Roku info page "How to set up your Roku streaming player for surround sound" now has a new Note under the "More about audio formats" subhead that says:
- Note: The Roku Ultra* provides an additional option to transcode Dolby Digital Plus to Dolby Digital. This allows you to connect your Roku Ultra to an AVR, sound bar, or TV that supports Dolby Digital, but not Dolby Digital Plus, and still hear the audio in surround sound.
* Not available in all countries.
That is only the 2016 Ultra, model 4640. The current Ultra version is model 4660, and does not convert DD+ to DD. If the Ultra has an optical port, it's a 4640. Otherwise it's a 4660.
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.