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SLM-3650 Satellite Modem
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A.3.2
Encoder
The encoder inserts the unique word into the data stream for synchronization of the
descrambler and the deinterleaver on the decode side. The unique word consists of four
hex bytes Ox5AOFBE66, Ox5A being the first bit in time. The unique word is inserted
into the last two bytes of the last two blocks of the last page of the interleave frame.
The RS rate exchange is performed by the RS Xilinx using the 25 MHz system clock.
The muxclk is converted to 40nS wide pulses at the TX terrestrial data rate. This
generated clock is used by the Xilinx circuitry for data processing at the none RS
overhead rate. The RS overhead clock is generated by inserting more 40nS pulses into the
none overhead clock pulse stream. The required insertions are determined by the RS n, k,
t values programmed into the Xilinx.
The RS overhead pulse insertions are always 80nS after a detected none RS overhead
pulse. Keeping in mind that an Overhead card may also be inserting pulses prior to the
RS card insertions, a method of monitoring the insertion rate is necessary to keep from
disturbing the MOD encoder operation. The MOD can handle only one insertion per
symbol clk/4. An up/down counter is used to keep track of proposed insertion points
(either overhead or Reed-Solomon). Everytime a proposed insertion point is detected the
up/down counter is bumped up. When the up/down counter is at a none-zero point a real
insertion takes place every 5 terrestrial data clocks. Insertions continue at a rate of every
5 terrestrial clocks until the up/down counter is zeroed, at that point no more insertions
take place until the counter is bumped again.
On the encode side, dummy Reed-Solomon check bytes are clocked into the Reed-
Solomon codec as place holders for the real check bytes. While the dummy check bytes
are inserted, the erase line on the Reed-Solomon codec is asserted. Asserting the erase
line at this time increases the correction capability of the Reed-Solomon codec on the
decode side, because the position information of the erasures is known. Correcting
erasures takes half as much correction capability as correcting errors. See Figure A-21 for
a block diagram of the R-S Encoder.
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A–73
Revision 3
MN/SLM3650.IOM
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