copy (so lossless), while only the audio is re-encoded from AAC 5.1 to AC3. Any tips for a noob?
NS19 = Nvidia Shield 2019 (two versions, regular and Pro) - AndroidTV OS-based streaming device with excellent design/support for local streaming (USB & UPnP/DLNA, etc etc) using Kodi, etc.
As for re-encoding or transcoding:
1) You can convert (re-encode) your files from AAC to EAC3/AC3 using all sorts of tools (e.g. ffmpeg)
2) You didn't mention your file types or playback method (USB or UPnP/DLNA, etc), but you could use the built-in transcoding function of a UPnP/DLNA server (e.g. Serviio) to automatically transcode @ playback time.
3) Plex (server & client) can also transcode the AAC to AC3 @ playback time, etc.
Ah, I had not seen the Shield called NS19. Thanks, I'll take a closer look at it.
I play my video two ways:
1) From a simple DLNA server on my router, reading data from an SSD plugged into it. The router of course, doesn't do any transcoding.
2) Using a USB flash drive plugged into the Roku Ultra (the reason I got the Ultra instead of a cheaper version).
I tried Plex but don't want to keep a computer on all the time to run a server that does transcoding. At least not yet...
My video runs the gamut: mp4, mkv, even old avi files. Video encoded using AVC and HEVC. Audio in AAC stereo and 5.1, AC3, and E-AC-3.
