MPEG Null Packet Description

When carrying MPEG-2 or MPEG-4 video via MPEG-2 Transport Streams, something called Null Packets are regularly inserted by encoders and distribution systems. These Null Packets are inserted for many reasons and sometimes for no reason at all.   One reason they are inserted is to pad-up variable bit rate content so as to produce a constant bit rate transport stream. 

What is an MPEG Null Packet? It is a regular sized MPEG transport stream packet of 188 bytes in length with a few header bits set so to indicate that the contents of the remaining 184 bytes is meaningless.  Null packets are not permitted to contain the PCR timing fields used for synchronization.

It is defined in the system layer MPEG-2 Specification ISO/IEC 13818-1.

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