Poor quality audio transcode from DTS on Plex for LG

Plex Media Server
,
1

Server Version#: 1.31.2.6810
Player Version#: Plex for LG / webOS 5.4.2 / LG OLED55CXPUA

I watch Plex using the webOS app on my LG CX. Unfortunately, this TV is missing the ability to decode or passthrough DTS due to licensing issues, so I have to uncheck DTS as a playback option in the Plex for LG app’s settings on the TV - otherwise it plays silence for those files. I use HDMI ARC to pass audio to a Yamaha 5.1 receiver.

Since I have DTS compatibility unchecked on the TV’s app, my Plex server automatically transcodes DTS audio to a different format. Here is an example of one source audio track on an MKV before playing it in Plex:

  • DTS 48000Hz 6ch 1536kbps

And here are specs of the audio that the TV ended up playing back after transcoding (found by inspecting a media-#####.ts file that Plex produced):

  • AAC 48000Hz 6ch 133kbps

The audio sounds noticeably poor after being transcoded - sloshy like an old RealMedia stream. You can see the bitrate is winding up at under 10% of the original when I try watching DTS content on my TV.

Is there a way to configure my Plex server to transcode the audio at a higher bitrate, or maybe to another format like AC3 or something lossless instead of AAC? Thank you!

2

If this is a copy from a DVD, there are normally 2 tracks available.

  1. DTS

  2. AC3 or EAC

3

This issue pertains to hundreds of files I have that do not have alternative surround sound tracks, just DTS. But thank you!

4

No.

Check the log files to find the bitrate (Settings → Troubleshooting). Look in Plex Media Server.log for lines with Reached Decision.

If I disable DTS on my B7, it transcodes to AAC 5.1, 774kbps: Audio=(id=65460 decision=transcode bitrate=774 encoder=aac channels=6 rate=48000.

Note: If TV is remote from server, and Plex is transcoding to get under a bandwidth limit, it will transcode both the video and the audio. This will impact the bitrate chosen by Plex.


Mar 25, 2023 12:55:04.528 [0x7f7c02197b38] DEBUG - [Req#763ba/Transcode] Streaming Resource: Reached Decision id=1361 codes=(General=1001,Direct play not available; Conversion OK. Direct Play=3000,App cannot direct play this item. Direct play is disabled. Transcode=1001,Direct play not available; Conversion OK.) media=(id=30245 part=(id=30259 decision=transcode container=mpegts protocol=hls streams=(Video=(id=65459 decision=copy width=1920 height=1080) Audio=(id=65460 decision=transcode bitrate=774 encoder=aac channels=6 rate=48000))))

5

Thanks for helping me troubleshoot!

My log looks like yours, it reports that it’s encoding to AAC at 774kbps, but the quality sounds dismal to the point where it’s difficult to make out dialog at times (even when listening back on my PC to the media-#####.ts files).

Mar 24, 2023 23:58:53.900 [26756] DEBUG - [Req#b93fa/Transcode] Streaming Resource: Reached Decision id=32732 codes=(General=1001,Direct play not available; Conversion OK. Direct Play=3000,App cannot direct play this item. Direct play is disabled. Transcode=1001,Direct play not available; Conversion OK.) media=(id=36541 part=(id=38387 decision=transcode container=mpegts protocol=hls streams=(Video=(id=117061 decision=copy width=1920 height=816) Audio=(id=117062 decision=transcode bitrate=774 encoder=aac_mf channels=6 rate=48000))))

I can confirm the TV is on the same LAN and the video is not being encoded.

6

I’m still hoping someone has a suggestion on how to configure the bitrate or format that Plex is re-encoding DTS audio to.

I tested a workaround involving using ffmpeg to manually re-encode a DTS track to AC3, and that file sounded drastically better on the TV client end than the equivalent Plex AAC transcode. But it’s just not practical or feasible to alter hundreds of gigabytes of video files to do that across the board.

7

Multichannel AAC has a different channel mapping than DTS or AC3. If Plex isn’t explicitly defining the channelmap during transcode, it’s probably incorrectly mapping them, leading to an incorrect output.

If you have a full server log of the issue, you can find the actual transcode command. You can then see exactly how it’s converting the audio since it’s essentially an ffmpeg command.

As far as telling Plex to transcode to something else (like AC3), there are custom profiles you can look into using.

8

Thanks, I’ll look into the channel mapping - but I can confirm the center channel with dialogue is coming out of the correct speaker after transcode (just at an audible quality loss).

Could you point me in the right direction for where I could learn to set up a custom profile on Plex Server for Windows for the LG WebOS client?

1 Like
9

Settings - Server - ‘Show Advanced’ - Transcoder

  • “Transcoder Quality”
    (NOT “Background transcoding quality”)
    Set that to “Prefer Higher Quality” or “Make my CPU hurt”.
10

Thanks - unfortunately that’s already set to “Make my CPU hurt.”

11

I’ve been digging into client profile today and found these. Be great if Plex had some documentation on this.

https://freetime.mikeconnelly.com/archives/7213

12

^^ This is old info.

There is no documentation needed.
Just press Play.
Works for instance in Chrome on Windows, using the hosted web app. No changes required – provided there is enough bandwidth available to transport the usually high-bitrate files from UHD Blurays.

grafik

But expect it to activate transcoding if you are selecting a picture-based subtitle stream (PGS, VOBSUB), because browsers don’t support that.

It does work up to HDR10 color. But not with DoVi.

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