[x264-devel] CRF + VBV vs CBR for real time steaming applications

[eloise vidal]

Hello everybody,

I try to undersand why the CRF+VBV is preferable to the CBR mode for live
streaming applications.

I read in the post "Making sense out of x264 rate control
methods"<http://mailman.videolan.org/pipermail/x264-devel/2010-February/006945.html>,
that CRF+VBV performs better quality than CBR with the following
configuration :

CBR : x264 --vbv-bufsize 2000 --vbv-maxrate 1000 --bitrate 1000 -o
<output> <input>
--> here we constraint the bitrate and the quality fluctuates with the
frame complexity

CRF+VBV : x264 --vbv-bufsize 2000 --vbv-maxrate 1000 --crf 18 -o
<output> <input>
--> here we constraint the quality and the maximum bitrate, and the
final bitrate of the sequence depends on its complexity but can be
superior to the vbv-maxrate.

Here, the both modes have the same vbv-maxrate, so the CRF+VBV mode cannot
allocate more bits than the CBR mode to a frame(?)
So if we compare one frame encoded with the both modes, the quality cannot
be better with the CRF+VBV mode.
So the CRF+VBV mode is preferable only because the quality is constant over
all the frames of the sequence (No flickering effect).

Are My assumptions true?

Thanks for your help

Eloïse
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