Information Technology Reference
In-Depth Information
Figure 6.3: An example of using PTS/DTS to synchronize bidirectional decoding.
In practice the time between input pictures is constant and so there is a certain amount of redundancy in the time
stamps. Consequently PTS/ DTS need not appear in every PES packet. Time stamps can be up to 700 ms apart in
program streams and up to 100 ms apart in transport streams. As each picture type ( I , P or B ) is flagged in the
bitstream, the decoder can infer the PTS/DTS for every picture from the ones actually transmitted.
Figure 6.4 shows that one or more PES packets can be assembled in a pack whose header contains a sync
pattern and a system clock reference code which allows the decoder to re-create the encoder clock. Clock
reference operation is described in section 6.4 .
Figure 6.4: A pack is a set of PES packets. The pack header contains a clock reference code.
6.3 Transport streams
The MPEG-2 transport stream is intended to be a multiplex of many TV programs with their associated sound and
data channels, although a single program transport stream (SPTS) is possible. The transport stream is based upon
packets of constant size so that adding error-correction codes and interleaving [ 1 ] in a higher layer is eased. Figure
6.5 shows that these are always 188 bytes long. Transport stream packets should not be confused with PES
packets which are larger and vary in size.
 
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