Transcoding 4k to 1080p changes the bitrate, but leaves resolution at 4k

Plex Media Server
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Server Version#: 1.27.2.5929

Hi All,

I’ve encountered, and at least partially debugged, a weird 4k transcoding issue.

First, some basics: I’m running my plex server on a synology 920+ with hardware acceleration on. I have two main clients for my plex server - namely two chromecast with google TVs, one hooked up to a 4k tv and one hooked up to a 1080p tv. In general, things have been working pretty flawlessly.

But I’ve intermittently encountered a weird issue and decided to drill down further. Occasionally, when streaming a 4k source to my 1080p TV, the thing will buffer like mad. I have it set up to transcode to 1080p (I typically choose 20 mbps), but it still buffers. This doesn’t happen for all 4k content, and doesn’t seem to depend on bitrate either. This is all local streaming and so bandwidth is not an issue. Playing the same content on my 4k TV has no issues.

Upon some more investigation, it would appear that transcoding to 1080p (20mbps bitrate) changes the bitrate to 20 mbps, but leaves the resolution at 4k. So my suspicion is that this is what is causing the buffering. I semi-confirmed this suspicion when I transcoded to 1080p 20mbps on my 4k TV and IT began buffering, whereas playing at original quality it had no issues. Additionally, I’d like to point out that I don’t think this is a transcoding throughput issue, either, as my CPU usage doesn’t even go over 50% and I have no issue transcoding 4k to 1080p for the majority of files.

After more testing, I think I’ve figured out what the problem is, although I don’t fully understand the why yet. It seems like with some 4k files, transcoding them to 1080p High Quality, which is a 20 mbps transcode, only downgrades the bitrate, but leaves the resolution at 4k. I don’t know why this happens only with specific files and not others. but it feels like a bug to me. If I choose medium quality (12 mbps) it downgrades both.

This seems like a really odd problem, and only seems specific to certain files. Anyone have any insight here? Happy to post specifics about the files if that would be helpful.

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DEBUG log files which capture this happening are needed.

Can you recreate (making certain DEBUG logging is set before starting) and then attach the logs after recreating?

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Thanks for the reply. I’ve uploaded the log file here, although redacted anything to do with my external IP, server name, user name, email, etc.

Edit: I initially attached the player debug log, not the server debug log. Posting an excerpt of the debug log now.
plexdebug.log (41.4 KB)

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I’m sorry, I don’t see the ZIP file containing your logs.
Did you remove it?

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Sorry, it’s up now. Just being an idiot. The entire log file was 10 mb, so I clipped an excerpt of what I was doing specifically that I suspect has the useful information.

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UPDATE: I see your revised log.

Unfortunately you edited out too much (it’s too short).
I do not see the sequence from when “Play” is pressed until the actual playback starts.

I will send you a PM so you may attach the full, unedited ZIP file (Settings - Server - Troubleshooting - Download Logs)

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