DSP56300 datasheet(46/108 Pages) FREESCALE

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3-18

Viterbi Decoder Implementation

Expanding the Viterbi Algorithm

Main: Gluing the Pieces Together

Next, we begin the executable code. The first two lines initialize address registers m2

and n2 so that r2 addressing wraps around and increments by 3ÕsÑboth for branch

metric storage. The next 5 lines initialize address registers r4 and r5 to wrap around for

path metric/path storage to begin them pointing to the correct places in the storage

table. Offset register n5 is set to access path metrics spaced halfway apart in the storage

table.

The initialization finishes by pointing n0 to the path storage table start, making sure r3

is in linear mode for traceback, and pointing r1 to the input data. Note that the r1

addressing mode is not set. We assume that an outside calling routine does this, and

permit circular input buffers if desired.

These next two sections do much of the decoding work. The first loop processes

ENCBITS of input data. We process 1 pair of decoder inputs for each iteration of the

macros FindMetrics and ACS. This preprocessing loads ENCBITS-1 of data into the

paths. Thereafter, we process the decoder input pairs in groups of eight, producing 8

path bits for each state. For every 8 decoder input pairs processed, we invoke the

STOREPATHS macro to store the paths.

Example 3-6

Main Viterbi Decoding Routine: Patch Metric Update

;

;***********************PREPARATION LOOP*************************

;

This loop iterates by the number of bits used in the

;

encoder to pre load the bit decisions. Thereafter,

;

the paths are updated and stored off in bytes. We need

;

the preload bits so that the stored path metrics point

;

correctly to their previous paths for traceback

;****************************************************************

;

#ENCBITS-1,PRELP

;once for each encoder bit

;

FindMetrics

ACS

PRELP

;*****************MAIN LOOP--PROCESS BYTES OF DATA*****************

#NUMINPUTS/8-1,DATALP

;process bytes of output

#8,SYMLP

;8 bits per byte

;

FindMetrics

ACS

SYMLP

STOREPATHS#NUMSTATES

DATALP

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