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Vitterbi decoding is not in the recommendation. However, at the present
moment, Vitterbi decoding is generally used as it is the most efficient
decoding scheme for trellis coding.
(1) 8-state trellis coding
Fig. 5-53 shows the trellis coder in V.17.
Q
6n
Q
5n
Q
4n
Q
3n
Q
Y
2n
2n
Q
Y
1n
1n
T
Y
2n-1
T
T
Y
1n-1
Differential coder
The trellis coder comprises a mapping circuit for placing (mapping) the
differential coder, convolutional coder and signals onto a signal spatial dia-
gram (constellation). The data that is transmitted from the terminal device
is sorted into data (symbols) every six bits by the parallelizing circuit, and
is input to the trellis coder. In the coder, first of all, specific two bits Q1n
and Q2n from among six bits Q1n to Q6n that were input are input to the
differential coder to be differentially coded. After the two bits Q1n and
Q2n are input to the differential coder, they are differentially coded as
shown in Table 5-11, and are further input to the convolutional coder as 2-
bit coded data Y1n and Y2n.
G3 FACSIMILE COMMUNICATIONS
W
W
n1
n3
T
Convolutional coder
a
b
Fig. 5-53 Trellis Coder
Q
6n
Q
5n
Q
4n
Q
3n
Y
2n
Y
1n
W
Y
n2
0n
T
a b
a
b
S
0
0
S
1
0
1
1
0
S
2
1
1
1
2
3
P
Q
4
5
S
6
1
2
0
0
1
0
1
1
0
1
ït
çi
5–63
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