[M4IF Technotes] BVOPs question

[Christoph Lampert]

On Wed, 18 Sep 2002, Ricardo Torres wrote:
> You can try to use higher quantization step for the B-frame. (I don´t  know
> if it is
> 
> possible with the Microsoft MPEG-4 coder).
> 
> If you use higher quantization steps for the B-frames  the overall file size
> of the video will decrease,
> 
> but the global quality hardly decrease, due to the B-Frames are not used as
> reference for coding
> 
> the video sequence.

_If_ B-frames result in lower overall bitrate at constant quality
at all depends heavily on the input material. If you are lucky, even 
B-frames at same quantizer as P/I-frames will give you lower bitrate. 
If you are unlucky, even more than doubling the quantizer of B-frames
will give higher bitrate than no B-frames at all. 
The problem is that even though B-frames are smaller than P-frames, the
P-frames tend to get larger when the interval in which they are set
increases. This might have been different for MPEG-2, but for MPEG-4 the
main advantage of B-frames is not really reduced filesize. 
Christoph
-- 
Christoph H. Lampert chl   math,uni-bonn,de | Diese Signature wurde maschi-     
Beringstr. 6, Zi. 14, 53115 Bonn, Germany | nell erstellt und bedarf
Tel. (0228) 73-2948  Fax. +49 228 73-7916 | keiner Unterschrift. AZ 27B-6