I've recently upgraded my Plex instance to separate out the storage side and the compute side. In general, I couldn't be happier - the compute side is a little HP prodesk with a quicksync-capable CPU, and the results have been phenomenal.
I have one user who has been complaining about stuttering playback when doing Direct Play on a Roku 3 (2015). When they switch to transcoding, it works perfectly. The only time I've seen it, it has been doing a direct stream of MKV -> MPEGTS for the video container with a transcode of DCA 7.1 -> Stereo audio.
Bandwidth is not the issue and they were the only ones streaming at the time.
Do you think the Roku choking on it somehow? My 2017 Shield TV can fully direct play it without issue.
What's the full codec and format breakdown of an example problematic file?
Is it something barely supported like 10bit H264? Or perhaps an ass subtitle incompatibility?
Could also just be as simple as bad wifi to the roku.
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