[Mp4-tech] PSNR of B frames.

[Barzilaij]

Dear Experts,
It turns out in all my simulations (scalable mode) with the JSVM, that the B frames in the hierarchical B frames structure always have a higher initial QP and therefore a lower PSNR value than the I or P frames (e.g. I or P frames have a PSNR of 38 while B frames have a PSNR of 34).
During simulation with VQM Software I analyzed different frame rates with similar bitrates (CIF format -- 7.5, 15 and 30fps on 100, 200, 500 and 1000kbit/s). In general, even for rather slow moving video content, the highest framerate was preferred over others for any bitrate setting. It seems that with extra bitrate budget, increasing framerate is always preferred over increasing PSNR of lower temporal levels. Since the B frames can be constructed with little data, jerky or unnatural motion is very easy to combat.
Currently I am unable to find good encoder settings such that a lowering the framerate becomes more useful. My question is the following;
Why do B frames have a lower QP and is there a way to adjust the initial QP value of the B frames? More general, is there a way to increase the PSNR of these frames to the PSNR of the I or P frames.
Any feedback is appreciated.
Kind Regards,
Mark Barzilay
VQM Software: http://www.its.bldrdoc.gov/n3/video/vqmsoftware.htm
If asked, I can send some results or more info about my research.
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