Log in

View Full Version : Want to understand bitrate destribution.


junglemike
15th June 2006, 00:47
Hi.
I encoded a DVD which consists of 11 different video clips with x264.. I first encoded the whole DVD at 2 pass, and set avarage bitrate in 2nd pass to 450kbps (pretty low especially for clips, want to test the limits). Than i cut up the resulting file to 11 separate clips.
Than I discovered target bitrate very a great deal. for example there were couple of clips encoded at ~200kbps, and couple at 800kbps, rest in between.
I watched them all - quality is OK for all of them.

How can bitrate very so much for different clips?
What if I try to encode same dvd with 1-pass average bitrate (same 450kbps) - How codec is going to know what clips require high bitrate, and which low? - Will it just give 450kbps to all of them?
Thanks.

Sharktooth
15th June 2006, 01:16
in 2 pass mode the bitrate wildly jumps along the whole clip unless you limit the variability with the --ratetol option.
however you will always (well... almost always) get your desired average.
more info on x264 rate control: http://students.washington.edu/lorenm/src/x264/ratecontrol.txt