The following discussion applies to constant bit rate operation.  

When a decoder is started in the middle of a sequence, it does not know how full its buffer should be.  If it decodes the first access unit at the wrong time, its fullness will not be the mirror image of the encoder buffer fullness, and it may later underflow or overflow. This problem would be solved if the decoder could use the VBV model, but the required vbv_buffer_size_value is not available except in the sequence header.  Fortunately, equivalent information is carried in the PCR and PTS/DTS values which are present throughout the sequence.






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