6.1 Introduction

There are two basic applications of MPEG bitstreams recording and transmission and these have quite different requirements. In the multi- channel recording application the encoders and decoders can all share the same clock. The transport speed can be altered so that the incoming bit rate can be slaved to a reference generated by the decoder itself.

In the case of multichannel transmission the program sources are not necessarily synchronous. The source of timing is the encoder and the decoder must synchronize or genlock to that. Thus the difference between a program stream and a transport stream is that the latter must contain additional synchronizing information to lock the encoder and decoder clocks together independently in each program.

6.2 Packets and time stamps

The video elementary stream is an endless bitstream representing pictures which are not necessarily in the correct order and which take a variable length of time to transmit. Storage and transmission systems prefer discrete blocks of data and so elementary streams are packetized to form a PES (packetized elementary stream). Audio elementary streams are also packetized. A packet is shown in Figure 6.1. It begins with a header containing an unique packet start code and a code which identifies the type of...