4 -High-Density
Bipolar (HDB) Signaling
In this
scheme, the problem of the bipolar signal being nontransparent is eliminated by
adding pulses when the number of consecutive 0's exceeds n. Such a
modified coding is called high-density bipolar (HDB) coding and denoted
by HDBN, where N can take on any value 1, 2, 3,... The most important of the
HDB codes is the HDB3 format, which has been adopted as an international
standard.
The basic
idea of the HDBN code is that when a run of N+1 zeros occurs, this group
of zeros is replaced by one of the special N+1 binary digit sequences.
The sequences are chosen to include some binary 1's in order to increase
the timing content of the signal. The 1's included violate the bipolar rule for
easy identification of the substituted sequence. In HDB3 coding, for example,
the special sequences used are 000V and BOOV, where B = l conforms
to the bipolar rule and V=l violates the bipolar rule. The choice of
sequence 000V or BOOV is made in such a way that consecutive V
pulses alternate signs in order to avoid dc wander and to maintain the dc null
in the PSD. This requires that the sequence BOOV be used when there is
an even number of 1's following the last special sequence and the sequence 000V
be used when there is an odd number of 1's following the last sequence. Note
that in the sequence BOOV, B and V are both encoded by the
same pulse. The decoder has to check two
things, the
bipolar violations and the number of 0's preceding each violation to determine
if the previous 1 is also a substitution. HDB signaling retains error detecting
capability.
Four Zero
bits in a row are coded in the following way
|
|
Number of pulses since last
replacement |
|
|
last pulse |
Odd |
Even |
|
Negative |
000- |
+00+ |
|
Positive |
000+ |
-00- |
Table - High-Density Bipolar (HDB) Signaling
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