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SDM-300A Satellite Modem
The corrected data is buffered through the output RAM and re-timing circuit, which
provides a data stream to the differential decoder and descrambler at the constant rate of
the data clock. The data and the clock are then output from the card.
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.
A.1.4.8
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 A-9.
TO
M IC RO C O M PUTER
IN TE R FAC E
FRO M
IN PUT B UFFE R
FRO M
CLK RE CO VE RY
Rev. 4
CO NTR O L I/O
DE PU NCTUR E
PR O CE SS O R
AM BIG U ITY
RE SO LV ER
Figure A-9. Viterbi Decoder Block Diagram
VITE RB I D EC O D ER
W ITH
V.35 DE SCR AM BLE R
CO STAS
AN D C HAN NE L
BE R DETEC TIO N
Options
LO CK DE TE CT
FRO M
PR O C.
RX DATA
RX CLO C K
A–43
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