110 Why do some people hate B-frames? (MPEG-2)
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
110 Why do some people hate B-frames? (MPEG-2)
Computational complexity, bandwidth, end-to-end delay, and picture
buffer size are the four B-frame Pet Peeves. Computational complexity
in the decoder is increased since some macroblock modes require
averaging between two block predictions (macroblock_motion_forward==1
&& macroblock_motion_backward==1).
Worst case, memory bandwidth is increased an extra 15.2 MByte/s
(assuming 4:2:0 chroma_format at Main Level), not including any half
pel or page-mode overhead) for this extra directional prediction. To
really rub it in, an extra picture buffer is needed to store the future
reference picture (backwards prediction frame). Finally, an extra
picture delay is introduced in the decoder since the frame used for
backwards prediction needs to be transmitted to the decoder and
reconstructed before the intermediate B-pictures in display order can
be decoded.
Cable television have been particularly adverse to B-frames since, for
CCIR 601 rate video, the extra picture buffer pushes the decoder DRAM
memory requirements past the magic 8- Mbit (1 Mbyte) threshold into the
evil realm of 16 Mbits (2 Mbyte).---- although 8-Mbits is fine for 352
x 480 B picture sequence. However, cable often forgets that DRAM does
not come in convenient high-volume (low cost) 8- Mbit packages as does
friendly 4-Mbit and 16-Mbit packages. In a few years, the cost
difference between 16 Mbit and 8 Mbit will become insignificant
compared to the bandwidth savings gain through higher compression. For
the time being, some cable boxes will start with 8-Mbit and allow
future drop-in upgrades to the full 16-Mbit.
Continue to:
My Books
