Question about Transcoding

General Discussions
1

Hello,

I’ve been investigating a bit on Transcoding but I still have a question I don’t find answered. If I missed it, please refer me to the existing answer and sorry for reposting.

I wanted to know what has an impact in CPU usage when transcoding. Is it only the Bitrate and the Resolution of the original file or also to what quality this is transcoded to. For example: If you have a 10Mbps 1080p movie and you only slightly downscale it to let’s say 8Mbps 1080p will this stress the CPU more or less then for example taking a 4Mbps 720p movie but downscaling it all the way to let’s say 64Kbps.

In the first case the origin is a bigger but it only needs to downscale 2Mbps while in the second case the origin is smaller but it needs to downscale almost 4Mbps.

I’m tented to think that it is only the original size of the file that marks the workload and that very high bitrates and resolutions put a high workload on the CPU, regardless if it has to be taken to 8Mbps or 64Kbps, but I’m not sure at all.

Can anyone clarify this for me?

Thank you very much!

2

Transcoder is a 2 step process. 1 - decode which is reading the original file and extracting the audio and video info from it. In most cases this is pretty quick and does not put a lot of stress on your computer. 2 - encode is turning the audio and video back into a file. At a high bitrate, the process has to be more careful to produce the high quality results. With a lower quality, the encoder can guess a bit so it will work faster.

3

Thanks for the reply!

So if I understood correctly, the main factor is the quality plex needs to transcode to, not so much the quality of the origin. In other words, the first example (10Mbps to 8Mpbs) would stress the system considerable more than the second example (4Mbps to 64Kbps).

That is good to know. I use transcoding basically for viewing on mobile devices (tablets, phones). Since they have small screens, I normally choose a quite low quality output, 720p at the most.

Thanks again.

4

It is still a difference if the original file has a higher resolution (or bitrate!) or a lower resolution.
The ~2000 passmark number which is told in this support document
https://support.plex.tv/hc/en-us/articles/201774043-What-kind-of-CPU-do-I-need-for-my-Server-
for instance is only valid if your original file has 1080p resolution and a bitrate of ~8mbps and is encoded in the H.264 video codec.

If your file is a direct rip from BluRay with up to ~35 mbps, you’ll need ~ double the passmarks!

Further additional cpu performance is needed if you ‘burn in’ a subtitle during transcoding.
Or if your file on disk has a different codec (like H.265 requires almost triple the cpu power for transcoding).

5

@Eneko Prins said:
So if I understood correctly, the main factor is the quality plex needs to transcode to, not so much the quality of the origin. In other words, the first example (10Mbps to 8Mpbs) would stress the system considerable more than the second example (4Mbps to 64Kbps).
True for the most part. There are a few codecs that decoding can be a chore (i.e. VC-1 and HEVC), so in these cases the decoding can play a significant factor. But the overall stress still follows the above statement. 10Mbps HEVC to 8 Mbps H264 is more stressful than 4Mbps HEVC to 64Kbps H264.

6

Thanks again for the great information.

I recently bought an HP Microserver to use as a NAS/PMS and it has an Intel Celeron G1610T (2.30GHz). This CPU has a Passmark score of 2349, not too much over the 2000 mentioned as a recommendation in the support documents. I’m asking this question to try to stress this CPU and see where it can go. I have read some comments in forums that say this CPU is not really powerful enough to do transcoding.

However, I have seen entire movies of around 30Mbps (BluRay Remux) transcoded to 10Mbps and it didn’t skip a beat! The CPU goes up to 99% usage but I don’t know how Plex does it, but the movies didn’t stop in any moment. I’ve tried other lower resolutions and bitrates to transcode to and no difference. 99% of CPU usage but the video plays absolutely fine.

I don’t have much HEVC (H.265) material and the files I have are only around 1Mbps. Nevertheless I already notice the difference. Some skips with the HEVC transcoding to the original quality (so 1Mbps H.265 to 1Mbps H.264), normally at the beginning of starting playback. But still, very very watchable!!

So I’m pretty impressed both by the little HP Microserver and with PMS which for me transcodes a lot more efficient than I initially thought it would. Before buying the server I read all kinds of horror stories about how much power is needed for transcoding.

7

Activate a (PGS or VOBSUB) subtitle then or find a bluray remux with the VC-1 codec :smiley: