Comtech EF Data SDM-9000 - Manual (Page 204)

Options SDM-9000 Satellite Modem
A–18 Rev. 4
A.2.5 SYNC Decoder (Correlater)
In DBS mode, an external sync pulse provides the reference for the frame structure used
throughout the transmission process. This external sync pulse must be provided
coincident with the first bit of the MPEG-2 sync byte.
A.2.5.1 DBS Mode SYNC Decoder for 204-Byte Packets
A.2.5.1.1 TX Functions of the DBS Mode SYNC Decoder
During the transmission process, the DBS mode sync-detect logic ignores the sync byte
data in the data packet. Instead, the external sync pulse is used. Upon receiving the
external sync pulse, a new sync byte is written over the existing sync byte in the data
packet. In addition, every eighth sync byte is inverted.
Use of the external sync pulse means the data packet is already correlated, because its
framing structure is in a known relationship to the scrambler, the Reed-Solomon Codec,
and the interleaver.
If the external sync pulse is not supplied, the condition is detected. In this case, a new
framing structure is written over the 204-byte packets in no particular relationship to the
original packets. This preservation of the framing structure allows the modem to operate
correctly; however, the original MPEG-2 packets will be corrupted.
If the clock input is not supplied, the internal clock is used, and a new framing structure
is supplied. The internal Alarm Indication Signal (AIS) generator fills the 187-byte
payload with all 1s, and the last 16 bytes of the packet are filled with 0s.
A.2.5.1.2 RX Functions of the DBS Mode SYNC Decoder
During the reception process, the sync decoder always operates as a correlater because
the frame structure must be determined entirely from the MPEG-2 sync bytes (no
external sync pulse is available). Once the frame structure is detected, the de-interleaver,
Reed-Solomon decoder, and descrambler are synchronized to the 204-byte packets.
The sync decoder also recovers the
π
phase ambiguity of the QPSK demodulator that the
Viterbi decoder cannot detect.