<s>
Pulse-code	B-Algorithm
modulation	I-Algorithm
(	O
PCM	B-Algorithm
)	O
is	O
a	O
method	O
used	O
to	O
digitally	O
represent	O
sampled	B-Algorithm
analog	O
signals	O
.	O
</s>
<s>
It	O
is	O
the	O
standard	O
form	O
of	O
digital	O
audio	O
in	O
computers	O
,	O
compact	O
discs	O
,	O
digital	B-General_Concept
telephony	I-General_Concept
and	O
other	O
digital	O
audio	O
applications	O
.	O
</s>
<s>
In	O
a	O
PCM	B-Algorithm
stream	B-Architecture
,	O
the	O
amplitude	B-Application
of	O
the	O
analog	O
signal	O
is	O
sampled	B-Algorithm
at	O
uniform	O
intervals	O
,	O
and	O
each	O
sample	B-Algorithm
is	O
quantized	B-Algorithm
to	O
the	O
nearest	O
value	O
within	O
a	O
range	O
of	O
digital	O
steps	O
.	O
</s>
<s>
Linear	B-Algorithm
pulse-code	I-Algorithm
modulation	I-Algorithm
(	O
LPCM	B-Algorithm
)	O
is	O
a	O
specific	O
type	O
of	O
PCM	B-Algorithm
in	O
which	O
the	O
quantization	B-Algorithm
levels	O
are	O
linearly	O
uniform	O
.	O
</s>
<s>
This	O
is	O
in	O
contrast	O
to	O
PCM	B-Algorithm
encodings	O
in	O
which	O
quantization	B-Algorithm
levels	O
vary	O
as	O
a	O
function	O
of	O
amplitude	B-Application
(	O
as	O
with	O
the	O
A-law	B-Algorithm
algorithm	I-Algorithm
or	O
the	O
μ-law	B-Algorithm
algorithm	I-Algorithm
)	O
.	O
</s>
<s>
Though	O
PCM	B-Algorithm
is	O
a	O
more	O
general	O
term	O
,	O
it	O
is	O
often	O
used	O
to	O
describe	O
data	O
encoded	O
as	O
LPCM	B-Algorithm
.	O
</s>
<s>
A	O
PCM	B-Algorithm
stream	B-Architecture
has	O
two	O
basic	O
properties	O
that	O
determine	O
the	O
stream	B-Architecture
's	O
fidelity	O
to	O
the	O
original	O
analog	O
signal	O
:	O
the	O
sampling	O
rate	O
,	O
which	O
is	O
the	O
number	O
of	O
times	O
per	O
second	O
that	O
samples	O
are	O
taken	O
;	O
and	O
the	O
bit	O
depth	O
,	O
which	O
determines	O
the	O
number	O
of	O
possible	O
digital	O
values	O
that	O
can	O
be	O
used	O
to	O
represent	O
each	O
sample	B-Algorithm
.	O
</s>
<s>
Early	O
electrical	O
communications	O
started	O
to	O
sample	B-Algorithm
signals	O
in	O
order	O
to	O
multiplex	B-Architecture
samples	O
from	O
multiple	O
telegraphy	O
sources	O
and	O
to	O
convey	O
them	O
over	O
a	O
single	O
telegraph	O
cable	O
.	O
</s>
<s>
The	O
American	O
inventor	O
Moses	O
G	O
.	O
Farmer	O
conceived	O
telegraph	O
time-division	B-Protocol
multiplexing	I-Protocol
(	O
TDM	O
)	O
as	O
early	O
as	O
1853	O
.	O
</s>
<s>
Electrical	O
engineer	O
W	O
.	O
M	O
.	O
Miner	O
,	O
in	O
1903	O
,	O
used	O
an	O
electro-mechanical	O
commutator	O
for	O
time-division	B-Protocol
multiplexing	I-Protocol
multiple	O
telegraph	O
signals	O
;	O
he	O
also	O
applied	O
this	O
technology	O
to	O
telephony	O
.	O
</s>
<s>
He	O
obtained	O
intelligible	O
speech	O
from	O
channels	O
sampled	B-Algorithm
at	O
a	O
rate	O
above	O
3500	O
–	O
4300Hz	O
;	O
lower	O
rates	O
proved	O
unsatisfactory	O
.	O
</s>
<s>
In	O
1920	O
,	O
the	O
Bartlane	O
cable	O
picture	O
transmission	O
system	O
used	O
telegraph	O
signaling	O
of	O
characters	O
punched	O
in	O
paper	O
tape	O
to	O
send	O
samples	O
of	O
images	O
quantized	B-Algorithm
to	O
5	O
levels	O
.	O
</s>
<s>
In	O
1926	O
,	O
Paul	O
M	O
.	O
Rainey	O
of	O
Western	O
Electric	O
patented	O
a	O
facsimile	O
machine	O
which	O
transmitted	O
its	O
signal	O
using	O
5-bit	O
PCM	B-Algorithm
,	O
encoded	O
by	O
an	O
opto-mechanical	O
analog-to-digital	O
converter	O
.	O
</s>
<s>
British	O
engineer	O
Alec	O
Reeves	O
,	O
unaware	O
of	O
previous	O
work	O
,	O
conceived	O
the	O
use	O
of	O
PCM	B-Algorithm
for	O
voice	O
communication	O
in	O
1937	O
while	O
working	O
for	O
International	O
Telephone	O
and	O
Telegraph	O
in	O
France	O
.	O
</s>
<s>
In	O
1943	O
the	O
Bell	O
Labs	O
researchers	O
who	O
designed	O
the	O
SIGSALY	O
system	O
became	O
aware	O
of	O
the	O
use	O
of	O
PCM	B-Algorithm
binary	O
coding	O
as	O
already	O
proposed	O
by	O
Reeves	O
.	O
</s>
<s>
In	O
1949	O
,	O
for	O
the	O
Canadian	O
Navy	O
's	O
DATAR	B-Device
system	O
,	O
Ferranti	O
Canada	O
built	O
a	O
working	O
PCM	B-Algorithm
radio	O
system	O
that	O
was	O
able	O
to	O
transmit	O
digitized	O
radar	O
data	O
over	O
long	O
distances	O
.	O
</s>
<s>
PCM	B-Algorithm
in	O
the	O
late	O
1940s	O
and	O
early	O
1950s	O
used	O
a	O
cathode-ray	B-Device
coding	O
tube	O
with	O
a	O
plate	O
electrode	O
having	O
encoding	O
perforations	O
.	O
</s>
<s>
As	O
in	O
an	O
oscilloscope	O
,	O
the	O
beam	O
was	O
swept	O
horizontally	O
at	O
the	O
sample	B-Algorithm
rate	O
while	O
the	O
vertical	O
deflection	O
was	O
controlled	O
by	O
the	O
input	O
analog	O
signal	O
,	O
causing	O
the	O
beam	O
to	O
pass	O
through	O
higher	O
or	O
lower	O
portions	O
of	O
the	O
perforated	O
plate	O
.	O
</s>
<s>
Rather	O
than	O
natural	O
binary	O
,	O
the	O
grid	O
of	O
Goodall	O
's	O
later	O
tube	O
was	O
perforated	O
to	O
produce	O
a	O
glitch-free	O
Gray	B-Device
code	I-Device
and	O
produced	O
all	O
bits	O
simultaneously	O
by	O
using	O
a	O
fan	O
beam	O
instead	O
of	O
a	O
scanning	O
beam	O
.	O
</s>
<s>
as	O
the	O
inventors	O
of	O
PCM	B-Algorithm
,	O
</s>
<s>
as	O
described	O
in	O
"	O
Communication	O
System	O
Employing	O
Pulse	B-Algorithm
Code	I-Algorithm
Modulation	I-Algorithm
"	O
,	O
filed	O
in	O
1946	O
and	O
1952	O
,	O
granted	O
in	O
1956	O
.	O
</s>
<s>
The	O
three	O
of	O
them	O
published	O
"	O
The	O
Philosophy	O
of	O
PCM	B-Algorithm
"	O
in	O
1948	O
.	O
</s>
<s>
The	O
T-carrier	O
system	O
,	O
introduced	O
in	O
1961	O
,	O
uses	O
two	O
twisted-pair	O
transmission	O
lines	O
to	O
carry	O
24	O
PCM	B-Algorithm
telephone	O
calls	O
sampled	B-Algorithm
at	O
8kHz	O
and	O
8-bit	O
resolution	O
.	O
</s>
<s>
This	O
development	O
improved	O
capacity	O
and	O
call	O
quality	O
compared	O
to	O
the	O
previous	O
frequency-division	B-Protocol
multiplexing	I-Protocol
schemes	O
.	O
</s>
<s>
In	O
1973	O
,	O
adaptive	B-Algorithm
differential	I-Algorithm
pulse-code	I-Algorithm
modulation	I-Algorithm
(	O
ADPCM	B-Algorithm
)	O
was	O
developed	O
,	O
by	O
P	O
.	O
Cummiskey	O
,	O
Nikil	O
Jayant	O
and	O
James	O
L	O
.	O
Flanagan	O
.	O
</s>
<s>
In	O
1967	O
,	O
the	O
first	O
PCM	B-Algorithm
recorder	O
was	O
developed	O
by	O
NHK	O
's	O
research	O
facilities	O
in	O
Japan	O
.	O
</s>
<s>
The	O
30kHz	O
12-bit	O
device	O
used	O
a	O
compander	B-Algorithm
(	O
similar	O
to	O
DBX	O
Noise	O
Reduction	O
)	O
to	O
extend	O
the	O
dynamic	O
range	O
,	O
and	O
stored	O
the	O
signals	O
on	O
a	O
video	O
tape	O
recorder	O
.	O
</s>
<s>
In	O
January	O
1971	O
,	O
using	O
NHK	O
's	O
PCM	B-Algorithm
recording	O
system	O
,	O
engineers	O
at	O
Denon	O
recorded	O
the	O
first	O
commercial	O
digital	O
recordings	O
.	O
</s>
<s>
In	O
1972	O
,	O
Denon	O
unveiled	O
the	O
first	O
8-channel	O
digital	O
recorder	O
,	O
the	O
DN-023R	O
,	O
which	O
used	O
a	O
4-head	O
open	O
reel	O
broadcast	O
video	O
tape	O
recorder	O
to	O
record	O
in	O
47.25kHz	O
,	O
13-bit	O
PCM	B-Algorithm
audio	I-Algorithm
.	O
</s>
<s>
In	O
1977	O
,	O
Denon	O
developed	O
the	O
portable	O
PCM	B-Algorithm
recording	O
system	O
,	O
the	O
DN-034R	O
.	O
</s>
<s>
Like	O
the	O
DN-023R	O
,	O
it	O
recorded	O
8	O
channels	O
at	O
47.25kHz	O
,	O
but	O
it	O
used	O
14-bits	O
"	O
with	O
emphasis	B-Algorithm
,	O
making	O
it	O
equivalent	O
to	O
15.5	O
bits.	O
"	O
</s>
<s>
It	O
was	O
recorded	O
in	O
50kHz	O
,	O
16-bit	O
linear	B-Algorithm
PCM	I-Algorithm
using	O
a	O
3M	O
digital	O
tape	O
recorder	O
.	O
</s>
<s>
The	O
compact	O
disc	O
(	O
CD	O
)	O
brought	O
PCM	B-Algorithm
to	O
consumer	O
audio	O
applications	O
with	O
its	O
introduction	O
in	O
1982	O
.	O
</s>
<s>
The	O
CD	O
uses	O
a	O
44,100	O
Hz	O
sampling	B-Algorithm
frequency	I-Algorithm
and	O
16-bit	O
resolution	O
and	O
stores	O
up	O
to	O
80	O
minutes	O
of	O
stereo	O
audio	O
per	O
disc	O
.	O
</s>
<s>
The	O
rapid	O
development	O
and	O
wide	O
adoption	O
of	O
PCM	B-Algorithm
digital	B-General_Concept
telephony	I-General_Concept
was	O
enabled	O
by	O
metal	B-Architecture
–	I-Architecture
oxide	I-Architecture
–	I-Architecture
semiconductor	I-Architecture
(	O
MOS	O
)	O
switched	B-Algorithm
capacitor	I-Algorithm
(	O
SC	O
)	O
circuit	O
technology	O
,	O
developed	O
in	O
the	O
early	O
1970s	O
.	O
</s>
<s>
This	O
led	O
to	O
the	O
development	O
of	O
PCM	B-Algorithm
codec-filter	O
chips	O
in	O
the	O
late	O
1970s	O
.	O
</s>
<s>
The	O
silicon-gate	O
CMOS	B-Device
(	O
complementary	O
MOS	O
)	O
PCM	B-Algorithm
codec-filter	O
chip	O
,	O
developed	O
by	O
David	O
A	O
.	O
Hodges	O
and	O
W.C.	O
Black	O
in	O
1980	O
,	O
has	O
since	O
been	O
the	O
industry	O
standard	O
for	O
digital	B-General_Concept
telephony	I-General_Concept
.	O
</s>
<s>
By	O
the	O
1990s	O
,	O
telecommunication	O
networks	O
such	O
as	O
the	O
public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
had	O
been	O
largely	O
digitized	O
with	O
very-large-scale	O
integration	O
(	O
VLSI	O
)	O
CMOS	B-Device
PCM	B-Algorithm
codec-filters	O
,	O
widely	O
used	O
in	O
electronic	B-Device
switching	I-Device
systems	I-Device
for	O
telephone	O
exchanges	O
,	O
user-end	O
modems	B-Application
and	O
a	O
wide	O
range	O
of	O
digital	O
transmission	O
applications	O
such	O
as	O
the	O
integrated	B-Protocol
services	I-Protocol
digital	I-Protocol
network	I-Protocol
(	O
ISDN	B-Protocol
)	O
,	O
cordless	O
telephones	O
and	O
cell	O
phones	O
.	O
</s>
<s>
PCM	B-Algorithm
is	O
the	O
method	O
of	O
encoding	O
typically	O
used	O
for	O
uncompressed	O
digital	O
audio	O
.	O
</s>
<s>
The	O
4ESS	B-Device
switch	I-Device
introduced	O
time-division	B-Protocol
switching	I-Protocol
into	O
the	O
US	O
telephone	O
system	O
in	O
1976	O
,	O
based	O
on	O
medium	O
scale	O
integrated	O
circuit	O
technology	O
.	O
</s>
<s>
LPCM	B-Algorithm
is	O
used	O
for	O
the	O
lossless	O
encoding	O
of	O
audio	O
data	O
in	O
the	O
compact	O
disc	O
Red	O
Book	O
standard	O
(	O
informally	O
also	O
known	O
as	O
Audio	O
CD	O
)	O
,	O
introduced	O
in	O
1982	O
.	O
</s>
<s>
AES3	O
(	O
specified	O
in	O
1985	O
,	O
upon	O
which	O
S/PDIF	O
is	O
based	O
)	O
is	O
a	O
particular	O
format	O
using	O
LPCM	B-Algorithm
.	O
</s>
<s>
LaserDiscs	O
with	O
digital	O
sound	O
have	O
an	O
LPCM	B-Algorithm
track	O
on	O
the	O
digital	O
channel	O
.	O
</s>
<s>
On	O
PCs	O
,	O
PCM	B-Algorithm
and	O
LPCM	B-Algorithm
often	O
refer	O
to	O
the	O
format	O
used	O
in	O
WAV	O
(	O
defined	O
in	O
1991	O
)	O
and	O
AIFF	O
audio	O
container	O
formats	O
(	O
defined	O
in	O
1988	O
)	O
.	O
</s>
<s>
LPCM	B-Algorithm
data	O
may	O
also	O
be	O
stored	O
in	O
other	O
formats	O
such	O
as	O
AU	O
,	O
raw	O
audio	O
format	O
(	O
header-less	O
file	O
)	O
and	O
various	O
multimedia	O
container	O
formats	O
.	O
</s>
<s>
LPCM	B-Algorithm
has	O
been	O
defined	O
as	O
a	O
part	O
of	O
the	O
DVD	O
(	O
since	O
1995	O
)	O
and	O
Blu-ray	O
(	O
since	O
2006	O
)	O
standards	O
.	O
</s>
<s>
LPCM	B-Algorithm
is	O
used	O
by	O
HDMI	B-Protocol
(	O
defined	O
in	O
2002	O
)	O
,	O
a	O
single-cable	O
digital	O
audio/video	O
connector	O
interface	O
for	O
transmitting	O
uncompressed	O
digital	O
data	O
.	O
</s>
<s>
RF64	O
container	O
format	O
(	O
defined	O
in	O
2007	O
)	O
uses	O
LPCM	B-Algorithm
and	O
also	O
allows	O
non-PCM	O
bitstream	O
storage	O
:	O
various	O
compression	O
formats	O
contained	O
in	O
the	O
RF64	O
file	O
as	O
data	O
bursts	O
(	O
Dolby	O
E	O
,	O
Dolby	O
AC3	O
,	O
DTS	O
,	O
MPEG-1/MPEG	O
-2	O
Audio	O
)	O
can	O
be	O
"	O
disguised	O
"	O
as	O
PCM	B-Algorithm
linear	O
.	O
</s>
<s>
In	O
the	O
diagram	O
,	O
a	O
sine	O
wave	O
(	O
red	O
curve	O
)	O
is	O
sampled	B-Algorithm
and	O
quantized	B-Algorithm
for	O
PCM	B-Algorithm
.	O
</s>
<s>
The	O
sine	O
wave	O
is	O
sampled	B-Algorithm
at	O
regular	O
intervals	O
,	O
shown	O
as	O
vertical	O
lines	O
.	O
</s>
<s>
For	O
each	O
sample	B-Algorithm
,	O
one	O
of	O
the	O
available	O
values	O
(	O
on	O
the	O
y-axis	O
)	O
is	O
chosen	O
.	O
</s>
<s>
The	O
PCM	B-Algorithm
process	O
is	O
commonly	O
implemented	O
on	O
a	O
single	O
integrated	O
circuit	O
called	O
an	O
analog-to-digital	O
converter	O
(	O
ADC	O
)	O
.	O
</s>
<s>
Several	O
PCM	B-Algorithm
streams	O
could	O
also	O
be	O
multiplexed	B-Architecture
into	O
a	O
larger	O
aggregate	O
data	B-General_Concept
stream	I-General_Concept
,	O
generally	O
for	O
transmission	O
of	O
multiple	O
streams	O
over	O
a	O
single	O
physical	O
link	O
.	O
</s>
<s>
One	O
technique	O
is	O
called	O
time-division	B-Protocol
multiplexing	I-Protocol
(	O
TDM	O
)	O
and	O
is	O
widely	O
used	O
,	O
notably	O
in	O
the	O
modern	O
public	O
telephone	O
system	O
.	O
</s>
<s>
To	O
recover	O
the	O
original	O
signal	O
from	O
the	O
sampled	B-Algorithm
data	O
,	O
a	O
demodulator	O
can	O
apply	O
the	O
procedure	O
of	O
modulation	O
in	O
reverse	O
.	O
</s>
<s>
After	O
each	O
sampling	B-Algorithm
period	I-Algorithm
,	O
the	O
demodulator	O
reads	O
the	O
next	O
value	O
and	O
transitions	O
the	O
output	O
signal	O
to	O
the	O
new	O
value	O
.	O
</s>
<s>
Common	O
sample	B-Algorithm
depths	O
for	O
LPCM	B-Algorithm
are	O
8	O
,	O
16	O
,	O
20	O
or	O
24	O
bits	O
per	O
sample	B-Algorithm
.	O
</s>
<s>
LPCM	B-Algorithm
encodes	O
a	O
single	O
sound	O
channel	O
.	O
</s>
<s>
Support	O
for	O
multichannel	O
audio	O
depends	O
on	O
file	O
format	O
and	O
relies	O
on	O
synchronization	O
of	O
multiple	O
LPCM	B-Algorithm
streams	O
.	O
</s>
<s>
Common	O
sampling	B-Algorithm
frequencies	I-Algorithm
are	O
48	O
kHz	O
as	O
used	O
with	O
DVD	O
format	O
videos	O
,	O
or	O
44.1kHz	O
as	O
used	O
in	O
CDs	O
.	O
</s>
<s>
Sampling	B-Algorithm
frequencies	I-Algorithm
of	O
96kHz	O
or	O
192kHz	O
can	O
be	O
used	O
on	O
some	O
equipment	O
,	O
but	O
the	O
benefits	O
have	O
been	O
debated	O
.	O
</s>
<s>
The	O
Nyquist	O
–	O
Shannon	O
sampling	O
theorem	O
shows	O
PCM	B-Algorithm
devices	O
can	O
operate	O
without	O
introducing	O
distortions	O
within	O
their	O
designed	O
frequency	O
bands	O
if	O
they	O
provide	O
a	O
sampling	B-Algorithm
frequency	I-Algorithm
at	O
least	O
twice	O
that	O
of	O
the	O
highest	O
frequency	O
contained	O
in	O
the	O
input	O
signal	O
.	O
</s>
<s>
For	O
effective	O
reconstruction	O
of	O
the	O
voice	O
signal	O
,	O
telephony	O
applications	O
therefore	O
typically	O
use	O
an	O
8000Hz	O
sampling	B-Algorithm
frequency	I-Algorithm
which	O
is	O
more	O
than	O
twice	O
the	O
highest	O
usable	O
voice	O
frequency	O
.	O
</s>
<s>
Regardless	O
,	O
there	O
are	O
potential	O
sources	O
of	O
impairment	O
implicit	O
in	O
any	O
PCM	B-Algorithm
system	O
:	O
</s>
<s>
Choosing	O
a	O
discrete	O
value	O
that	O
is	O
near	O
but	O
not	O
exactly	O
at	O
the	O
analog	O
signal	O
level	O
for	O
each	O
sample	B-Algorithm
leads	O
to	O
quantization	B-Algorithm
error	O
.	O
</s>
<s>
Between	O
samples	O
no	O
measurement	O
of	O
the	O
signal	O
is	O
made	O
;	O
the	O
sampling	O
theorem	O
guarantees	O
non-ambiguous	O
representation	O
and	O
recovery	O
of	O
the	O
signal	O
only	O
if	O
it	O
has	O
no	O
energy	O
at	O
frequency	O
fs/2	O
or	O
higher	O
(	O
one	O
half	O
the	O
sampling	B-Algorithm
frequency	I-Algorithm
,	O
known	O
as	O
the	O
Nyquist	O
frequency	O
)	O
;	O
higher	O
frequencies	O
will	O
not	O
be	O
correctly	O
represented	O
or	O
recovered	O
and	O
add	O
aliasing	O
distortion	O
to	O
the	O
signal	O
below	O
the	O
Nyquist	O
frequency	O
.	O
</s>
<s>
Some	O
forms	O
of	O
PCM	B-Algorithm
combine	O
signal	O
processing	O
with	O
coding	O
.	O
</s>
<s>
Linear	B-Algorithm
PCM	I-Algorithm
(	O
LPCM	B-Algorithm
)	O
is	O
PCM	B-Algorithm
with	O
linear	O
quantization	B-Algorithm
.	O
</s>
<s>
Differential	B-General_Concept
PCM	I-General_Concept
(	O
DPCM	B-General_Concept
)	O
encodes	O
the	O
PCM	B-Algorithm
values	O
as	O
differences	O
between	O
the	O
current	O
and	O
the	O
predicted	O
value	O
.	O
</s>
<s>
An	O
algorithm	O
predicts	O
the	O
next	O
sample	B-Algorithm
based	O
on	O
the	O
previous	O
samples	O
,	O
and	O
the	O
encoder	O
stores	O
only	O
the	O
difference	O
between	O
this	O
prediction	O
and	O
the	O
actual	O
value	O
.	O
</s>
<s>
For	O
audio	O
,	O
this	O
type	O
of	O
encoding	O
reduces	O
the	O
number	O
of	O
bits	O
required	O
per	O
sample	B-Algorithm
by	O
about	O
25%	O
compared	O
to	O
PCM	B-Algorithm
.	O
</s>
<s>
Adaptive	B-Algorithm
differential	I-Algorithm
pulse-code	I-Algorithm
modulation	I-Algorithm
(	O
ADPCM	B-Algorithm
)	O
is	O
a	O
variant	O
of	O
DPCM	B-General_Concept
that	O
varies	O
the	O
size	O
of	O
the	O
quantization	B-Algorithm
step	O
,	O
to	O
allow	O
further	O
reduction	O
of	O
the	O
required	O
bandwidth	O
for	O
a	O
given	O
signal-to-noise	O
ratio	O
.	O
</s>
<s>
Delta	B-General_Concept
modulation	I-General_Concept
is	O
a	O
form	O
of	O
DPCM	B-General_Concept
that	O
uses	O
one	O
bit	O
per	O
sample	B-Algorithm
to	O
indicate	O
whether	O
the	O
signal	O
is	O
increasing	O
or	O
decreasing	O
compared	O
to	O
the	O
previous	O
sample	B-Algorithm
.	O
</s>
<s>
The	O
default	O
signal	O
compression	O
encoding	O
on	O
a	O
DS0	O
is	O
either	O
μ-law	B-Algorithm
(	O
mu-law	B-Algorithm
)	O
PCM	B-Algorithm
(	O
North	O
America	O
and	O
Japan	O
)	O
or	O
A-law	B-Algorithm
PCM	B-Algorithm
(	O
Europe	O
and	O
most	O
of	O
the	O
rest	O
of	O
the	O
world	O
)	O
.	O
</s>
<s>
These	O
are	O
logarithmic	O
compression	O
systems	O
where	O
a	O
12	O
-	O
or	O
13-bit	O
linear	B-Algorithm
PCM	I-Algorithm
sample	B-Algorithm
number	O
is	O
mapped	O
into	O
an	O
8-bit	O
value	O
.	O
</s>
<s>
This	O
system	O
is	O
described	O
by	O
international	O
standard	O
G.711	B-Algorithm
.	O
</s>
<s>
An	O
ADPCM	B-Algorithm
algorithm	O
is	O
used	O
to	O
map	O
a	O
series	O
of	O
8-bit	O
μ-law	B-Algorithm
or	O
A-law	B-Algorithm
PCM	B-Algorithm
samples	O
into	O
a	O
series	O
of	O
4-bit	O
ADPCM	B-Algorithm
samples	O
.	O
</s>
<s>
The	O
technique	O
is	O
detailed	O
in	O
the	O
G.726	B-Algorithm
standard	O
.	O
</s>
<s>
Audio	O
coding	O
formats	O
and	O
audio	B-Algorithm
codecs	I-Algorithm
have	O
been	O
developed	O
to	O
achieve	O
further	O
compression	O
.	O
</s>
<s>
Advanced	O
compression	O
techniques	O
,	O
such	O
as	O
modified	O
discrete	O
cosine	O
transform	O
(	O
MDCT	O
)	O
and	O
linear	B-Algorithm
predictive	I-Algorithm
coding	I-Algorithm
(	O
LPC	O
)	O
,	O
are	O
now	O
widely	O
used	O
in	O
mobile	O
phones	O
,	O
voice	B-Application
over	I-Application
IP	I-Application
(	O
VoIP	B-Application
)	O
and	O
streaming	O
media	O
.	O
</s>
<s>
PCM	B-Algorithm
can	O
be	O
either	O
return-to-zero	B-Protocol
(	O
RZ	O
)	O
or	O
non-return-to-zero	B-Protocol
(	O
NRZ	O
)	O
.	O
</s>
<s>
For	O
binary	O
PCM	B-Algorithm
systems	O
,	O
the	O
density	O
of	O
1-symbols	O
is	O
called	O
ones-density	O
.	O
</s>
<s>
Ones-density	O
is	O
often	O
controlled	O
using	O
precoding	O
techniques	O
such	O
as	O
run-length	B-Protocol
limited	I-Protocol
encoding	I-Protocol
,	O
where	O
the	O
PCM	B-Algorithm
code	O
is	O
expanded	O
into	O
a	O
slightly	O
longer	O
code	O
with	O
a	O
guaranteed	O
bound	O
on	O
ones-density	O
before	O
modulation	O
into	O
the	O
channel	O
.	O
</s>
<s>
In	O
other	O
cases	O
,	O
extra	O
framing	O
bits	O
are	O
added	O
into	O
the	O
stream	B-Architecture
,	O
which	O
guarantees	O
at	O
least	O
occasional	O
symbol	O
transitions	O
.	O
</s>
<s>
Another	O
technique	O
used	O
to	O
control	O
ones-density	O
is	O
the	O
use	O
of	O
a	O
scrambler	O
on	O
the	O
data	O
,	O
which	O
will	O
tend	O
to	O
turn	O
the	O
data	B-General_Concept
stream	I-General_Concept
into	O
a	O
stream	B-Architecture
that	O
looks	O
pseudo-random	B-Error_Name
,	O
but	O
where	O
the	O
data	O
can	O
be	O
recovered	O
exactly	O
by	O
a	O
complementary	O
descrambler	O
.	O
</s>
<s>
Many	O
of	O
these	O
codes	O
are	O
bipolar	B-Protocol
codes	I-Protocol
,	O
where	O
the	O
pulses	O
can	O
be	O
positive	O
,	O
negative	O
or	O
absent	O
.	O
</s>
<s>
The	O
word	O
pulse	O
in	O
the	O
term	O
pulse-code	B-Algorithm
modulation	I-Algorithm
refers	O
to	O
the	O
pulses	O
to	O
be	O
found	O
in	O
the	O
transmission	O
line	O
.	O
</s>
<s>
This	O
perhaps	O
is	O
a	O
natural	O
consequence	O
of	O
this	O
technique	O
having	O
evolved	O
alongside	O
two	O
analog	O
methods	O
,	O
pulse-width	B-Algorithm
modulation	I-Algorithm
and	O
pulse-position	B-Device
modulation	I-Device
,	O
in	O
which	O
the	O
information	O
to	O
be	O
encoded	O
is	O
represented	O
by	O
discrete	O
signal	O
pulses	O
of	O
varying	O
width	O
or	O
position	O
,	O
respectively	O
.	O
</s>
<s>
In	O
this	O
respect	O
,	O
PCM	B-Algorithm
bears	O
little	O
resemblance	O
to	O
these	O
other	O
forms	O
of	O
signal	O
encoding	O
,	O
except	O
that	O
all	O
can	O
be	O
used	O
in	O
time-division	B-Protocol
multiplexing	I-Protocol
,	O
and	O
the	O
numbers	O
of	O
the	O
PCM	B-Algorithm
codes	O
are	O
represented	O
as	O
electrical	O
pulses	O
.	O
</s>
