<s>
Standard-definition	B-Device
television	I-Device
(	O
SDTV	B-Device
,	O
SD	O
,	O
often	O
shortened	O
to	O
standard	B-Device
definition	I-Device
)	O
is	O
a	O
television	O
system	O
which	O
uses	O
a	O
resolution	O
that	O
is	O
not	O
considered	O
to	O
be	O
either	O
high	B-Device
or	O
enhanced	O
definition	O
.	O
</s>
<s>
The	O
two	O
common	O
SDTV	B-Device
signal	O
types	O
are	O
576i	O
,	O
with	O
576	O
interlaced	O
lines	O
of	O
resolution	O
,	O
derived	O
from	O
the	O
European-developed	O
PAL	B-Language
and	O
SECAM	O
systems	O
,	O
and	O
480i	O
based	O
on	O
the	O
American	O
NTSC	O
system	O
.	O
</s>
<s>
Common	O
SDTV	B-Device
refresh	O
rates	O
are	O
25	O
,	O
29.97	O
and	O
30	O
frames	O
per	O
second	O
.	O
</s>
<s>
a	O
4:3	O
aspect	B-Device
ratio	I-Device
.	O
</s>
<s>
Standards	O
that	O
support	O
digital	O
SDTV	B-Device
broadcast	O
include	O
DVB	B-Application
,	O
ATSC	B-Device
,	O
and	O
ISDB	B-Protocol
.	O
</s>
<s>
The	O
last	O
two	O
were	O
originally	O
developed	O
for	O
HDTV	B-Device
,	O
but	O
are	O
also	O
used	O
for	O
their	O
ability	O
to	O
deliver	O
multiple	O
SD	B-Device
video	I-Device
and	O
audio	O
streams	O
via	O
multiplexing	B-Architecture
.	O
</s>
<s>
In	O
North	O
America	O
,	O
digital	O
SDTV	B-Device
is	O
broadcast	O
in	O
the	O
same	O
4:3	O
aspect	B-Device
ratio	I-Device
as	O
NTSC	O
signals	O
,	O
with	O
widescreen	O
content	O
often	O
being	O
center	O
cut	O
.	O
</s>
<s>
However	O
,	O
the	O
aspect	B-Device
ratio	I-Device
of	O
widescreen	O
content	O
may	O
be	O
preserved	O
in	O
a	O
4:3	O
frame	O
through	O
letterboxing	O
.	O
</s>
<s>
In	O
other	O
parts	O
of	O
the	O
world	O
that	O
used	O
the	O
PAL	B-Language
or	O
SECAM	O
color	O
systems	O
,	O
digital	O
standard-definition	B-Device
television	I-Device
is	O
now	O
usually	O
shown	O
with	O
a	O
16:9	O
aspect	B-Device
ratio	I-Device
,	O
with	O
the	O
transition	O
occurring	O
between	O
the	O
mid-1990s	O
and	O
late-2000s	O
depending	O
on	O
region	O
.	O
</s>
<s>
Older	O
programs	O
with	O
a	O
4:3	O
aspect	B-Device
ratio	I-Device
are	O
broadcast	O
with	O
a	O
flag	O
that	O
switches	O
the	O
display	O
to	O
4:3	O
.	O
</s>
<s>
Digital	O
SDTV	B-Device
eliminates	O
the	O
ghosting	O
and	O
noisy	O
images	O
associated	O
with	O
analog	O
systems	O
.	O
</s>
<s>
However	O
,	O
if	O
the	O
reception	O
has	O
interference	O
or	O
is	O
poor	O
,	O
where	O
the	O
error	O
correction	O
cannot	O
compensate	O
one	O
will	O
encounter	O
various	O
other	O
artifacts	O
such	O
as	O
image	O
freezing	O
,	O
stuttering	O
or	O
dropouts	O
from	O
missing	O
intra-frames	B-General_Concept
or	O
blockiness	O
from	O
missing	O
macroblocks	B-General_Concept
.	O
</s>
<s>
The	O
table	O
below	O
summarizes	O
pixel	B-Device
aspect	I-Device
ratios	I-Device
for	O
the	O
scaling	O
of	O
various	O
kinds	O
of	O
SDTV	B-Device
video	O
lines	O
.	O
</s>
<s>
The	O
pixel	B-Device
aspect	I-Device
ratio	I-Device
is	O
the	O
same	O
for	O
720	O
-	O
and	O
704-pixel	O
resolutions	O
because	O
the	O
visible	O
image	O
(	O
be	O
it	O
4:3	O
or	O
16:9	O
)	O
is	O
contained	O
in	O
the	O
center	O
704	O
horizontal	O
pixels	O
of	O
the	O
digital	O
frame	O
.	O
</s>
<s>
For	O
SMPTE	O
259M-C	O
compliance	O
,	O
an	O
SDTV	B-Device
broadcast	O
image	O
is	O
scaled	O
to	O
720	O
pixels	O
wide	O
for	O
every	O
480	O
NTSC	O
(	O
or	O
576	O
PAL	B-Language
)	O
lines	O
of	O
the	O
image	O
with	O
the	O
amount	O
of	O
non-proportional	O
line	O
scaling	O
dependent	O
on	O
either	O
the	O
display	O
or	O
pixel	B-Device
aspect	I-Device
ratio	I-Device
.	O
</s>
<s>
An	O
SDTV	B-Device
image	O
outside	O
the	O
constraints	O
of	O
the	O
SMPTE	O
standards	O
requires	O
no	O
non-proportional	O
scaling	O
with	O
640	O
pixels	O
for	O
every	O
line	O
of	O
the	O
image	O
.	O
</s>
<s>
The	O
display	O
and	O
pixel	B-Device
aspect	I-Device
ratio	I-Device
is	O
generally	O
not	O
required	O
with	O
the	O
line	O
height	O
defining	O
the	O
aspect	O
.	O
</s>
