<s>
A	O
codec	B-General_Concept
is	O
a	O
device	O
or	O
computer	B-Application
program	I-Application
that	O
encodes	O
or	O
decodes	O
a	O
data	B-General_Concept
stream	I-General_Concept
or	O
signal	O
.	O
</s>
<s>
Codec	B-General_Concept
is	O
a	O
portmanteau	O
of	O
coder/decoder	B-General_Concept
.	O
</s>
<s>
In	O
electronic	O
communications	O
,	O
an	O
endec	B-General_Concept
is	O
a	O
device	O
that	O
acts	O
as	O
both	O
an	O
encoder	O
and	O
a	O
decoder	O
on	O
a	O
signal	O
or	O
data	B-General_Concept
stream	I-General_Concept
,	O
and	O
hence	O
is	O
a	O
type	O
of	O
codec	B-General_Concept
.	O
</s>
<s>
Endec	B-General_Concept
is	O
a	O
portmanteau	O
of	O
encoder/decoder	B-General_Concept
.	O
</s>
<s>
A	O
coder	O
or	O
encoder	O
encodes	O
a	O
data	B-General_Concept
stream	I-General_Concept
or	O
a	O
signal	O
for	O
transmission	O
or	O
storage	O
,	O
possibly	O
in	O
encrypted	O
form	O
,	O
and	O
the	O
decoder	O
function	O
reverses	O
the	O
encoding	O
for	O
playback	O
or	O
editing	O
.	O
</s>
<s>
Codecs	B-General_Concept
are	O
used	O
in	O
videoconferencing	B-Application
,	O
streaming	O
media	O
,	O
and	O
video	O
editing	O
applications	O
.	O
</s>
<s>
In	O
the	O
mid-20th	O
century	O
,	O
a	O
codec	B-General_Concept
was	O
a	O
device	O
that	O
coded	O
analog	O
signals	O
into	O
digital	O
form	O
using	O
pulse-code	B-Algorithm
modulation	I-Algorithm
(	O
PCM	B-Algorithm
)	O
.	O
</s>
<s>
Later	O
,	O
the	O
name	O
was	O
also	O
applied	O
to	O
software	O
for	O
converting	O
between	O
digital	O
signal	O
formats	O
,	O
including	O
companding	B-Algorithm
functions	O
.	O
</s>
<s>
An	O
audio	B-Algorithm
codec	I-Algorithm
converts	O
analog	O
audio	O
signals	O
into	O
digital	O
signals	O
for	O
transmission	O
or	O
encodes	O
them	O
for	O
storage	O
.	O
</s>
<s>
A	O
receiving	O
device	O
converts	O
the	O
digital	O
signals	O
back	O
to	O
analog	O
form	O
using	O
an	O
audio	B-Algorithm
decoder	I-Algorithm
for	O
playback	O
.	O
</s>
<s>
An	O
example	O
of	O
this	O
are	O
the	O
codecs	B-General_Concept
used	O
in	O
the	O
sound	O
cards	O
of	O
personal	O
computers	O
.	O
</s>
<s>
A	O
video	B-Application
codec	I-Application
accomplishes	O
the	O
same	O
task	O
for	O
video	O
signals	O
.	O
</s>
<s>
An	O
Emergency	O
Alert	O
System	O
unit	O
is	O
usually	O
an	O
endec	B-General_Concept
,	O
but	O
sometimes	O
just	O
a	O
decoder	O
.	O
</s>
<s>
When	O
implementing	O
the	O
Infrared	O
Data	O
Association	O
(	O
IrDA	O
)	O
protocol	O
,	O
an	O
endec	B-General_Concept
may	O
be	O
used	O
between	O
the	O
UART	O
and	O
the	O
optoelectronics	O
.	O
</s>
<s>
In	O
addition	O
to	O
encoding	O
a	O
signal	O
,	O
a	O
codec	B-General_Concept
may	O
also	O
compress	O
the	O
data	O
to	O
reduce	O
transmission	O
bandwidth	O
or	O
storage	O
space	O
.	O
</s>
<s>
Compression	O
codecs	B-General_Concept
are	O
classified	O
primarily	O
into	O
lossy	B-Algorithm
codecs	B-General_Concept
and	O
lossless	B-Algorithm
codecs	I-Algorithm
.	O
</s>
<s>
Lossless	B-Algorithm
codecs	I-Algorithm
are	O
often	O
used	O
for	O
archiving	O
data	O
in	O
a	O
compressed	O
form	O
while	O
retaining	O
all	O
information	O
present	O
in	O
the	O
original	O
stream	O
.	O
</s>
<s>
If	O
preserving	O
the	O
original	O
quality	O
of	O
the	O
stream	O
is	O
more	O
important	O
than	O
eliminating	O
the	O
correspondingly	O
larger	O
data	O
sizes	O
,	O
lossless	B-Algorithm
codecs	I-Algorithm
are	O
preferred	O
.	O
</s>
<s>
This	O
is	O
especially	O
true	O
if	O
the	O
data	O
is	O
to	O
undergo	O
further	O
processing	O
(	O
for	O
example	O
editing	O
)	O
in	O
which	O
case	O
the	O
repeated	O
application	O
of	O
processing	O
(	O
encoding	O
and	O
decoding	O
)	O
on	O
lossy	B-Algorithm
codecs	B-General_Concept
will	O
degrade	O
the	O
quality	O
of	O
the	O
resulting	O
data	O
such	O
that	O
it	O
is	O
no	O
longer	O
identifiable	O
(	O
visually	O
,	O
audibly	O
or	O
both	O
)	O
.	O
</s>
<s>
Using	O
more	O
than	O
one	O
codec	B-General_Concept
or	O
encoding	O
scheme	O
successively	O
can	O
also	O
degrade	O
quality	O
significantly	O
.	O
</s>
<s>
The	O
decreasing	O
cost	O
of	O
storage	O
capacity	O
and	O
network	O
bandwidth	O
has	O
a	O
tendency	O
to	O
reduce	O
the	O
need	O
for	O
lossy	B-Algorithm
codecs	B-General_Concept
for	O
some	O
media	O
.	O
</s>
<s>
Many	O
popular	O
codecs	B-General_Concept
are	O
lossy	B-Algorithm
.	O
</s>
<s>
Often	O
,	O
this	O
type	O
of	O
compression	O
is	O
virtually	O
indistinguishable	O
from	O
the	O
original	O
uncompressed	O
sound	O
or	O
images	O
,	O
depending	O
on	O
the	O
codec	B-General_Concept
and	O
the	O
settings	O
used	O
.	O
</s>
<s>
The	O
most	O
widely	O
used	O
lossy	B-Algorithm
data	I-Algorithm
compression	I-Algorithm
technique	O
in	O
digital	O
media	O
is	O
based	O
on	O
the	O
discrete	B-General_Concept
cosine	I-General_Concept
transform	I-General_Concept
(	O
DCT	B-General_Concept
)	O
,	O
used	O
in	O
compression	O
standards	O
such	O
as	O
JPEG	O
images	O
,	O
H.26x	B-Application
and	O
MPEG	O
video	O
,	O
and	O
MP3	B-Application
and	O
AAC	B-Application
audio	O
.	O
</s>
<s>
Smaller	O
data	O
sets	O
ease	O
the	O
strain	O
on	O
relatively	O
expensive	O
storage	O
sub-systems	O
such	O
as	O
non-volatile	B-General_Concept
memory	I-General_Concept
and	O
hard	B-Device
disk	I-Device
,	O
as	O
well	O
as	O
write-once-read-many	B-Device
formats	O
such	O
as	O
CD-ROM	B-Device
,	O
DVD	O
and	O
Blu-ray	O
Disc	O
.	O
</s>
<s>
Two	O
principal	O
techniques	O
are	O
used	O
in	O
codecs	B-General_Concept
,	O
pulse-code	B-Algorithm
modulation	I-Algorithm
and	O
delta	B-General_Concept
modulation	I-General_Concept
.	O
</s>
<s>
Codecs	B-General_Concept
are	O
often	O
designed	O
to	O
emphasize	O
certain	O
aspects	O
of	O
the	O
media	O
to	O
be	O
encoded	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
digital	O
video	O
(	O
using	O
a	O
DV	O
codec	B-General_Concept
)	O
of	O
a	O
sports	O
event	O
needs	O
to	O
encode	O
motion	O
well	O
but	O
not	O
necessarily	O
exact	O
colors	O
,	O
while	O
a	O
video	O
of	O
an	O
art	O
exhibit	O
needs	O
to	O
encode	O
color	O
and	O
surface	O
texture	O
well	O
.	O
</s>
<s>
Audio	B-Algorithm
codecs	I-Algorithm
for	O
cell	O
phones	O
need	O
to	O
have	O
very	O
low	O
latency	O
between	O
source	O
encoding	O
and	O
playback	O
.	O
</s>
<s>
In	O
contrast	O
,	O
audio	B-Algorithm
codecs	I-Algorithm
for	O
recording	O
or	O
broadcast	O
can	O
use	O
high-latency	O
audio	O
compression	O
techniques	O
to	O
achieve	O
higher	O
fidelity	O
at	O
a	O
lower	O
bit-rate	O
.	O
</s>
<s>
There	O
are	O
thousands	O
of	O
audio	O
and	O
video	B-Application
codecs	I-Application
,	O
ranging	O
in	O
cost	O
from	O
free	O
to	O
hundreds	O
of	O
dollars	O
or	O
more	O
.	O
</s>
<s>
This	O
variety	O
of	O
codecs	B-General_Concept
can	O
create	O
compatibility	O
and	O
obsolescence	O
issues	O
.	O
</s>
<s>
For	O
example	O
,	O
raw	O
uncompressed	O
PCM	B-Algorithm
audio	I-Algorithm
(	O
44.1kHz	O
,	O
16	O
bit	O
stereo	O
,	O
as	O
represented	O
on	O
an	O
audio	O
CD	O
or	O
in	O
a	O
.wav	O
or	O
.aiff	O
file	O
)	O
has	O
long	O
been	O
a	O
standard	O
across	O
multiple	O
platforms	O
,	O
but	O
its	O
transmission	O
over	O
networks	O
is	O
slow	O
and	O
expensive	O
compared	O
with	O
more	O
modern	O
compressed	O
formats	O
,	O
such	O
as	O
Opus	B-Algorithm
and	O
MP3	B-Application
.	O
</s>
<s>
Many	O
multimedia	O
data	B-General_Concept
streams	I-General_Concept
contain	O
both	O
audio	O
and	O
video	O
,	O
and	O
often	O
some	O
metadata	O
that	O
permits	O
synchronization	O
of	O
audio	O
and	O
video	O
.	O
</s>
<s>
Each	O
of	O
these	O
three	O
streams	O
may	O
be	O
handled	O
by	O
different	O
programs	O
,	O
processes	O
,	O
or	O
hardware	O
;	O
but	O
for	O
the	O
multimedia	O
data	B-General_Concept
streams	I-General_Concept
to	O
be	O
useful	O
in	O
stored	O
or	O
transmitted	O
form	O
,	O
they	O
must	O
be	O
encapsulated	O
together	O
in	O
a	O
container	O
format	O
.	O
</s>
<s>
Lower	O
bitrate	O
codecs	B-General_Concept
allow	O
more	O
users	O
,	O
but	O
they	O
also	O
have	O
more	O
distortion	O
.	O
</s>
<s>
Beyond	O
the	O
initial	O
increase	O
in	O
distortion	O
,	O
lower	B-General_Concept
bit	I-General_Concept
rate	I-General_Concept
codecs	I-General_Concept
also	O
achieve	O
their	O
lower	O
bit	O
rates	O
by	O
using	O
more	O
complex	O
algorithms	O
that	O
make	O
certain	O
assumptions	O
,	O
such	O
as	O
those	O
about	O
the	O
media	O
and	O
the	O
packet	O
loss	O
rate	O
.	O
</s>
<s>
Other	O
codecs	B-General_Concept
may	O
not	O
make	O
those	O
same	O
assumptions	O
.	O
</s>
<s>
When	O
a	O
user	O
with	O
a	O
low	O
bitrate	O
codec	B-General_Concept
talks	O
to	O
a	O
user	O
with	O
another	O
codec	B-General_Concept
,	O
additional	O
distortion	O
is	O
introduced	O
by	O
each	O
transcoding	B-Algorithm
.	O
</s>
<s>
Audio	O
Video	O
Interleave	O
(	O
AVI	O
)	O
is	O
sometimes	O
erroneously	O
described	O
as	O
a	O
codec	B-General_Concept
,	O
but	O
AVI	O
is	O
actually	O
a	O
container	O
format	O
,	O
while	O
a	O
codec	B-General_Concept
is	O
a	O
software	O
or	O
hardware	O
tool	O
that	O
encodes	O
or	O
decodes	O
audio	O
or	O
video	O
into	O
or	O
from	O
some	O
audio	O
or	O
video	O
format	O
.	O
</s>
<s>
Audio	O
and	O
video	O
encoded	O
with	O
many	O
codecs	B-General_Concept
might	O
be	O
put	O
into	O
an	O
AVI	O
container	O
,	O
although	O
AVI	O
is	O
not	O
an	O
ISO	O
standard	O
.	O
</s>
<s>
There	O
are	O
also	O
other	O
well-known	O
container	O
formats	O
,	O
such	O
as	O
Ogg	O
,	O
ASF	B-Application
,	O
QuickTime	B-Application
,	O
RealMedia	O
,	O
Matroska	O
,	O
and	O
DivX	O
Media	O
Format	O
.	O
</s>
<s>
are	O
used	O
when	O
an	O
online	O
user	O
takes	O
a	O
type	O
of	O
codec	B-General_Concept
and	O
installs	O
viruses	O
and	O
other	O
malware	O
into	O
whatever	O
data	O
is	O
being	O
compressed	O
and	O
uses	O
it	O
as	O
a	O
disguise	O
.	O
</s>
<s>
This	O
disguise	O
appears	O
as	O
a	O
codec	B-General_Concept
download	O
through	O
a	O
pop-up	O
alert	O
or	O
ad	O
.	O
</s>
<s>
When	O
a	O
user	O
goes	O
to	O
click	O
or	O
download	O
that	O
codec	B-General_Concept
,	O
the	O
malware	O
is	O
then	O
installed	O
on	O
the	O
computer	O
.	O
</s>
<s>
Once	O
a	O
fake	O
codec	B-General_Concept
is	O
installed	O
it	O
is	O
often	O
used	O
to	O
access	O
private	O
data	O
,	O
corrupt	O
an	O
entire	O
computer	O
system	O
or	O
to	O
keep	O
spreading	O
the	O
malware	O
.	O
</s>
<s>
One	O
of	O
the	O
previous	O
most	O
used	O
ways	O
to	O
spread	O
malware	O
was	O
fake	O
AV	O
pages	O
and	O
with	O
the	O
rise	O
of	O
codec	B-General_Concept
technology	O
both	O
have	O
been	O
used	O
in	O
combination	O
to	O
take	O
advantage	O
of	O
online	O
users	O
.	O
</s>
<s>
This	O
combination	O
allows	O
fake	O
codecs	B-General_Concept
to	O
be	O
automatically	O
downloaded	O
to	O
a	O
device	O
through	O
a	O
website	O
linked	O
in	O
a	O
pop-up	O
ad	O
,	O
virus/codec	O
alerts	O
or	O
articles	O
as	O
well	O
.	O
</s>
