Dbs Mode (De)Scrambler For 204-Byte Packets; Figure A-16. Dbs Scrambler Sequence - Comtech EF Data SDM-9000 Installation And Operation Manual

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A.2.6.1 DBS Mode (De)Scrambler for 204-Byte Packets
The DVB specifications require that the inverted sync byte detection be used to load the
PRBS generator every eight sync bytes, and that the PRBS generator run continuously
until the next load (eight packet period).
The first bit at the output of the PRBS generator is applied to the first bit of the first byte
following the inverted MPEG-2 sync byte (i.e., 0xB8). To aid other synchronization
functions during the MPEG-2 sync bytes of the subsequent seven transport packets, the
PRBS generation continues, but its output is gated off, leaving these bytes unscrambled.
In DBS mode, however, there is an additional 16 bytes of padding at the end of each
transport packet. The padding is reserved for Reed-Solomon (RS) overhead. (To be
compatible with the DVB specification, the PRBS generator must be halted and the
scrambler disabled during these 16 padding bytes.) The period of the PRBS generator for
DBS mode is:
PERIOD
Figure A-16 illustrates the DBS scrambler sequence.
DBS PACKET STREAM
PRBS LOAD
SCRAMBLER ENABLE
PRBS CLOCK ENABLE
SCRAMBLED
DBS PACKET STREAM
Note: The scrambler and the descrambler work in the same way, except that scrambled
data is input and descrambled data is output.
A–20

204 BYTES

= ∗
8 PACKETS

PACKET
SYNC 1
ZERO PADDING
BYTE
16 BYTES
SYNC 1
ZERO PADDING
16 BYTES BYTE

Figure A-16. DBS Scrambler Sequence


16 PAD BYTES

− −
1 INVERTED SYNC BYTE

PACKET
204 BYTES
(1,632 BITS)
PAYLOAD (187 BYTES)
SCRAMBLED PAYLOAD (187 BYTES)
SDM-9000 Satellite Modem
=
1503 BYTES
SYNC 2
ZERO PADDING
BYTE
16 BYTES
SYNC 2
ZERO PADDING
BYTE
16 BYTES
Rev. 4