[Mp4-tech] B frame bit-rate in H.264 stream

[changfeng niu]

Hello everyone:
 First, I would like to ask a question about H.264 video coding. When I use
JM
8.6 to code, I find that for almost all sequences the second one of  two
successive B frames mostly has fewer bits than the first one without B frame
as a
reference. Why?
  Now, I am working on transcoding from MPEG2 to H.264. I meet such a
similar
question. When coding with MPEG2 decoder reconstruction picture as JM 8.6
inputfile, I frame, P frame and the first one of two successive B frames
without B
frame as a reference have almost the same bits and PSNR as tanscoing
procedure and
coding bits of the second one of two successive B frames are fewer notably
than
the first one of two successive B frames .However two successive B frames
mostly
have the same bits without B frame as a reference in our transcoding
procedure .
  Our transcoding procedure has the same configuration parameters except
following
two differences  with JM 8.6.
1. Whether a macroblock is coded as a intra one or a inter one lies on MPEG2
encoder in transcoding  procedure while it is decided by mode selection in
JM 8.6.
2. In transcoing procedure mv of a macroblock constitutes of MPEG2 mv
integer and
inserted subpixell using H.264 arithmetic while it is get by full searching
restricted to 16 pixels in JM 8.6.
I don't know whether mv makes an influence or not.
Waiting for your answer.Thanks very much!
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