Reasonable questions, with fairly simple answers.
For the first question, Roku has supported Dolby Digital bitstreaming through the HDMI cable almost from the beginning. Actually I think even their very first player did so. By the way, pass through is another way of saying it will pass the digital audio bitstream intact to a AV Receiver or the TV display. Virtually any modern (within the last 20 years) TV is capable of decoding DD and playing it through their speakers. DTS is a bit different, in that few TVs have DTS decoders, so the DTS passthrough is really only useful when using an AVR that can decode the digital audio. Also, Roku only supports DTS when it's a video within the MKV or TS container, so only applicable if you're playing media from a USB drive or a DLNA server.
Your second question is in reference to players that have a Dolby encoder, so it is capable on converting an audio stream into DD. Only the Ultra 4640 and the Ultra 4800/4801/4802 have that feature. So if the incoming audio stream isn't in DD, the Ultra can convert it to DD which again is supported by all modern TVs.
Dan
Roku Community Streaming Expert
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