Buffering on LG WebOS only

Plex Players Smart TVs
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Short Answer: When using the LG Plex app, avoid subtitles, especially when the audio is transcoding. If you wish to use subtitles, use the SRT format and only when audio direct plays (Dolby Digital and AAC). Other combinations of audio & subtitles result in transcoded video.

The AppleTV is a more capable platform than the LG TV. That is why you do not experience the problems when streaming with it.

Long Answer:

  1. You do not mention the Synology model and the information is not in the log file. If you have a unit with the J4xxx Celeron (some 2020 models), be aware there is a problem with the Intel supplied video driver. Due to the driver issue, hardware accelerated transcoding does not work correctly. See Synology FAQ #26 for a workaround.

  2. The physical Ethernet port on the TV is 100 Mbps. The TV’s WiFi is capable of faster communication when using 802.11 AC on 5 GHz.

Some 4K video can burst above 100 Mbps, resulting in buffering, even when the media is direct playing. If you’ve a strong 5 GHz 802.11AC signal at the TV, you may have better streaming performance via WiFi than with wired Ethernet when streaming high bit-rate content.

  1. Your TV does not support dts audio (specifications). LG removed support for dts beginning with the 2019 models. As you discovered you must deselect dts audio in the Plex app. This forces Plex to transcode the audio to a supported format.

  2. Be aware of the limitations of the Plex app on LG (and Samsung) Smart TVs:

  • Enabling image based subtitles, PGS & VOBSUB, causes Plex to transcode the video.
  • Enabling SSA/ASS subtitles that contain color, position, etc information, causes Plex to transcode the video.
  • If the audio is transcoding, enabling SRT subtitles causes Plex to transcode the video.
  • If the audio is not transcoding, SRT subtitles direct play.

  1. Burning subtitles into the video stream occurs on the CPU, even when hardware accelerated transcoding is used. The process is single threaded, using only one core/thread on the CPU. Furthermore, the Celeron CPUs used in Synology NAS have low single threaded passmark ratings (ex: J3455). As a result, transcoding video to burn in subtitles on a Synology NAS is problematic and can lead to issues such as buffering.

  2. You can monitor playback via the Plex Dashboard or the Plex Dash mobile app. This will let you know if the video or audio is transcoding.

Bottom Line:

When using the LG Plex app, avoid subtitles if the audio is transcoding (TrueHD, dts). Use only SRT subtitles if the audio is not transcoding (Dolby Digital, Dolby Digital Plus, AAC). Other combinations of audio & subtitle types result in Plex burning the subtitles into the video stream. This can cause problems given the limited power of the Intel Celeron CPU in Synology NAS.

If you use subtitles on a regular basis, consider using a streaming device such as an AppleTV 4K or 2019 Nvidia Shield Pro. Either is a more capable Plex client than the LG Plex app.

The Plex Android TV app on the Shield direct plays subtitles even when the audio is transcoding.

You can use your current ATV 4K to test various video/audio/subtitle combinations to make sure it fits your needs.

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