DSP56300 datasheet(47/108 Pages) FREESCALE

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Expanding the Viterbi Algorithm

Memory Organization

Viterbi Decoder Implementation

3-19

By preloading ENCBITS-1 and storing the leftmost eight every time we store paths, we

ensure that the paths always have (ENCBITS-1)+8 bits at the time we store paths.The

extra ENCBITS-1 bits are, as noted above, the current state, and are kept till the next

path store time to allow us to traceback. The main loop processes all but the last

8-ENCBITS+1 sets of decoder input data.

Example 3-7

shows the final processing for the data block. At this time, we have

processed all but the last byte, i.e., 8 bits less the ENCBITS-1 processing we did in

preprocessing. This means we have 3 bits of data left to process. After processing the

last decoder input, the TRACEBACK macro is invoked to obtain the decoder output.

The decoded data is memory at y:DECOUT, in 16-bit word form.

3.8

MEMORY ORGANIZATION

Example 3-8

shows all memory organization except the input data defines used to test

the code. The first reserved spaces are dedicated to the storage for the path metrics (in X

memory) and their respective paths (in Y memory). Note that these must be collocated.

For this code, we initialized the metrics for that state 00 has a large path metric, and the

rest are 0. We do not initialize the path metrics in executable code! This allows the

decoder to operate on data over multiple invocations if desired. If the decoder is

operating on independent data blocks, each started assuming the encoder starts in the 0

state, then this memory will have to be initialized accordingly. Another alternative in

this case would be to modify the butterfly loop in the preprocessor to use the fact that

the encoder starts in the 0 state. This option is explored in Section 4.

Example 3-7

Main Viterbi Decoding Routine: Termination and Traceback

;

;********POSTPROCESSING,LAST INFO BYTE ********************************

;

; ENCBITS PREPROCESSED, SO WE HAVE 8-ENCBITS BITS LEFT.

; FOR THIS EXAMPLE, WE HAVE 3INPUTS LEFT TO PROCESS

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

;

THE LAST THREE BITS------

;

#8-ENCBITS+1,FLSH1;process last 3 bits to get last byte

;

FindMetrics

ACS

FLSH1

;*******Traceback the path data to obtain the decoder output**********

TRACEBACK

FIN

nop

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