<s>
A	O
zero-crossing	B-Algorithm
is	O
a	O
point	O
where	O
the	O
sign	O
of	O
a	O
mathematical	O
function	O
changes	O
(	O
e.g.	O
</s>
<s>
from	O
positive	O
to	O
negative	O
)	O
,	O
represented	O
by	O
an	O
intercept	O
of	O
the	O
axis	O
(	O
zero	O
value	O
)	O
in	O
the	O
graph	B-Application
of	O
the	O
function	O
.	O
</s>
<s>
It	O
is	O
a	O
commonly	O
used	O
term	O
in	O
electronics	O
,	O
mathematics	O
,	O
acoustics	O
,	O
and	O
image	B-Algorithm
processing	I-Algorithm
.	O
</s>
<s>
In	O
alternating	O
current	O
,	O
the	O
zero-crossing	B-Algorithm
is	O
the	O
instantaneous	O
point	O
at	O
which	O
there	O
is	O
no	O
voltage	O
present	O
.	O
</s>
<s>
The	O
zero-crossing	B-Algorithm
is	O
important	O
for	O
systems	O
that	O
send	O
digital	B-General_Concept
data	I-General_Concept
over	O
AC	O
circuits	O
,	O
such	O
as	O
modems	B-Application
,	O
X10	O
home	O
automation	O
control	O
systems	O
,	O
and	O
Digital	B-General_Concept
Command	O
Control	O
type	O
systems	O
for	O
Lionel	O
and	O
other	O
AC	O
model	O
trains	O
.	O
</s>
<s>
Counting	O
zero-crossings	B-Algorithm
is	O
also	O
a	O
method	O
used	O
in	O
speech	B-Algorithm
processing	I-Algorithm
to	O
estimate	O
the	O
fundamental	O
frequency	O
of	O
speech	O
.	O
</s>
<s>
At	O
audio	O
frequencies	O
,	O
such	O
as	O
in	O
modern	O
consumer	O
electronics	O
like	O
digital	B-General_Concept
audio	O
players	O
,	O
these	O
effects	O
are	O
clearly	O
audible	O
,	O
resulting	O
in	O
a	O
'	O
zipping	O
 '	O
sound	O
when	O
rapidly	O
ramping	O
the	O
gain	O
or	O
a	O
soft	O
'	O
click	O
 '	O
when	O
a	O
single	O
gain	O
change	O
is	O
made	O
.	O
</s>
<s>
If	O
changes	O
are	O
made	O
only	O
at	O
zero-crossings	B-Algorithm
of	O
the	O
input	O
signal	O
,	O
then	O
no	O
matter	O
how	O
the	O
amplifier	O
gain	O
setting	O
changes	O
,	O
the	O
output	O
also	O
remains	O
at	O
zero	O
,	O
thereby	O
minimizing	O
the	O
change	O
.	O
</s>
<s>
If	O
electrical	O
power	O
is	O
to	O
be	O
switched	O
,	O
no	O
electrical	O
interference	O
is	O
generated	O
if	O
switched	O
at	O
an	O
instant	O
when	O
there	O
is	O
no	O
current	O
—	O
a	O
zero	B-Algorithm
crossing	I-Algorithm
.	O
</s>
<s>
Early	O
light	O
dimmers	O
and	O
similar	O
devices	O
generated	O
interference	O
;	O
later	O
versions	O
were	O
designed	O
to	O
switch	O
at	O
the	O
zero	B-Algorithm
crossing	I-Algorithm
.	O
</s>
<s>
In	O
the	O
field	O
of	O
Digital	B-Algorithm
Image	I-Algorithm
Processing	I-Algorithm
,	O
great	O
emphasis	O
is	O
placed	O
on	O
operators	O
that	O
seek	O
out	O
edges	O
within	O
an	O
image	O
.	O
</s>
<s>
They	O
are	O
called	O
'	O
Edge	B-Algorithm
Detection	I-Algorithm
 '	O
or	O
'	O
Gradient	O
filters	O
 '	O
.	O
</s>
<s>
A	O
gradient	B-Algorithm
filter	I-Algorithm
is	O
a	O
filter	O
that	O
seeks	O
out	O
areas	O
of	O
rapid	O
change	O
in	O
pixel	O
value	O
.	O
</s>
<s>
It	O
seeks	O
out	O
points	O
in	O
the	O
signal	O
stream	O
where	O
the	O
digital	B-General_Concept
signal	O
of	O
an	O
image	O
passes	O
through	O
a	O
pre-set	O
'	O
0	O
 '	O
value	O
,	O
and	O
marks	O
this	O
out	O
as	O
a	O
potential	O
edge	O
point	O
.	O
</s>
<s>
Because	O
the	O
signal	O
has	O
crossed	O
through	O
the	O
point	O
of	O
zero	O
,	O
it	O
is	O
called	O
a	O
zero-crossing	B-Algorithm
.	O
</s>
