High CPU usage during HW transcoding with low performance on LG TVs

[Fabio-Cecamore]

Hello,
this is not my first experience with PLEX but it’s my first time using hw transcoding.
I just bought a QNAP TS-464eU powered by an Intel Celeron N5095 with 8GB of RAM and I’m running the latest official docker version of PLEX with hw transcoding enabled (atm limited at 50% CPU).

Everything runs perfectly except when I use my LG television with transcoding.
If I watch a movie without transcoding it works properly (low CPU usage with high bandwidth and perfectly smooth)

but if I ask for a transcoding, the NAS’s CPU runs at 100% and the streaming starts buffering:

Since the transcoding works perfectly with other devices (Plex for windows, Chromecast, Samsung TV) I think it’s because of the audio codec that LG TV requires that uses much more CPU resources.

In comparison, this is the same file transcoded for Chromecast:

(look at the audio codec with the Chromecast)

What do you think?
Thanks

Server Version#:1.31.2.6810
Player Version#: 5.58.3

[FordGuy61]

@Fabio-Cecamore

Everything runs perfectly except when I use my LG television with transcoding

You are running into the limitations of the Plex LG app and the Celeron CPU.

With the Plex LG app, if the media is direct streaming, such as when audio is transcoding, enabling any subtitle results in a video transcode to burn the subtitles into the video stream.

Burning subtitles uses the CPU, even when using hardware accelerated transcoding. Furthermore, it is a single threaded process, so it cannot take advantage of multiple CPU cores.

If the CPU is not fast enough to burn subtitles in real time, you will experience buffering.

Other Plex clients do not have the same limitations, which is why you do not experience the buffering & high CPU when using them.

[Fabio-Cecamore]

First of all, thanks for your reply.

I just tried again with my LG, disabling subtitle + transcoding and now only 20% of CPU and no more buffering.

So I double checked on my Chromecast with subtitle ON + transcoding and everything work flowlessy. So it’s the LG app the issue, but why?

I don’t understand, why PLEX decides to burn the subtitle only when transcoding with the LG app?

[FordGuy61]

@Fabio-Cecamore

I don’t understand, why PLEX decides to burn the subtitle only when transcoding with the LG app?

It is a limitation of the streaming protocol used between Plex Media Server and the Plex app on the LG.

If Plex were to direct stream the subtitles, they would become out of sync with the video and audio.

To get around the problem, Plex Media Server burns the subtitles into the video stream.

On most platforms, Plex uses the video player supplied by the manufacturer. Each video player has its own capabilities / limitations. That is why a movie/show may direct stream on one platform and transcode on another.

Plex for LG is one of the more limited clients, especially regarding subtitle support.

The Plex Android TV app is more capable. It can direct stream subtitles even when the video or audio is transcoding.