TABLE OF CONTENTS After compression, audio and video signals are simply data and only differ from generic data in that they relate to real-time signals. MPEG bitstreams are basically means of transporting data whilst allowing the time axis of the original signal to be re-created at the decoder.
There are two basic applications of MPEG bitstreams - recording and transmission - and these have quite different requirements. In the multichannel recording application the encoders and decoders can all share the same clock. The read data rate of the storage device can be adjusted so that the decoded frame rate or audio sampling rate can be slaved to any required reference signal. In the case of multichannel transmission, such as a digital television service, 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.
Transmission techniques are steadily becoming more complex as new applications are found. In the earliest systems a dedicated signal path with constant bandwidth was specified and one compression factor would be employed. The use of statistical multiplexing allowed variable compression factors to be used subject to the overall bit rate of the multiplex remaining the same.
In network applications different users have access to different bit rates according to geographic and economic constraints.
