An MPEG data stream may multiplex streams from several sources, each having a slight difference in clock rate from the others. In general, it will not be possible to synchronize all these sources to the transport multiplexer.
In order to multiplex several incoming MPEG streams into one, the individual streams must have some null packets that can be removed if necessary to adjust the over-all multiplex data rate. The null packet removal, as well as the interleaving of packets from different sources, violates the assumption of a constant-delay transmission channel, and introduces jitter into the packet timing.
The MPEG time-stamping process provides sufficient information to carry synchronization through such a multiplexer to a decoder, but the MPEG standard does not specifically provide methods to take care of packet jitter effects. In general, the resulting packet jitter may exceed the MPEG tolerance, and then methods must be developed to correct this in order to feed a standard decoder. Some commercial equipment avoids this problem by physically co-locating all encoders that feed a given multiplex (for example a satellite transponder), and driving them all from a common clock and time reference.
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