SYSTEM LAYER AND MPEG-2 PACKETS
The system layer permits a datastream to be integrated into a programme multiplex. Fig. 12.10 indicates the structure of a MPEG-2 packet: the sync word, the data itself and a checksum. The packets are separate for video, audio and programme data, and are combined into a programme stream (Fig. 12.11) together with Programme Specific Information (PSI), the key to the demultiplexing and decoding operations at the receiving end.
PSI
Programme Specific Information is added to the transport stream, and describes its composition, including information on which data- streams (audio, video, data) go together. The decoder depends on PSI to demultiplex, decode, assemble and present MPEG programmes; also for the extraction of conditional access information to descramble and de-encrypt programme elementary streams. Components of the PSI are look-up tables for Programme Association; Programme Map; Network Information; and Conditional Access.
