ETSI
F
inal draft ETSI EN 300 744 V1.5.1 (2004
-
0
6)
1
1
4.3 Channel coding and modulation
4.3.1 Transport multiplex adaptation and randomization for energy
dispersal
The System input stream shall be organized in fixed length packets (see figure 3), following the MPEG-2 transport
multiplexer. The total packet length of the MPEG-2 transport multiplex (MUX) packet is 188 bytes.
This includes 1 sync-word byte (i.e. 47
HEX
). The processing order at the transmitting side shall always start from the
MSB (i.e. "0") of the sync-word byte (i.e. 01 000 111). In order to ensure adequate binary transitions, the data of the
input MPEG-2 multiplex shall be randomized in accordance with the configurations depicted in figure 2.
Figure 2: Scrambler/descrambler schematic diagram
The polynomial for the Pseudo Random Binary Sequence (PRBS) generator shall be (see note):
1 + X
14
+ X
15
NOTE: The polynomial description given here is in the form taken from the Satellite baseline specification
EN 300 421 [2]. Elsewhere, in both the Satellite baseline specification and in the present document, a
different polynomial notation is used which conforms with the standard textbook of Peterson and Weldon
(Error correcting codes, second edition, MIT Press, 1972).
Loading of the sequence "100101010000000" into the PRBS registers, as indicated in figure 2, shall be initiated at the
start of every eight transport packets. To provide an initialization signal for the descrambler, the MPEG-2 sync byte of
the first transport packet in a group of eight packets is bit-wise inverted from 47
HEX
(SYNC) to B8
HEX
(SYNC ).
This process is referred to as "transport multiplex adaptation" (see figure 3b).
The first bit at the output of the PRBS generator shall be applied to the first bit (i.e. MSB) of the first byte following the
inverted MPEG-2 sync byte (i.e. B8
HEX
). To aid other synchronization functions, during the MPEG-2 sync bytes of the
subsequent 7 transport packets, the PRBS generation shall continue, but its output shall be disabled, leaving these bytes
unrandomized. Thus, the period of the PRBS sequence shall be 1 503 bytes.
The randomization process shall be active also when the modulator input bit-stream is non-existent, or when it is
non-compliant with the MPEG-2 transport stream format (i.e. 1 sync byte + 187 packet bytes).
4.3.2 Outer coding and outer interleaving
The outer coding and interleaving shall be performed on the input packet structure (see figure 3a).
Reed-Solomon RS (204,188, t = 8) shortened code (see note 1), derived from the original systematic RS (255,239, t = 8)
code, shall be applied to each randomized transport packet (188 byte) of figure 3b to generate an error protected packet
(see figure 3c). Reed-Solomon coding shall also be applied to the packet sync byte, either non-inverted (i.e. 47
HEX
) or
inverted (i.e. B8
HEX
).
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