<s>
Digital	B-General_Concept
data	I-General_Concept
,	O
in	O
information	O
theory	O
and	O
information	O
systems	O
,	O
is	O
information	O
represented	O
as	O
a	O
string	O
of	O
discrete	O
symbols	O
,	O
each	O
of	O
which	O
can	O
take	O
on	O
one	O
of	O
only	O
a	O
finite	O
number	O
of	O
values	O
from	O
some	O
alphabet	B-Application
,	O
such	O
as	O
letters	O
or	O
digits	O
.	O
</s>
<s>
An	O
example	O
is	O
a	O
text	B-General_Concept
document	I-General_Concept
,	O
which	O
consists	O
of	O
a	O
string	O
of	O
alphanumeric	O
characters	O
.	O
</s>
<s>
The	O
most	O
common	O
form	O
of	O
digital	B-General_Concept
data	I-General_Concept
in	O
modern	O
information	O
systems	O
is	O
binary	O
data	O
,	O
which	O
is	O
represented	O
by	O
a	O
string	O
of	O
binary	O
digits	O
(	O
bits	O
)	O
each	O
of	O
which	O
can	O
have	O
one	O
of	O
two	O
values	O
,	O
either	O
0	O
or	O
1	O
.	O
</s>
<s>
Digital	B-General_Concept
data	I-General_Concept
can	O
be	O
contrasted	O
with	O
analog	O
data	O
,	O
which	O
is	O
represented	O
by	O
a	O
value	O
from	O
a	O
continuous	O
range	O
of	O
real	O
numbers	O
.	O
</s>
<s>
The	O
term	O
is	O
most	O
commonly	O
used	O
in	O
computing	O
and	O
electronics	O
,	O
especially	O
where	O
real-world	O
information	O
is	O
converted	O
to	O
binary	O
numeric	O
form	O
as	O
in	O
digital	O
audio	O
and	O
digital	B-Application
photography	I-Application
.	O
</s>
<s>
Instead	O
of	O
sampling	B-Algorithm
and	O
quantization	B-Algorithm
as	O
in	O
analog-to-digital	O
conversion	O
,	O
such	O
techniques	O
as	O
polling	B-General_Concept
and	O
encoding	B-General_Concept
are	O
used	O
.	O
</s>
<s>
Data	O
will	O
be	O
lost	O
if	O
,	O
within	O
a	O
single	O
polling	B-General_Concept
interval	O
,	O
two	O
switches	O
are	O
pressed	O
,	O
or	O
a	O
switch	O
is	O
pressed	O
,	O
released	O
,	O
and	O
pressed	O
again	O
.	O
</s>
<s>
This	O
polling	B-General_Concept
can	O
be	O
done	O
by	O
a	O
specialized	O
processor	O
in	O
the	O
device	O
to	O
prevent	O
burdening	O
the	O
main	O
CPU	B-General_Concept
.	O
</s>
<s>
When	O
a	O
new	O
symbol	O
has	O
been	O
entered	O
,	O
the	O
device	O
typically	O
sends	O
an	O
interrupt	B-Application
,	O
in	O
a	O
specialized	O
format	O
,	O
so	O
that	O
the	O
CPU	B-General_Concept
can	O
read	O
it	O
.	O
</s>
<s>
For	O
devices	O
with	O
only	O
a	O
few	O
switches	O
(	O
such	O
as	O
the	O
buttons	O
on	O
a	O
joystick	B-Device
)	O
,	O
the	O
status	O
of	O
each	O
can	O
be	O
encoded	O
as	O
bits	O
(	O
usually	O
0	O
for	O
released	O
and	O
1	O
for	O
pressed	O
)	O
in	O
a	O
single	O
word	O
.	O
</s>
<s>
This	O
is	O
useful	O
when	O
combinations	O
of	O
key	O
presses	O
are	O
meaningful	O
,	O
and	O
is	O
sometimes	O
used	O
for	O
passing	O
the	O
status	O
of	O
modifier	O
keys	O
on	O
a	O
keyboard	B-Device
(	O
such	O
as	O
shift	O
and	O
control	O
)	O
.	O
</s>
<s>
Devices	O
with	O
many	O
switches	O
(	O
such	O
as	O
a	O
computer	B-Device
keyboard	I-Device
)	O
usually	O
arrange	O
these	O
switches	O
in	O
a	O
scan	O
matrix	O
,	O
with	O
the	O
individual	O
switches	O
on	O
the	O
intersections	O
of	O
x	O
and	O
y	O
lines	O
.	O
</s>
<s>
Polling	B-General_Concept
(	O
often	O
called	O
scanning	O
in	O
this	O
case	O
)	O
is	O
done	O
by	O
activating	O
each	O
x	O
line	O
in	O
sequence	O
and	O
detecting	O
which	O
y	O
lines	O
then	O
have	O
a	O
signal	O
,	O
thus	O
which	O
keys	O
are	O
pressed	O
.	O
</s>
<s>
When	O
the	O
keyboard	B-Device
processor	O
detects	O
that	O
a	O
key	O
has	O
changed	O
state	O
,	O
it	O
sends	O
a	O
signal	O
to	O
the	O
CPU	B-General_Concept
indicating	O
the	O
scan	O
code	O
of	O
the	O
key	O
and	O
its	O
new	O
state	O
.	O
</s>
<s>
The	O
symbol	O
is	O
then	O
encoded	O
or	O
converted	O
into	O
a	O
number	O
based	O
on	O
the	O
status	O
of	O
modifier	O
keys	O
and	O
the	O
desired	O
character	B-General_Concept
encoding	I-General_Concept
.	O
</s>
<s>
A	O
custom	O
encoding	B-General_Concept
can	O
be	O
used	O
for	O
a	O
specific	O
application	O
with	O
no	O
loss	O
of	O
data	O
.	O
</s>
<s>
However	O
,	O
using	O
a	O
standard	O
encoding	B-General_Concept
such	O
as	O
ASCII	B-Protocol
is	O
problematic	O
if	O
a	O
symbol	O
such	O
as	O
'	O
ß	O
 '	O
needs	O
to	O
be	O
converted	O
but	O
is	O
not	O
in	O
the	O
standard	O
.	O
</s>
<s>
Digital	B-General_Concept
data	I-General_Concept
come	O
in	O
these	O
three	O
states	O
:	O
data	B-General_Concept
at	I-General_Concept
rest	I-General_Concept
,	O
data	B-General_Concept
in	I-General_Concept
transit	I-General_Concept
,	O
and	O
data	B-General_Concept
in	I-General_Concept
use	I-General_Concept
.	O
</s>
<s>
All	O
digital	B-General_Concept
information	I-General_Concept
possesses	O
common	O
properties	O
that	O
distinguish	O
it	O
from	O
analog	O
data	O
with	O
respect	O
to	O
communications	O
:	O
</s>
<s>
Synchronization	O
:	O
Since	O
digital	B-General_Concept
information	I-General_Concept
is	O
conveyed	O
by	O
the	O
sequence	O
in	O
which	O
symbols	O
are	O
ordered	O
,	O
all	O
digital	O
schemes	O
have	O
some	O
method	O
for	O
determining	O
the	O
beginning	O
of	O
a	O
sequence	O
.	O
</s>
<s>
Errors	O
:	O
Disturbances	O
(	O
noise	B-Algorithm
)	O
in	O
analog	O
communications	O
invariably	O
introduce	O
some	O
,	O
generally	O
small	O
deviation	O
or	O
error	O
between	O
the	O
intended	O
and	O
actual	O
communication	O
.	O
</s>
<s>
Further	O
,	O
techniques	O
such	O
as	O
check	O
codes	O
may	O
be	O
used	O
to	O
detect	B-Error_Name
errors	I-Error_Name
and	I-Error_Name
correct	I-Error_Name
them	I-Error_Name
through	O
redundancy	O
or	O
re-transmission	O
.	O
</s>
<s>
Copying	O
:	O
Because	O
of	O
the	O
inevitable	O
presence	O
of	O
noise	B-Algorithm
,	O
making	O
many	O
successive	O
copies	O
of	O
an	O
analog	O
communication	O
is	O
infeasible	O
because	O
each	O
generation	O
increases	O
the	O
noise	B-Algorithm
.	O
</s>
<s>
Granularity	B-Operating_System
:	O
The	O
digital	O
representation	O
of	O
a	O
continuously	O
variable	O
analog	O
value	O
typically	O
involves	O
a	O
selection	O
of	O
the	O
number	O
of	O
symbols	O
to	O
be	O
assigned	O
to	O
that	O
value	O
.	O
</s>
<s>
The	O
difference	O
between	O
the	O
actual	O
analog	O
value	O
and	O
the	O
digital	O
representation	O
is	O
known	O
as	O
quantization	B-Algorithm
error	O
.	O
</s>
<s>
This	O
property	O
of	O
digital	O
communication	O
is	O
known	O
as	O
granularity	B-Operating_System
.	O
</s>
<s>
Compressible	B-General_Concept
:	O
According	O
to	O
Miller	O
,	O
"	O
Uncompressed	B-General_Concept
digital	B-General_Concept
data	I-General_Concept
is	O
very	O
large	O
,	O
and	O
in	O
its	O
raw	O
form	O
,	O
it	O
would	O
actually	O
produce	O
a	O
larger	O
signal	O
(	O
therefore	O
be	O
more	O
difficult	O
to	O
transfer	O
)	O
than	O
analog	O
data	O
.	O
</s>
<s>
However	O
,	O
digital	B-General_Concept
data	I-General_Concept
can	O
be	O
compressed	O
.	O
</s>
<s>
This	O
makes	O
it	O
possible	O
to	O
send	O
much	O
more	O
information	O
and	O
results	O
in	O
,	O
for	O
example	O
,	O
digital	B-Application
television	I-Application
signals	O
offering	O
more	O
room	O
on	O
the	O
airwave	O
spectrum	O
for	O
more	O
television	O
channels.	O
"	O
</s>
<s>
DNA	O
genetic	O
code	O
is	O
a	O
naturally	O
occurring	O
form	O
of	O
digital	B-General_Concept
data	I-General_Concept
storage	O
.	O
</s>
<s>
The	O
abacus	B-Application
was	O
created	O
sometime	O
between	O
1000	O
BC	O
and	O
500	O
BC	O
,	O
it	O
later	O
became	O
a	O
form	O
of	O
calculation	O
frequency	O
.	O
</s>
<s>
A	O
beacon	B-Protocol
is	O
perhaps	O
the	O
simplest	O
non-electronic	O
digital	O
signal	O
,	O
with	O
just	O
two	O
states	O
(	O
on	O
and	O
off	O
)	O
.	O
</s>
<s>
In	O
particular	O
,	O
smoke	O
signals	O
are	O
one	O
of	O
the	O
oldest	O
examples	O
of	O
a	O
digital	O
signal	O
,	O
where	O
an	O
analog	O
"	O
carrier	O
"	O
(	O
smoke	O
)	O
is	O
modulated	B-Application
with	O
a	O
blanket	O
to	O
generate	O
a	O
digital	O
signal	O
(	O
puffs	O
)	O
that	O
conveys	O
information	O
.	O
</s>
<s>
International	O
maritime	O
signal	O
flags	O
have	O
distinctive	O
markings	O
that	O
represent	O
letters	O
of	O
the	O
alphabet	B-Application
to	O
allow	O
ships	O
to	O
send	O
messages	O
to	O
each	O
other	O
.	O
</s>
<s>
More	O
recently	O
invented	O
,	O
a	O
modem	B-Application
modulates	O
an	O
analog	O
"	O
carrier	O
"	O
signal	O
(	O
such	O
as	O
sound	O
)	O
to	O
encode	O
binary	O
electrical	O
digital	B-General_Concept
information	I-General_Concept
,	O
as	O
a	O
series	O
of	O
binary	O
digital	O
sound	O
pulses	O
.	O
</s>
<s>
"	O
sound	O
"	O
and	O
"	O
silence	O
"	O
)	O
on	O
magnetic	B-Device
cassette	I-Device
tape	I-Device
for	O
use	O
with	O
early	O
home	O
computers	O
.	O
</s>
