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154
Baseband Pulse and Digital Signaling Chap. 3
Code
words
Sign
bit
11010000
Code
words
PCM code structure
Chord bits
0 1 2 3 4 5 6 7
Segment 3
Step bits
A total of
8 chords along
positive axis
11011111
4
Chord 4
Negative
(just like
positive)
0
Segment 1
16 steps
step size
Chord 1
Chord 2
Segment 2
16 steps
step size
2
Figure 3–9
Chord 3
Segment 1
16 steps
step size
4
Code
word
+ Full scale
+ Zero
– Zero
– Full scale
Continued
Sign
bit
1
1
0
0
Chord
bits
000
111
111
000
Segment B
(last segment)
16 steps
step size
128
Step
bits
0000
1111
1111
0010
(including a half-width step on each side of zero) of width ¢ are used for Segment 1, 16 steps
of width 2¢
are used for Segment 2, 16 steps of width 4¢
for Segment 3, etc. The value of ¢
is chosen so that the full scale value (last step of Segment 8) matches the peak value of the
input analog signal. This segmenting technique is now accepted worldwide for the µ 255
characteristic. As shown in Fig. 3–9d, the 8-bit PCM code word consists of a sign bit that
denotes a positive or negative input voltage, three chord bits that denote the segment number,
and four step bits that denote the particular step within the segment.
Another compression law, used mainly in Europe, is the A-law characteristic, defined
[Cattermole, 1969] by
A |w 1 (t)|
1 + ln A ,
|w 2 (t)| = d
1 + ln (A |w 1 (t)|)
1 + ln A
,
0 … |w 1 (t)| … 1 A
1
A … |w 1(t)| … 1
(3–24)