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By inverting on each occurrence of 1, AMI
accomplishes 2 things:
-The DC
component is zero
-Long sequence
of 1's stay synchronized
No mechanism of ensuring synch is there
for long stream of 0's
o Two
variations are developed
to solve the
problem of synchronization of
sequential
0's
-B8ZS Æ used in North
America
-HDB3 Æ used in
Europe & Japan
o Both modify original pattern of AMI only
on case of long stream of zeroes
ƒ B8ZS
Convention
adopted in North America to provide synch for long string of
zeros
Difference b/w
AMI and B8ZS
occurs only when
8 or more
consecutive
zeros
are encountered
Forces
artificial signal changes called VIOLATIONS
Each
time eight 0's occur , B8ZS introduces changes in pattern based on
polarity
of previous 1 (the '1' occurring just before zeros)
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ƒ HDB3
Alteration
of AMI adopted in Europe and Japan
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Introduces
changes into AMI, every time four consecutive zeros are encountered
instead
of waiting for eight zeros as in the case of B8ZS
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As
in B8ZS, the pattern of violations is based on the polarity of the previous 1
bit
Unlike
B8ZS, HDB3 also looks at the no. of 1's that have occurred since the last
substitution
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