Comtech EF Data SDM-9000, Installation And Operation Manual

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Options SDM-9000 Satellite Modem
A–20 Rev. 4
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 8 PACKETS
204 BYTES
PACKET
16 PAD BYTES
PACKET
1 INVERTED SYNC BYTE 1503 BYTES
= ∗ − − =




Figure A-16 illustrates the DBS scrambler sequence.
SYNC 1
BYTE
PAYLOAD (187 BYTES)
204 BYTES
(1,632 BITS)
ZERO PADDING
16 BYTES
ZERO PADDING
16 BYTES
SYNC 2
BYTE
DBS PACKET STREAM
SCRAMBLER ENABLE
PRBS LOAD
PRBS CLOCK ENABLE
SYNC 1
BYTE
SCRAMBLED PAYLOAD (187 BYTES)
ZERO PADDING
16 BYTES
ZERO PADDING
16 BYTES
SYNC 2
BYTE
SCRAMBLED
DBS PACKET STREAM
Figure A-16. DBS Scrambler Sequence
Note: The scrambler and the descrambler work in the same way, except that scrambled
data is input and descrambled data is output .