Add-Compare-Select (Acs); Window Error Detection (Wed); Figure 13-4 Window Error Detection Function - Motorola DSP56305 User Manual

VITERBI CO-PROCESSOR
Block Description
13.3.4

Add-Compare-Select (ACS)

The ACS block performs the Add-Compare-Select function:
• Add (add a branch metric to its associated path metric)
• Compare (compare two updated path metrics)
• Select (select the optimal path of the two states and save the survivor path metric
for every state)
The ACS is performed for every pair of states, according to the ACS butterfly scheme
The ACS adds to each state metric (from the path metric RAM) its associated branch
metric and compares the two sums. In the scheme employed in the VCOP, the greater
value determines the survivor path and is written into the path metric RAM. The Trellis
RAM is updated with the survivor decision bit (0 or 1). The two ACS operations
composing a full ACS butterfly are done sequentially using the same pair of state
metrics.
13.3.5

Window Error Detection (WED)

The Window Error Detection (WED) block performs the WED computation on a
window of the decoded block of bits (as specified in the VWES register). This block runs
in lockstep with the ACS block. For every ACS operation in the window, the difference
in path metrics is passed to WED. The block updates WED RAM with the
minimal-difference decision along every path within the WED window.
WED RAM
Update State's
Minimal Difference
13-8
Previous
State's
Minimal
Difference

Figure 13-4 Window Error Detection Function

DSP56305 User's Manual
CMP
Decision
Difference
MUX
2
.
ACS
AA1314
MOTOROLA