Comtech EF Data SDR-54A - Manual (Page 52)

SDR-54A Satellite Demodulator Revision 4
Reed-Solomon Option MN/SDR54A.IOM
6.6 Functionality
6.6.1 Correlator
On the Rx (decoder) side, the correlator searches for unique words from the R-S frame.
The correlator starts in hunt mode, searching for the first two bytes of the unique word.
Upon detection of the first two bytes of the unique word, the correlator searches for the
next two bytes of the unique word. When all four bytes of the unique word are found, an
up/down counter is incremented. The correlator acknowledges a locked condition when
the up/down counter reaches a maximum count of 16.
Once the counter starts counting up, a missing byte of the unique word causes the counter
to count down. The counter must count down to zero before entering the hunt mode again,
offering some compliance for noise in the system.
After a locked condition is reached, the correlator uses the known frame sync position to
synchronize the counters that generate the deinterleaver addresses, W/R selects, and the
descrambler synchronization. The correlator converts the input data stream to an 8-bit
parallel byte format for processing by the deinterleaver and R-S decoder.
6.6.2 Error Detection
The decoder provides a means to check for uncorrectable R-S blocks. The decode Xilinx
monitors the CRTN line out of the R-S decoder. CRTN is valid during the first byte of
each R-S block. CRTN is programmed such that a low indicates that the incoming R-S
block of data was correctable.
• If CRTN is high during the first byte, then the upcoming R-S block of data
exceeded the correction capability of the R-S decoder.
• If the block is uncorrectable, the data is passed through the decoder as
received, meaning no correction was performed on that block of data.
The decode Xilinx monitors the CRTN line and latches a status line on any high state.
Approximately once every second, the M&C reads the status line and reports any UAS
(unavailable seconds) as a stored fault on the modems front panel. The status line is
cleared every time the M&C reads the status.
Refer to Figure 6-5 for a block diagram of the Reed-Solomon Decoder section.
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