<s>
Flicker-free	B-General_Concept
is	O
a	O
term	O
given	O
to	O
video	O
displays	O
,	O
primarily	O
cathode	B-Device
ray	I-Device
tubes	I-Device
,	O
operating	O
at	O
a	O
high	O
refresh	O
rate	O
to	O
reduce	O
or	O
eliminate	O
the	O
perception	O
of	O
screen	B-General_Concept
flicker	I-General_Concept
.	O
</s>
<s>
For	O
televisions	O
,	O
this	O
involves	O
operating	O
at	O
a	O
100	O
Hz	O
or	O
120	O
Hz	O
hertz	O
field	O
rate	O
to	O
eliminate	O
flicker	B-General_Concept
,	O
compared	O
to	O
standard	O
televisions	O
that	O
operate	O
at	O
50	O
Hz	O
(	O
PAL	B-Language
,	O
SÉCAM	O
systems	O
)	O
or	O
60	O
Hz	O
(	O
NTSC	O
)	O
,	O
most	O
simply	O
done	O
by	O
displaying	O
each	O
field	O
twice	O
,	O
rather	O
than	O
once	O
.	O
</s>
<s>
This	O
should	O
not	O
be	O
confused	O
with	O
motion	B-General_Concept
interpolation	I-General_Concept
,	O
though	O
they	O
may	O
be	O
combined	O
–	O
see	O
implementation	O
,	O
below	O
.	O
</s>
<s>
Televisions	O
operating	O
at	O
these	O
frequencies	O
are	O
often	O
labelled	O
as	O
being	O
100	O
or	O
120	O
Hz	O
without	O
using	O
the	O
words	O
flicker-free	B-General_Concept
in	O
the	O
description	O
.	O
</s>
<s>
The	O
term	O
is	O
primarily	O
used	O
for	O
CRTs	O
,	O
especially	O
televisions	O
in	O
50	O
Hz	O
countries	O
(	O
PAL	B-Language
or	O
SECAM	O
)	O
and	O
computer	O
monitors	O
from	O
the	O
1990s	O
and	O
early	O
2000s	O
–	O
the	O
50	O
Hz	O
rate	O
of	O
PAL/SECAM	O
video	O
(	O
compared	O
with	O
60	O
Hz	O
NTSC	O
video	O
)	O
and	O
the	O
relatively	O
large	O
computer	O
monitors	O
close	O
to	O
the	O
viewer	O
's	O
peripheral	O
vision	O
make	O
flicker	B-General_Concept
most	O
noticeable	O
on	O
these	O
devices	O
.	O
</s>
<s>
Contrary	O
to	O
popular	O
belief	O
,	O
modern	O
LCD	O
monitors	O
are	O
not	O
flicker	B-General_Concept
free	I-General_Concept
,	O
since	O
most	O
of	O
them	O
use	O
pulse-width	B-Algorithm
modulation	I-Algorithm
(	O
PWM	O
)	O
for	O
brightness	O
control	O
.	O
</s>
<s>
As	O
the	O
brightness	O
setting	O
is	O
lowered	O
,	O
the	O
flicker	B-General_Concept
becomes	O
more	O
noticeable	O
,	O
since	O
the	O
period	O
when	O
the	O
backlight	O
is	O
active	O
in	O
each	O
PWM	O
duty	O
cycle	O
shortens	O
.	O
</s>
<s>
The	O
goal	O
is	O
to	O
display	O
images	O
sufficiently	O
frequently	O
to	O
exceed	O
the	O
human	O
flicker	B-General_Concept
fusion	O
threshold	O
,	O
and	O
hence	O
create	O
the	O
impression	O
of	O
a	O
constant	O
(	O
non-flickering	O
)	O
source	O
.	O
</s>
<s>
In	O
computer	O
displays	O
this	O
consists	O
of	O
changing	O
the	O
frame	O
rate	O
of	O
the	O
produced	O
signal	O
in	O
the	O
video	B-Device
card	I-Device
(	O
and	O
in	O
sync	O
with	O
this	O
,	O
the	O
displayed	O
image	O
on	O
the	O
display	O
)	O
.	O
</s>
<s>
This	O
is	O
limited	O
by	O
the	O
clock	O
speed	O
of	O
the	O
video	B-Device
adapter	I-Device
and	O
frame	O
rate	O
required	O
of	O
the	O
program	O
–	O
for	O
a	O
given	O
pixel	O
clock	O
speed	O
,	O
higher	O
refresh	O
rates	O
require	O
lower	O
resolution	O
or	O
color	O
depth	O
,	O
and	O
higher	O
frame	O
rates	O
require	O
that	O
the	O
program	O
producing	O
the	O
video	O
recalculate	O
the	O
screen	O
more	O
frequently	O
.	O
</s>
<s>
For	O
these	O
reasons	O
,	O
refresh	O
rates	O
above	O
90	O
–	O
100	O
Hz	O
to	O
reduce	O
flicker	B-General_Concept
are	O
uncommon	O
on	O
computers	O
–	O
these	O
rates	O
are	O
sufficient	O
to	O
eliminate	O
flicker	B-General_Concept
.	O
</s>
<s>
Most	O
simply	O
,	O
the	O
frame	O
rate	O
can	O
be	O
doubled	O
by	O
simply	O
displaying	O
the	O
same	O
broadcast	O
image	O
twice	O
in	O
rapid	O
succession	O
,	O
as	O
is	O
done	O
with	O
movie	B-Application
projectors	I-Application
(	O
which	O
display	O
each	O
frame	O
of	O
24	O
fps	O
film	O
two	O
or	O
more	O
times	O
)	O
–	O
either	O
displaying	O
each	O
field	O
twice	O
or	O
alternating	O
fields	O
.	O
</s>
<s>
Alternatively	O
,	O
this	O
can	O
involve	O
motion	B-General_Concept
interpolation	I-General_Concept
,	O
where	O
rather	O
than	O
displaying	O
the	O
original	O
fields	O
twice	O
,	O
creates	O
interpolated	O
images	O
between	O
the	O
original	O
frames	O
.	O
</s>
<s>
This	O
may	O
be	O
combined	O
with	O
deinterlacing	O
,	O
converting	O
the	O
image	O
to	O
progressive	B-Device
scan	I-Device
(	O
attempting	O
to	O
create	O
a	O
full	O
picture	O
from	O
the	O
two	O
half	O
images	O
)	O
.	O
</s>
<s>
Higher	O
refresh	O
rates	O
,	O
while	O
they	O
reduce	O
flicker	B-General_Concept
,	O
may	O
cause	O
other	O
problems	O
.	O
</s>
<s>
See	O
motion	B-General_Concept
interpolation	I-General_Concept
for	O
further	O
discussion	O
.	O
</s>
