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The H.261 Bitstream structure may be summarised as follows:
- Need to
delineate boundaries between pictures, so send Picture Start Code ->
- Need timestamp for picture (used later for audio
synchronization), so send Temporal Reference -> TR
this a P-frame or an I-frame? Send Picture Type -> PType
- Picture is divided into regions of 11x3 macroblocks called Groups
of Blocks -> GOB
- Might want to skip whole groups, so
send Group Number (Grp #)
- Might want to use one quantization value for whole group, so send Group Quantization Value -> GQuant
- Overall, bitstream is designed so we can skip data whenever possible while still unambiguous.
The overall H.261 Codec is summarised in Fig 7.8.4.