<s>
SCH	O
(	O
Subcarrier	O
to	O
Horizontal	O
)	O
phase	O
refers	O
to	O
the	O
phase	O
relationship	O
between	O
the	O
leading	O
edge	O
of	O
horizontal	B-General_Concept
sync	I-General_Concept
(	O
at	O
the	O
50%	O
amplitude	O
point	O
)	O
and	O
the	O
zero	O
crossings	O
of	O
the	O
color	O
burst	O
(	O
by	O
extrapolating	O
the	O
color	O
burst	O
to	O
the	O
leading	O
edge	O
of	O
sync	O
)	O
.	O
</s>
<s>
For	O
PAL	B-Language
,	O
the	O
definition	O
of	O
SCH	O
phase	O
is	O
slightly	O
different	O
due	O
to	O
the	O
more	O
complicated	O
relationship	O
between	O
the	O
sync	O
and	O
subcarrier	O
frequencies	O
—	O
the	O
SCH	O
phase	O
relationship	O
for	O
a	O
given	O
line	O
repeats	O
only	O
once	O
every	O
eight	O
fields	O
.	O
</s>
<s>
SCH	O
phase	O
is	O
important	O
when	O
merging	O
two	O
or	O
more	O
video	B-Application
signals	I-Application
.	O
</s>
<s>
To	O
avoid	O
color	O
shifts	O
or	O
“	O
picture	O
jumps	O
,	O
”	O
the	O
video	B-Application
signals	I-Application
must	O
have	O
the	O
same	O
horizontal	O
,	O
vertical	O
,	O
and	O
subcarrier	O
timing	O
and	O
the	O
phases	O
must	O
be	O
closely	O
matched	O
.	O
</s>
<s>
To	O
achieve	O
these	O
timing	O
constraints	O
,	O
the	O
video	B-Application
signals	I-Application
must	O
have	O
the	O
same	O
SCH	O
phase	O
relationship	O
since	O
the	O
horizontal	B-General_Concept
sync	I-General_Concept
and	O
subcarrier	O
are	O
continuous	O
signals	O
with	O
a	O
defined	O
relationship	O
.	O
</s>
<s>
It	O
is	O
common	O
for	O
an	O
encoder	O
to	O
allow	O
adjustment	O
of	O
the	O
SCH	O
phase	O
to	O
simplify	O
merging	O
two	O
or	O
more	O
video	B-Application
signals	I-Application
.	O
</s>
<s>
Maintaining	O
proper	O
SCH	O
phase	O
is	O
also	O
important	O
since	O
NTSC	O
and	O
PAL	B-Language
decoders	O
may	O
monitor	O
the	O
SCH	O
phase	O
to	O
determine	O
which	O
color	O
field	O
is	O
being	O
decoded	O
.	O
</s>
