Here's the quote from a Roku media playback support page:
Support for AAC (Advanced Audio Coding)
Roku devices do not support AAC pass-through. Instead, Stereo AAC is transcoded to PCM stereo.
Roku TVs and some Roku players decode 5.1 AAC to PCM stereo for headphone, internal speaker, and HDMI/ S/PDIF (optical) output. However, Roku devices that support Dolby Audio decode connected via HDMI or S/PDIF (optical) to an A/V receiver that also supports Dolby Audio, will convert the audio to Dolby Digital 5.1.
https://support.roku.com/article/208754908
That last bit about "some Roku players" only applies to the Ultra 4640 and 4800, to the best of my knowledge. Those two players and Roku TVs are the only devices I know of with a Dolby encoder.
If you want a player that will bitstream the audio used on UHD disc rips (Dolby TrueHD, DTS Master Audio), you need the Nvidia Shield. I don't know of any other streaming player that will do that.
Dan
Roku Community Streaming Expert
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