Comtech EF Data SNM-1001L - Manual (Page 229)

SNM-1001L Satellite Modem Revision 1
Decoder MN/SNM1001L.IOM
14–3
The sequential decoder also provides a lock detect signal to the M&C when the error rate has
dropped below a threshold level. The M&C monitors these signals and takes appropriate action.
The raw BER count is made by comparing the input and output decoder data. Because the
output data contains many fewer errors than the input, differences in the two can be counted to
yield the raw BER. The raw BER is sent to the M&C for further processing.
14.1.2 Viterbi Decoder
The Viterbi decoder operates in conjunction with the convolutional encoder at the transmit
modem. The decoder uses a decoding algorithm to provide FEC on the received data stream for
errors occurring in the transmission channel. A block diagram of the Viterbi decoder is shown
in Figure 14-2.
TO
MICROCOMPUTER
INTERFACE
CONTROL I/O
DEPUNCTURE
PROCESSOR
VITERBI DECODER
WITH
V.35 DESCRAMBLER
AND CHANNEL
BER DETECTION
LOCK DETECT
FROM
COSTAS
PROC. RX DATA
RX CLOCK
AMBIGUITY
RESOLVER
FROM
INPUT BUFFER
FROM
CLK RECOVERY
Figure 14-2. Viterbi Decoder Block Diagram