<s>
H.261	B-Algorithm
is	O
an	O
ITU-T	O
video	B-Algorithm
compression	I-Algorithm
standard	I-Algorithm
,	O
first	O
ratified	O
in	O
November	O
1988	O
.	O
</s>
<s>
It	O
is	O
the	O
first	O
member	O
of	O
the	O
H.26x	B-Application
family	O
of	O
video	B-Algorithm
coding	I-Algorithm
standards	I-Algorithm
in	O
the	O
domain	O
of	O
the	O
ITU-T	O
Study	O
Group	O
16	O
Video	B-Application
Coding	I-Application
Experts	I-Application
Group	I-Application
(	O
VCEG	B-Application
,	O
then	O
Specialists	O
Group	O
on	O
Coding	O
for	O
Visual	O
Telephony	O
)	O
.	O
</s>
<s>
It	O
was	O
the	O
first	O
video	B-Algorithm
coding	I-Algorithm
standard	I-Algorithm
that	O
was	O
useful	O
in	O
practical	O
terms	O
.	O
</s>
<s>
H.261	B-Algorithm
was	O
originally	O
designed	O
for	O
transmission	O
over	O
ISDN	B-Protocol
lines	I-Protocol
on	O
which	O
data	O
rates	O
are	O
multiples	O
of	O
64	O
kbit/s	O
.	O
</s>
<s>
The	O
standard	O
supports	O
two	O
video	O
frame	O
sizes	O
:	O
CIF	B-Application
(	O
352×288	O
luma	O
with	O
176×144	O
chroma	O
)	O
and	O
QCIF	B-Application
(	O
176×144	O
with	O
88×72	O
chroma	O
)	O
using	O
a	O
4:2:0	O
sampling	O
scheme	O
.	O
</s>
<s>
The	O
first	O
digital	O
video	B-Algorithm
coding	I-Algorithm
standard	I-Algorithm
was	O
H.120	B-Algorithm
,	O
created	O
by	O
the	O
CCITT	O
(	O
now	O
ITU-T	O
)	O
in	O
1984	O
.	O
</s>
<s>
H.120	B-Algorithm
was	O
not	O
usable	O
in	O
practice	O
,	O
as	O
its	O
performance	O
was	O
too	O
poor	O
.	O
</s>
<s>
H.120	B-Algorithm
was	O
based	O
on	O
differential	B-General_Concept
pulse-code	I-General_Concept
modulation	I-General_Concept
(	O
DPCM	B-General_Concept
)	O
,	O
which	O
had	O
inefficient	O
compression	O
.	O
</s>
<s>
During	O
the	O
late	O
1980s	O
,	O
a	O
number	O
of	O
companies	O
began	O
experimenting	O
with	O
the	O
much	O
more	O
efficient	O
DCT	B-General_Concept
compression	O
for	O
video	B-Algorithm
coding	I-Algorithm
,	O
with	O
the	O
CCITT	O
receiving	O
14	O
proposals	O
for	O
DCT-based	O
video	B-Algorithm
compression	I-Algorithm
formats	I-Algorithm
,	O
in	O
contrast	O
to	O
a	O
single	O
proposal	O
based	O
on	O
vector	B-Algorithm
quantization	I-Algorithm
(	O
VQ	O
)	O
compression	O
.	O
</s>
<s>
The	O
H.261	B-Algorithm
standard	I-Algorithm
was	O
subsequently	O
developed	O
based	O
on	O
DCT	B-General_Concept
compression	O
.	O
</s>
<s>
H.261	B-Algorithm
was	O
developed	O
by	O
the	O
CCITT	O
Study	O
Group	O
XV	O
Specialists	O
Group	O
on	O
Coding	O
for	O
Visual	O
Telephony	O
(	O
which	O
later	O
became	O
part	O
of	O
ITU-T	O
SG16	O
)	O
,	O
chaired	O
by	O
Sakae	O
Okubo	O
of	O
NTT	O
.	O
</s>
<s>
Since	O
H.261	B-Algorithm
,	O
DCT	B-General_Concept
compression	O
has	O
been	O
adopted	O
by	O
all	O
the	O
major	O
video	B-Algorithm
coding	I-Algorithm
standards	I-Algorithm
that	O
followed	O
.	O
</s>
<s>
Whilst	O
H.261	B-Algorithm
was	O
preceded	O
in	O
1984	O
by	O
H.120	B-Algorithm
(	O
which	O
also	O
underwent	O
a	O
revision	O
in	O
1988	O
of	O
some	O
historic	O
importance	O
)	O
as	O
a	O
digital	O
video	B-Algorithm
coding	I-Algorithm
standard	I-Algorithm
,	O
H.261	B-Algorithm
was	O
the	O
first	O
truly	O
practical	O
digital	O
video	B-Algorithm
coding	I-Algorithm
standard	I-Algorithm
(	O
in	O
terms	O
of	O
product	O
support	O
in	O
significant	O
quantities	O
)	O
.	O
</s>
<s>
In	O
fact	O
,	O
all	O
subsequent	O
international	O
video	B-Algorithm
coding	I-Algorithm
standards	I-Algorithm
(	O
MPEG-1	O
Part	O
2	O
,	O
H.262/MPEG	B-Algorithm
-2	I-Algorithm
Part	I-Algorithm
2	I-Algorithm
,	O
H.263	B-Algorithm
,	O
MPEG-4	B-Application
Part	I-Application
2	I-Application
,	O
H.264/MPEG	B-Application
-4	I-Application
Part	I-Application
10	I-Application
,	O
and	O
HEVC	B-Algorithm
)	O
have	O
been	O
based	O
closely	O
on	O
the	O
H.261	B-Algorithm
design	O
.	O
</s>
<s>
Additionally	O
,	O
the	O
methods	O
used	O
by	O
the	O
H.261	B-Algorithm
development	O
committee	O
to	O
collaboratively	O
develop	O
the	O
standard	O
have	O
remained	O
the	O
basic	O
operating	O
process	O
for	O
subsequent	O
standardization	O
work	O
in	O
the	O
field	O
.	O
</s>
<s>
Although	O
H.261	B-Algorithm
was	O
first	O
approved	O
as	O
a	O
standard	O
in	O
1988	O
,	O
the	O
first	O
version	O
was	O
missing	O
some	O
significant	O
elements	O
necessary	O
to	O
make	O
it	O
a	O
complete	O
interoperability	O
specification	O
.	O
</s>
<s>
The	O
1993	O
revision	O
added	O
an	O
Annex	O
D	O
entitled	O
"	O
Still	O
image	O
transmission	O
"	O
,	O
which	O
provided	O
a	O
backward-compatible	O
way	O
to	O
send	O
still	O
images	O
with	O
704×576	O
luma	O
resolution	O
and	O
352×288	O
chroma	O
resolution	O
by	O
using	O
a	O
staggered	O
2:1	O
subsampling	B-Algorithm
horizontally	O
and	O
vertically	O
to	O
separate	O
the	O
picture	O
into	O
four	O
sub-pictures	O
that	O
were	O
sent	O
sequentially	O
.	O
</s>
<s>
The	O
basic	O
processing	O
unit	O
of	O
the	O
design	O
is	O
called	O
a	O
macroblock	B-General_Concept
,	O
and	O
H.261	B-Algorithm
was	O
the	O
first	O
standard	O
in	O
which	O
the	O
macroblock	B-General_Concept
concept	O
appeared	O
.	O
</s>
<s>
Each	O
macroblock	B-General_Concept
consists	O
of	O
a	O
16×16	O
array	O
of	O
luma	O
samples	O
and	O
two	O
corresponding	O
8×8	O
arrays	O
of	O
chroma	O
samples	O
,	O
using	O
4:2:0	O
sampling	O
and	O
a	O
YCbCr	O
color	B-Algorithm
space	I-Algorithm
.	O
</s>
<s>
The	O
coding	O
algorithm	O
uses	O
a	O
hybrid	O
of	O
motion-compensated	B-Algorithm
inter-picture	B-Algorithm
prediction	I-Algorithm
and	O
spatial	O
transform	B-Algorithm
coding	I-Algorithm
with	O
scalar	B-Algorithm
quantization	I-Algorithm
,	O
zig-zag	O
scanning	O
and	O
entropy	B-Algorithm
encoding	I-Algorithm
.	O
</s>
<s>
The	O
inter-picture	B-Algorithm
prediction	I-Algorithm
reduces	O
temporal	O
redundancy	O
,	O
with	O
motion	O
vectors	O
used	O
to	O
compensate	O
for	O
motion	O
.	O
</s>
<s>
Whilst	O
only	O
integer-valued	O
motion	O
vectors	O
are	O
supported	O
in	O
H.261	B-Algorithm
,	O
a	O
blurring	O
filter	O
can	O
be	O
applied	O
to	O
the	O
prediction	O
signal	O
–	O
partially	O
mitigating	O
the	O
lack	O
of	O
fractional-sample	O
motion	O
vector	O
precision	O
.	O
</s>
<s>
Transform	B-Algorithm
coding	I-Algorithm
using	O
an	O
8×8	O
discrete	B-General_Concept
cosine	I-General_Concept
transform	I-General_Concept
(	O
DCT	B-General_Concept
)	O
reduces	O
the	O
spatial	O
redundancy	O
.	O
</s>
<s>
The	O
DCT	B-General_Concept
that	O
is	O
widely	O
used	O
in	O
this	O
regard	O
was	O
introduced	O
by	O
N	O
.	O
Ahmed	O
,	O
T	O
.	O
Natarajan	O
and	O
K	O
.	O
R	O
.	O
Rao	O
in	O
1974	O
.	O
</s>
<s>
Scalar	B-Algorithm
quantization	I-Algorithm
is	O
then	O
applied	O
to	O
round	O
the	O
transform	O
coefficients	O
to	O
the	O
appropriate	O
precision	O
determined	O
by	O
a	O
step	O
size	O
control	O
parameter	O
,	O
and	O
the	O
quantized	O
transform	O
coefficients	O
are	O
zig-zag	O
scanned	O
and	O
entropy-coded	O
(	O
using	O
a	O
"	O
run-level	O
"	O
variable-length	B-Algorithm
code	I-Algorithm
)	O
to	O
remove	O
statistical	O
redundancy	O
.	O
</s>
<s>
The	O
H.261	B-Algorithm
standard	I-Algorithm
actually	O
only	O
specifies	O
how	O
to	O
decode	O
the	O
video	O
.	O
</s>
<s>
One	O
effective	O
post-processing	O
technique	O
that	O
became	O
a	O
key	O
element	O
of	O
the	O
best	O
H.261-based	O
systems	O
is	O
called	O
deblocking	B-General_Concept
filtering	O
.	O
</s>
<s>
This	O
reduces	O
the	O
appearance	O
of	O
block-shaped	O
artifacts	O
caused	O
by	O
the	O
block-based	O
motion	B-Algorithm
compensation	I-Algorithm
and	O
spatial	O
transform	O
parts	O
of	O
the	O
design	O
.	O
</s>
<s>
Deblocking	B-General_Concept
filtering	O
has	O
since	O
become	O
an	O
integral	O
part	O
of	O
the	O
more	O
recent	O
standards	O
H.264	B-Application
and	O
HEVC	B-Algorithm
(	O
although	O
even	O
when	O
using	O
these	O
newer	O
standards	O
,	O
additional	O
post-processing	O
is	O
still	O
allowed	O
and	O
can	O
enhance	O
visual	O
quality	O
if	O
performed	O
well	O
)	O
.	O
</s>
<s>
Design	O
refinements	O
introduced	O
in	O
later	O
standardization	O
efforts	O
have	O
resulted	O
in	O
significant	O
improvements	O
in	O
compression	O
capability	O
relative	O
to	O
the	O
H.261	B-Algorithm
design	O
.	O
</s>
<s>
This	O
has	O
resulted	O
in	O
H.261	B-Algorithm
becoming	O
essentially	O
obsolete	O
,	O
although	O
it	O
is	O
still	O
used	O
as	O
a	O
backward-compatibility	O
mode	O
in	O
some	O
video-conferencing	O
systems	O
(	O
such	O
as	O
H.323	B-Protocol
)	O
and	O
for	O
some	O
types	O
of	O
internet	O
video	O
.	O
</s>
<s>
However	O
,	O
H.261	B-Algorithm
remains	O
a	O
major	O
historical	O
milestone	O
in	O
the	O
field	O
of	O
video	B-Algorithm
coding	I-Algorithm
development	O
.	O
</s>
<s>
The	O
LGPL-licensed	O
libavcodec	B-Language
includes	O
a	O
H.261	B-Algorithm
encoder	O
and	O
decoder	O
.	O
</s>
<s>
It	O
is	O
supported	O
by	O
the	O
free	O
VLC	B-Application
media	I-Application
player	I-Application
and	O
MPlayer	B-Application
multimedia	O
players	O
,	O
and	O
in	O
ffdshow	B-Language
and	O
FFmpeg	B-Library
decoders	O
projects	O
.	O
</s>
<s>
The	O
following	O
companies	O
contributed	O
patents	O
towards	O
the	O
development	O
of	O
the	O
H.261	B-Algorithm
format	O
:	O
</s>
<s>
PictureTel	B-Application
Corp	I-Application
.	I-Application
</s>
