Plex transcoding audio to opus when requesting video transcoding

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Trying to understand what is happening.

I am using a firestick max 4k and am streaming from plex server on linux Version 1.32.6.7371
I have the video encoded at 4k h.265 10 bit in mkv format I have AC3 audio included in the stream.

I can direct play the 4k video along with the AC3 audio just fine. When I choose a lower quality 1080p video stream I expect the video to HW transcode and the audio to continue to direct stream. However the audio is transcoding to OPUS. trying to understand why?

plexlog.zip (23.9 KB)

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It’s an intended behavior, because otherwise on some devices audio video sync can drift apart over time.

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So anytime the video transcodes we should expect the audio to transcode to opus? Is opus the only option? Is it 2 channel opus even if the original stream is 6 channel?

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Opus is used because it delivers the highest quality at very low bitrates.
If you pick a higher target bandwidth, you can get also higher audio data rates. Depending on the player device, this can also mean 6-channel sound.

The reasons for transcoding according to your log are:

  • audio has 8 channels, while your device only supports 6
  • the selected audio track is not the first audio track within the source file
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Actually aware of the 8 channel vs 6 channel issue. However I specifically changed to the 6 channel audio stream after the video had started. So you should see it go from 8 channel TrueHD to 6 channel AC3 audio. I then manually change the video quality from 4K to 1080 to force a HW transcode.

I’m not stating anything is wrong here just trying to understand why it does what it does. Thank you!

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