HC-5560_03 Datasheet, PDF (5/10 Pages) Intersil Corporation – PCM Transcoder

HC-5560
A method for insuring minimum logic 1 density by substituting
bipolar code in place of strings of 0’s is called BNZS or Bipolar
with N Zero Substitution. B6ZS is used commonly in North
American T2 (6.3212MHz) carriers. For every string of 6
zeros, bipolar code is substituted according to the following
rule:
• If the immediate preceding pulse is of (-) polarity, then
code each group of 6 zeros as 0+- 0+-, and if the
immediate preceding pulse is of (+) polarity, code each
group of 6 zeros as 0+- 0-+.
One can see the consecutive logic 1 pulses of the same
polarity violate the AMI coding scheme.
e.g.,
6
PCM CODE
0 0 0 1 0 1 110 0 0 0 0 0 1
0 ^- +0 + -
B6ZS (-)
V
V
0+-0 - +
B6ZS (+)
V = VIOLATION
V
V
B8ZS is used commonly in North American T1 (1.544MHz)
and T1C (3.152MHz) carriers. For every string of 8 zeros,
bipolar code is substituted according to the following rules:
1. If the immediate preceding pulse is of (-) polarity, then
code each group of 8 zeros as 000-+ 0+-.
2. If the immediate preceding pulse is of (+) polarity then
code each group of 8 zeros as 000+-0-+.
e.g.,
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PCM CODE
1 0 10 0 0 0 0 0 0 0 1 1 0
B8ZS (-)
00 0 - +0+ -
V
V
B8ZS (+)
0 0 0 + -0 - +
V
V = VIOLATION
The BNZS coding schemes, in addition to eliminating DC
wander, minimize timing jitter and allow a line error
monitoring capability.
Another coding scheme is HDB3, high density bipolar 3, used
primarily in Europe for 2.048MHz and 8.448MHz carriers. This
code is similar to BNZS in that it substitutes bipolar code for 4
consecutive zeros according to the following rule:
1. If the polarity of the immediate preceding pulse is (-) and
there have been an odd (even) number of logic 1 pulses
since the last substitution, each group of 4 consecutive
zeros is coded as 000-(+00+).
2. If the polarity of the immediate preceding pulse is (+) then
the substitution is 000+(-00-) for odd (even) number of
logic 1 pulses since the last substitution.
e.g.,
4
4
PCM CODE
0 0 0 0 1 01 11 0 0 0 0 0 0 1
HDB3 (-)
000 -
V
+ 0 0+
V
HDB3 (+)
000 +
- 0 0-
V
V
V = VIOLATION
The 3 in HDB3 refers to the coding format that precludes
strings of zeros greater than 3. Note that violations are
produced only in the fourth bit location of the substitution
code and that successive substitutions produce alternate
polarity violations.
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