<s>
In	O
telecommunication	O
,	O
bipolar	B-Protocol
encoding	I-Protocol
is	O
a	O
type	O
of	O
return-to-zero	B-Protocol
(	O
RZ	O
)	O
line	B-Protocol
code	I-Protocol
,	O
where	O
two	O
nonzero	O
values	O
are	O
used	O
,	O
so	O
that	O
the	O
three	O
values	O
are	O
+	O
,	O
−	O
,	O
and	O
zero	O
.	O
</s>
<s>
Such	O
a	O
signal	O
is	O
called	O
a	O
duobinary	B-Protocol
signal	I-Protocol
.	O
</s>
<s>
Standard	O
bipolar	B-Protocol
encodings	I-Protocol
are	O
designed	O
to	O
be	O
DC-balanced	O
,	O
spending	O
equal	O
amounts	O
of	O
time	O
in	O
the	O
+	O
and	O
−	O
states	O
.	O
</s>
<s>
The	O
reason	O
why	O
bipolar	B-Protocol
encoding	I-Protocol
is	O
classified	O
as	O
a	O
return	B-Protocol
to	I-Protocol
zero	I-Protocol
(	O
RZ	O
)	O
is	O
that	O
when	O
a	O
bipolar	O
encoded	O
channel	O
is	O
idle	O
the	O
line	O
is	O
held	O
at	O
a	O
constant	O
"	O
zero	O
"	O
level	O
,	O
and	O
when	O
it	O
is	O
transmitting	O
bits	O
the	O
line	O
is	O
either	O
in	O
a	O
+V	O
or	O
-V	O
state	O
corresponding	O
to	O
the	O
binary	O
bit	O
being	O
transmitted	O
.	O
</s>
<s>
One	O
kind	O
of	O
bipolar	B-Protocol
encoding	I-Protocol
is	O
a	O
paired	B-Protocol
disparity	I-Protocol
code	I-Protocol
,	O
of	O
which	O
the	O
simplest	O
example	O
is	O
alternate	O
mark	O
inversion	O
.	O
</s>
<s>
In	O
this	O
code	O
,	O
a	O
binary	O
0	O
is	O
encoded	O
as	O
zero	O
volts	O
,	O
as	O
in	O
unipolar	B-Protocol
encoding	I-Protocol
,	O
whereas	O
a	O
binary	O
1	O
is	O
encoded	O
alternately	O
as	O
a	O
positive	O
voltage	O
or	O
a	O
negative	O
voltage	O
.	O
</s>
<s>
The	O
use	O
of	O
a	O
bipolar	B-Protocol
code	I-Protocol
prevents	O
a	O
significant	O
build-up	O
of	O
DC	O
,	O
as	O
the	O
positive	O
and	O
negative	O
pulses	O
average	O
to	O
zero	O
volts	O
.	O
</s>
<s>
Bipolar	B-Protocol
encoding	I-Protocol
is	O
preferable	O
to	O
non-return-to-zero	B-Protocol
whenever	O
signal	O
transitions	O
are	O
required	O
to	O
maintain	O
synchronization	O
between	O
the	O
transmitter	O
and	O
receiver	O
.	O
</s>
<s>
A	O
bipolar	B-Protocol
encoding	I-Protocol
is	O
an	O
often	O
good	O
compromise	O
:	O
runs	O
of	O
ones	O
will	O
not	O
cause	O
a	O
lack	O
of	O
transitions	O
.	O
</s>
<s>
Where	O
frequent	O
transitions	O
are	O
a	O
requirement	O
,	O
a	O
self-clocking	O
encoding	O
such	O
as	O
return-to-zero	B-Protocol
or	O
some	O
other	O
more	O
complicated	O
line	B-Protocol
code	I-Protocol
may	O
be	O
more	O
appropriate	O
,	O
though	O
they	O
introduce	O
significant	O
overhead	O
.	O
</s>
<s>
The	O
coding	O
was	O
used	O
extensively	O
in	O
first-generation	O
PCM	B-Algorithm
networks	O
,	O
and	O
is	O
still	O
commonly	O
seen	O
on	O
older	O
multiplexing	B-Architecture
equipment	O
today	O
,	O
but	O
successful	O
transmission	O
relies	O
on	O
no	O
long	O
runs	O
of	O
zeroes	O
being	O
present	O
.	O
</s>
<s>
There	O
are	O
two	O
popular	O
ways	O
to	O
ensure	O
that	O
no	O
more	O
than	O
15	O
consecutive	O
zeros	O
are	O
ever	O
sent	O
:	O
robbed-bit	O
signaling	O
and	O
bit	B-Protocol
stuffing	I-Protocol
.	O
</s>
<s>
Another	O
benefit	O
of	O
bipolar	B-Protocol
encoding	I-Protocol
compared	O
to	O
unipolar	O
is	O
error	B-Error_Name
detection	I-Error_Name
.	O
</s>
<s>
Each	O
such	O
bipolar	B-Protocol
violation	I-Protocol
(	O
BPV	O
)	O
is	O
an	O
indication	O
of	O
a	O
transmission	O
error	O
.	O
</s>
<s>
For	O
data	O
channels	O
,	O
in	O
order	O
to	O
avoid	O
the	O
need	O
of	O
always	O
setting	O
bit	O
8	O
to	O
1	O
,	O
as	O
described	O
above	O
,	O
other	O
T1	O
encoding	B-Protocol
schemes	I-Protocol
(	O
Modified	B-Protocol
AMI	I-Protocol
codes	I-Protocol
)	O
ensure	O
regular	O
transitions	O
regardless	O
of	O
the	O
data	O
being	O
carried	O
.	O
</s>
<s>
B8ZS	B-Protocol
is	O
a	O
newer	O
format	O
for	O
North	O
America	O
,	O
where	O
HDB3	B-Protocol
is	O
the	O
original	O
line	B-Protocol
coding	I-Protocol
type	O
used	O
in	O
Europe	O
and	O
Japan	O
.	O
</s>
<s>
A	O
very	O
similar	O
encoding	B-Protocol
scheme	I-Protocol
,	O
with	O
the	O
logical	O
positions	O
reversed	O
,	O
is	O
also	O
used	O
and	O
is	O
often	O
referred	O
to	O
as	O
pseudoternary	B-Protocol
encoding	O
.	O
</s>
<s>
B-MAC	O
,	O
and	O
essentially	O
all	O
family	O
members	O
of	O
the	O
Multiplexed	B-Architecture
Analogue	O
Components	O
Television	O
Transmission	O
family	O
used	O
Duobinary	B-Protocol
to	O
encode	O
the	O
digital	O
audio	O
,	O
teletext	O
,	O
closed	O
captioning	O
and	O
selective	O
access	O
for	O
distribution	O
.	O
</s>
<s>
Because	O
of	O
the	O
way	O
Duobinary	B-Protocol
was	O
coupled	O
to	O
the	O
NICAM	O
like	O
digital	O
audio	O
subsystems	O
for	O
the	O
MAC	O
family	O
,	O
up	O
to	O
50%	O
of	O
data	O
reduction	O
was	O
possible	O
in	O
both	O
Stereo	O
and	O
Mono	O
transmission	O
modes	O
.	O
</s>
<s>
At	O
least	O
with	O
some	O
data	O
transmission	O
systems	O
,	O
duobinary	B-Protocol
can	O
perform	O
lossless	O
data	O
reduction	O
though	O
this	O
has	O
seldom	O
been	O
utilized	O
in	O
practice	O
.	O
</s>
