I'm confused about audio transcoding. especially stuff transcoded to dolby digital plus

Plex Media Server
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Im using the latest version of PMS and the latest apps as of may 31st 2020. On windows 10

Does transcoding the audio force video transcoding or not? I keep seeing conflicting posts.

I can direct play all my 4k video content without issue. And on both my 4k tvs it does appear to be properly directly played it even activates hdr. Some of them have dts ma audio and the rest 7.1 dolby true hd/ atmos auduo.

The true hd audio gets transcoded to 7.1 channel dd+ since I use roku, and it still seems to direct play 4k videos fine with hdr and all and it looks the same as when I throw the bluray on my 4k hdr blurays player.

And just to test I tried out transcoding the 5.1 channel dts ma track on other 4k hdr movies…once again just went to dd+ but this time 5.1 channels and the 4k hdr video direct played fine.

But I’ve seen a lot of posts saying thatd bad and I should use the 5.1 challenge ac3 core or dts core track since somehow transcoding the audio forces a video transcode. Is that true? I’ve only seen what looks to be evadince of the opposite.

Also I cant find anything that’s talks about this but what bitrate is the dolby digital plus that the audios are transcoded to, and does it support dd+ atmos or not?

2

It varies by client. Subtitles may come into play as well.

For Plex SmartTV app on LG & Samsung:

  • Enabling image based subtitles, PGS/VOBSUB, forces a video transcode.
  • If the audio is transcoding, enabling SRT subtitles forces a video transcode.
  • If the audio is direct playing/streaming, SRT subtitles direct play.

LG TVs direct play AAC, AC3, EAC3, dts, & dts-HD audio.
Samsung TVs direct play AAC, AC3, & EAC3 audio.
TrueHD audio transcodes on both.

With AndroidTV things just changed. Until recently, if the audio was transcoding, enabling subtitles forced a video transcode. This changed with the v8 client release.

I’ve no direct experience with a Roku. Not sure how it behaves.

During playback you can monitor status using the Plex Dashboard, Plex Dash app, or Tautulli.

It will be pretty obvious if you’re transcoding HDR material. Plex transcodes everything to H.264 SDR, so the HDR flag won’t show on your TV. Also, Plex does not tonemap HDR to SDR, so the colors will probably look washed out as well.

It is generally preferable to select tracks that direct play, as it puts less load on the server and there is less chance of an audio transcode causing a video transcode.

Movies with a TrueHD primary audio tracks also include an AC3 version of the primary audio track.

Movies with a dts-HD MA primary audio tracks also include a dts version of the primary audio track.

Atmos information is lost when transcoding, irrespective of source or destination codecs.

1 Like
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oh so its mostly a problem if i use subtitles. makes sense then ive never once used them in all my time of using plex. im not too worried about load on the server, ive got a 6 thread 12 core cpu and a gtx 960(for hardware encoding, modded drivers) for the only 2 things I have to encode for the back room tv, the 4k starwars projects which dont have hdr so no biggie there,

Good to know about atmos, I’ll be keeping my bluray player out for when i get my atmos avr then

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Consider the Nvidia Shield as well. The Shield is the only off the shelf Plex client that bitstreams TrueHD + Atmos. It also bitstreams AC3, EAC3, EAC3 + Atmos, and dts/dts-HD/dts:X.

Go with the 2019 Pro model. It has additional RAM and handles high bit rate media better than the non-Pro model.

My setup:
Shield Pro (2015) ↔ Denon AVR-X4300H ↔ LG B7 OLED
Everything direct plays. I have to work to find something that transcodes.

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