The H.261 Bitstream Structure

The H.261 Bitstream structure may be summarised as follows:

• Need to delineate boundaries between pictures, so send Picture Start Code -> PSC

• Need timestamp for picture (used later for audio synchronization), so send Temporal Reference -> TR

• Is 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.