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Recommendation H.261
(03/93)
11
4.2.3.1
Macroblock address (MBA) (Variable length)
A variable length codeword indicating the position of a macroblock within a group of blocks. The transmission order is as
shown in Figure 8. For the first transmitted macroblock in a GOB, MBA is the absolute address in Figure 8. For subsequent
macroblocks, MBA is the difference between the absolute addresses of the macroblock and the last transmitted macroblock.
The code table for MBA is given in Table 1.
An extra codeword is available in the table for bit stuffing immediately after a GOB header or a coded macroblock (MBA
stuffing). This codeword should be discarded by decoders.
The VLC for start code is also shown in Table 1.
MBA is always included in transmitted macroblocks.
Macroblocks are not transmitted when they contain no information for that part of the picture.
12
3456
789
10
11
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13
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FIGURE 8/H.261
Arrangement of macroblocks in a GOB
MBA
MTYPE
MQUANT
MVD
CBP
Block data
FIGURE 9/H.261
Structure of macroblock layer
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