HDSP-3400 Agilent(Hewlett-Packard), HDSP-3400 Datasheet - Page 24

• Manufacturer Part Number: HDSP-3400
• Description: 20 mm (0.8 inch) Seven Segment Displays
• Manufacturer: Agilent(Hewlett-Packard)

3.4 Viterbi Decoder
The Viterbi decoder accepts 3 bit soft decision samples of the in-phase (I) and quadrature (Q)
components of the received signal. Once QPSK lock has been achieved, the decoder searches for
the correct code rate, starting with rate 3/4, then proceeding to rate 2/3, 5/6, 7/8 and finally rate
1/2. Each of the possible synchronization phases at each rate is tested as well as the two
possible carrier phase ambiguity conditions. Polarity reversal is corrected in the word
synchronization logic. Viterbi lock is achieved when the trellis traceback algorithm converges, on
the average, within a prescribed number of symbols.
Although the algorithm automatically tests for carrier phase ambiguity, there is no provision to
automatically correct for phase reversal. Phase reversal can occur if the receiver chain, consisting
of an LNB and the tuner, provides an odd number of high side frequency translation operations. A
system may be required to operate with different LNBs, some of which provide phase reversal.
This condition may be corrected by the host processor, which can set a bit in the down converter
to correct for phase reversal.
The Viterbi decoder employs the radix two algorithm. The output buffer reserializes the data which
is made available, along with the Viterbi data clock as external signals. These signals permit
verification of the DVB specification which is referenced to the Viterbi decoder output.
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Q
I
3
3
Depuncturing
Change Carrier Phase
Logic
ACS Array
Change Puncture Phase
G1
G2
F
IGURE
Branch Metric
Calculator
18: V
ITERBI
D
ECODER
128
Decoder
Quality
Estimate
Traceback Memory
Trace-back
Controller
RAM
First-Out
Last-In
Buffer
Viterbi
Clock
Data
Out
Lock
Out