Variable Bit Rate Operation
It is not possible to arbitrarily vary the transmission bit rate without taking into account access units that are already in the encoder buffer. These previously encoded access units must be transferred to the decoder buffer entirely before their specified decoding time (PTS or DTS). The variable rate encoder thus may change the bit rate for the picture or audio block it is currently encoding to comply with external requests or internal conditions; but it must maintain the predetermined bit rate for any access units already in the buffer. Operation otherwise is similar to constant bit rate operation.
The video buffering verifier model uses additional transmitted information to determine operation of the receiver buffer depending on the type of picture being coded and whether it is to be displayed repeatedly (for example, for 3/2 pulldown), but this is made generally unnecessary by use of the time stamps in the decoder. (The repetition of fields or frames for 3/2 pulldown operation is still specified by the values of the flags top_field_first and repeat_first_field.)
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