Fundamentals and Evolution of MPEG-2 Systems: Paving the MPEG Road

13Data Structure and Design Considerations

How STC samples and PTS and DTS time stamps are coded. What the data structure of a PES header is. What descriptors are, how their data is structured and which types of descriptors are defined. The purpose and specific data structure of the various system descriptors and general content descriptors. The data structure of a program stream; the pack header with optional system header; the program stream map. Data structure of sections in general and of program association, program map and conditional access sections in particular. The data structure of transport packets with the adaptation field.

13.1 System Time Clock Samples and Time Stamps

Compatibility with MPEG-1 systems was one of the requirements when MPEG-2 systems was designed. For the use of PTS and DTS time stamps such compatibility was easy to achieve, as both 90 kHz resolution and the use of 33 bits for time stamp coding were considered suitable for MPEG-2 systems. However, for the STC (the System Time Clock) instead of 90 kHz, a resolution of 27 MHz was decided. For more background on this issue see Section 6.6.

On the other hand, there are also applications for which a 90 kHz STC resolution would have been sufficient. To allow applications to conveniently apply 90 kHz as the time base in their decoder implementations, as well as for compatibility with MPEG-1, it was decided to split the coding of STC samples in the PCR, OPCR, SCR, and ESCR fields in two parts. One part ...