Transport stream
synchronization
The transport stream packet consists of different substream data packets. The data
packets are transmitted with a length of 188 bytes. At the beginning of each packet is
the sync-byte (0x47). Because this value is not exclusively reserved for the sync-byte,
the repetetive occurrence of the sync-byte every 188 bytes has to be checked too to
ensure stable synchronization.
Packet identifying
To identify the individual packets there is a Packet Identity (PID) contained at the be-
ginning of each packet (after the sync-byte). Each substream
(e.g. video, audio) gets its own PID. Some PIDs are assigned automatically and can-
not be changed. (e.g.: PAT, CAT , .. siehe table 1)
Structure of the transport
stream
Normally a transport stream contains several programs. Each of this program contains
again a number of different substreams (elementary streams).
Program access
With the help of the information about the structure of a transport stream the required
program can be selected for decoding. If a program contains different equivalent ele-
mentary streams (e.g. different languages), the respective selection has to be made,
too. The selected elementary stream packets, which were detected by PID ,are passed
on from the demultiplexer to the decoder.
Decoding
The encoding of the received data can take place on various levels.
The entire transport stream or elementary streams (Packetized Elementary Streams
PES ) can be encoded. The header information of the transport stream always stays
uncoded, the header information of the PES is also encoded when the entire transport
stream is encoded.
The data necessary for decoding are transmitted in the Entitlement Control Messages
(ECM) and in the Entitlement Management Messages (EMM).
The ECMs also contain the necessary keycodes and the EMMs also contain the ac-
cess codes for the receiver.
Program
synchronization
A program consists of several elementary streams. To synchronize the decoding of the
individual elementary streams a common reference is necessary.
For that a Program Clock Reference (PCR) is included in each elementary stream.
This information is contained in the Adaption Field, which is also transmitted in a cycle
of max. 100ms. The Adaption Field always stays uncoded.
The information derived from the PCR-data is used to control the 27 MHz system
clock of the receiver. In this way the synchronization of multiplexer and demultiplexer
is ensured.
For syncronizing the elementary streams additional time reference values, Decoding
Time Stamps (DTS) and Presentation Time Stamps (PTS) are contained in the ele-
mentary streams.
Introduction
IRD 8500/..
Intergrated Receiver Decoder
1 - 5
