Introduction to Digital TV Transmission Technique
1–8
MTD200 MPEG Test Decoder User Manual
Two exceptions are permissible:
The Discontinuity Indicator is set in the Adaptation Field (for example,
during a program step). See Figure 1–5 on page 1–11. The Continuity
Counter can then have any value.
A packet may be transferred twice. The Continuity Counter must not be
incremented.
The received data can be scrambled. Scrambling is performed at the level of the
transport stream (TS) or of the packetized elementary stream (PES).
The corresponding header information (PES header or TS header) remains
unscrambled. If scrambling is required at the transport stream level, this also
includes the PES header, whereas the TS headers remain unscrambled in all
cases.
If a packet (TS packet or PES packet) is scrambled, this is indicated by the first
bit of the scrambling control field of the associated header (TS header or PES
header). The second bit in the scrambling control field is required, since the
scrambling code changes from time to time. This bit is set if a new scrambling
code is valid for the packet in question.
To be able to perform descrambling, the decoder requires specific control data
that are transmitted with the entitlement control messages (ECM) and the
entitlement management messages (EMM). The ECM contain the scrambling
codes and the EMM distribute the access authorizations for the receivers. ECM
and EMM are included and referred to in the program specific information (CAT
or PMT).
Descrambling is performed in a supplier-specific hardware, which can be
connected to the decoder via a DVB-defined interface.
In general, a program comprises several elementary streams. A common clock
reference is required to synchronize the decoding and display (or output) of the
individual elementary streams. This clock reference for each program comes as
Program Clock Reference (PCR) and is carried along in an elementary stream of
the program. To this effect, every 40 ms at maximum, the 4-byte header of a
transport packet is extended by an Adaptation Field (see Figure 1–5 on
page 1–11) which contains not only a variety of other signaling information that
cannot be dealt with here, but also the PCR. The Adaptation Field is always
unscrambled.
The PCR value (42 bits long) corresponds to the state of a counter with a
27 MHz clock at the time of arrival of the first TS-packet byte containing the
PCR value. It is used in the decoder for controlling the 27 MHz system clock
PLL. Thus, the synchronization of the multiplexer at the transmitter end and of
the demultiplexer at the receiver end is ensured.
Descrambling
Program Synchronization
Summary of Contents for MTD200
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