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G3 FACSIMILE COMMUNICATIONS
Here, the survivor paths in any step i exist individually in each of the
respective states, and the distances (pathmetric) with receive data strings
relating to each survivor path are assumed to Mai, Mbi, Mci and Mdi. At
this time, the following operation is repeated in Vitterbi decoding.
(1) In each of states A, B, C and D of step i+1, the distance up to the
(2) In each of the states of step i+1, the sum of branchmetric of the two
(3) The survival path and pathmetric in each of the states in step i+1
(4) The same operation is repeated in step i+2.
Fig. 5-52 shows the flow of the above operation. In the figure, operations
in states A and B in step i+1 are shown. This update operation is called
ACS (Add-Compare-Select) operation, and the distance is successively
compared to select the survival path
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receive data of the two branches arrived at from step i is calculated.
(branchmetric)
branches arrived at from step i and the distance (that is, pathmetric) up
to step i corresponding to this is calculated and compared. Whichever
of the paths smaller than the sum of the distance of these two is taken
to the survival path in step i+1. The sum of the distance in the survival
path is taken to the new pathmetric in step i+1
obtained by the above calculation result are stored.
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