Is there an ideal Buffer Size for Constant Bitrate?

Help/How To
1

The scenario: I’m recording the screen of a game to post on youtube with OBS. The file is set to be a mkv (so I won’t lost the video if there is any problem with OBS) with an average bit rate of 24Mb and maximum of 48Mb. To match the source, this video is 1080p.

After editing it on shotcut my export setting is “youtube”, so a mp4 file with constant bitrate of 48M and upscaled to 1440p (the choice for this resolution is to force youtube to use the better codec VP9 instead of the trash AVC1). I’ve used constant bitrate after some testing and got a better processing time on youtube than when using average bitrate, while still getting a decent size file.

All that just to ask if there is an ideal buffer size to set while exporting that video. Both in seconds and size.

Thank you all for the response in advance!

2

Are you open to “Quality-based VBR” mode instead of constant bitrate?

YouTube doesn’t reward anyone for fitting their video within a constant bitrate. All it does is compromise the encoding quality.

3

I tried that but the file size just skyrocketed. It went from 5 gb to 32gb. with a 75% quality. The bitrate went from an average 48 Mb to 190Mb. It is just excess.

I tested with a constant 48 Mb, and there was no quality difference on youtube But I don’t know what to do with the Buffer Size.

4

75% is way too high. Visually lossless is at 68%. Most videos are fine between 55% and 60%.

5

It does not. The reward comes with the 1440p in form of VP9 codec instead of the AVC1. But the processing time was cut shot with constant bitrate. I would say it cut a good 5 hours of processing. The video with 48 Mb average took 30 hours to process, the one with 48 Mb constant took 25ish.

And that is just processing time for a 1440p video. Not counting the upload, obviously.

6

Good to know. Do you think that setting it to 60% instead of 75% will make a huge difference on the size of the final video?

7

It doesn’t take 5 hours to decode average or constant bitrate video. Real-time playback would be impossible if that was the case. (Encoding is the same time either way since it’s always VP9.) Processing time is a matter of where you fall in the queue, and whether the guy ahead of you uploaded ten hours of Epic Sax Guy on a loop (I’m not even making that one up). Comparing format performance isn’t really possible.

If you do want to use a buffer, the default is usually fine. I think it is one second at whatever the bitrate is. Between 1 to 2 seconds is ideal. If the bitrate is 48M, then the buffer can be 48-96M.

Ginormous.

Also… are you using hardware or software encoding? File size can be heavily influenced by the model of GPU if using hardware encoding.

1 Like
8

I’m using the hardware option. H264.nvenc, since my gpu is a 3070.

Thank you for this information

I’ll try that too

9

Great card. You’ll be fine. Software encoding might make a slightly smaller file (not even guaranteed to when going up against a 3070), but even if it did, it probably wouldn’t be worth the extra export time required.

10

I will try to change the quality to 55%, 60% and see the difference. Also use a 1s buffer size.

Thank you again!

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11

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