<s>
Demodulation	B-Device
is	O
extracting	O
the	O
original	O
information-bearing	O
signal	O
from	O
a	O
carrier	O
wave	O
.	O
</s>
<s>
A	O
demodulator	B-Device
is	O
an	O
electronic	O
circuit	O
(	O
or	O
computer	B-Application
program	I-Application
in	O
a	O
software-defined	B-Architecture
radio	I-Architecture
)	O
that	O
is	O
used	O
to	O
recover	O
the	O
information	O
content	O
from	O
the	O
modulated	B-Application
carrier	O
wave	O
.	O
</s>
<s>
There	O
are	O
many	O
types	O
of	O
modulation	B-Application
so	O
there	O
are	O
many	O
types	O
of	O
demodulators	B-Device
.	O
</s>
<s>
The	O
signal	O
output	O
from	O
a	O
demodulator	B-Device
may	O
represent	O
sound	O
(	O
an	O
analog	O
audio	O
signal	O
)	O
,	O
images	O
(	O
an	O
analog	O
video	B-Application
signal	I-Application
)	O
or	O
binary	O
data	O
(	O
a	O
digital	O
signal	O
)	O
.	O
</s>
<s>
These	O
terms	O
are	O
traditionally	O
used	O
in	O
connection	O
with	O
radio	O
receivers	O
,	O
but	O
many	O
other	O
systems	O
use	O
many	O
kinds	O
of	O
demodulators	B-Device
.	O
</s>
<s>
For	O
example	O
,	O
in	O
a	O
modem	B-Application
,	O
which	O
is	O
a	O
contraction	O
of	O
the	O
terms	O
modulator/demodulator	O
,	O
a	O
demodulator	B-Device
is	O
used	O
to	O
extract	O
a	O
serial	O
digital	O
data	O
stream	O
from	O
a	O
carrier	O
signal	O
which	O
is	O
used	O
to	O
carry	O
it	O
through	O
a	O
telephone	O
line	O
,	O
coaxial	O
cable	O
,	O
or	O
optical	B-Architecture
fiber	I-Architecture
.	O
</s>
<s>
Demodulation	B-Device
was	O
first	O
used	O
in	O
radio	O
receivers	O
.	O
</s>
<s>
The	O
device	O
that	O
did	O
this	O
was	O
called	O
a	O
detector	B-Algorithm
.	O
</s>
<s>
The	O
first	O
detectors	B-Algorithm
were	O
coherers	O
,	O
simple	O
devices	O
that	O
acted	O
as	O
a	O
switch	O
.	O
</s>
<s>
The	O
term	O
detector	B-Algorithm
stuck	O
,	O
was	O
used	O
for	O
other	O
types	O
of	O
demodulators	B-Device
and	O
continues	O
to	O
be	O
used	O
to	O
the	O
present	O
day	O
for	O
a	O
demodulator	B-Device
in	O
a	O
radio	O
receiver	O
.	O
</s>
<s>
The	O
first	O
type	O
of	O
modulation	B-Application
used	O
to	O
transmit	O
sound	O
over	O
radio	O
waves	O
was	O
amplitude	B-Device
modulation	I-Device
(	O
AM	B-Device
)	O
,	O
invented	O
by	O
Reginald	O
Fessenden	O
around	O
1900	O
.	O
</s>
<s>
An	O
AM	B-Device
radio	O
signal	O
can	O
be	O
demodulated	O
by	O
rectifying	O
it	O
to	O
remove	O
one	O
side	O
of	O
the	O
carrier	O
,	O
and	O
then	O
filtering	O
to	O
remove	O
the	O
radio-frequency	O
component	O
,	O
leaving	O
only	O
the	O
modulating	O
audio	O
component	O
.	O
</s>
<s>
The	O
amplitude	B-Application
of	O
the	O
recovered	O
audio	O
frequency	O
varies	O
with	O
the	O
modulating	O
audio	O
signal	O
,	O
so	O
it	O
can	O
drive	O
an	O
earphone	O
or	O
an	O
audio	O
amplifier	O
.	O
</s>
<s>
Fessendon	O
invented	O
the	O
first	O
AM	B-Device
demodulator	B-Device
in	O
1904	O
called	O
the	O
electrolytic	O
detector	B-Algorithm
,	O
consisting	O
of	O
a	O
short	O
needle	O
dipping	O
into	O
a	O
cup	O
of	O
dilute	O
acid	O
.	O
</s>
<s>
The	O
same	O
year	O
John	O
Ambrose	O
Fleming	O
invented	O
the	O
Fleming	O
valve	O
or	O
thermionic	O
diode	O
which	O
could	O
also	O
rectify	O
an	O
AM	B-Device
signal	O
.	O
</s>
<s>
There	O
are	O
several	O
ways	O
of	O
demodulation	B-Device
depending	O
on	O
how	O
parameters	O
of	O
the	O
base-band	O
signal	O
such	O
as	O
amplitude	B-Application
,	O
frequency	O
or	O
phase	O
are	O
transmitted	O
in	O
the	O
carrier	O
signal	O
.	O
</s>
<s>
For	O
example	O
,	O
for	O
a	O
signal	O
modulated	B-Application
with	O
a	O
linear	O
modulation	B-Application
like	O
AM	B-Device
(	O
amplitude	B-Device
modulation	I-Device
)	O
,	O
we	O
can	O
use	O
a	O
synchronous	O
detector	B-Algorithm
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
for	O
a	O
signal	O
modulated	B-Application
with	O
an	O
angular	O
modulation	B-Application
,	O
we	O
must	O
use	O
an	O
FM	B-Application
(	O
frequency	B-Application
modulation	I-Application
)	O
demodulator	B-Device
or	O
a	O
PM	O
(	O
phase	B-Device
modulation	I-Device
)	O
demodulator	B-Device
.	O
</s>
<s>
Many	O
techniques	O
such	O
as	O
carrier	O
recovery	O
,	O
clock	O
recovery	O
,	O
bit	B-General_Concept
slip	I-General_Concept
,	O
frame	O
synchronization	O
,	O
rake	O
receiver	O
,	O
pulse	B-Algorithm
compression	I-Algorithm
,	O
Received	O
Signal	O
Strength	O
Indication	O
,	O
error	B-Error_Name
detection	I-Error_Name
and	I-Error_Name
correction	I-Error_Name
,	O
etc.	O
,	O
are	O
only	O
performed	O
by	O
demodulators	B-Device
,	O
although	O
any	O
specific	O
demodulator	B-Device
may	O
perform	O
only	O
some	O
or	O
none	O
of	O
these	O
techniques	O
.	O
</s>
<s>
Many	O
things	O
can	O
act	O
as	O
a	O
demodulator	B-Device
,	O
if	O
they	O
pass	O
the	O
radio	O
waves	O
on	O
nonlinearly	O
.	O
</s>
<s>
An	O
AM	B-Device
signal	O
encodes	O
the	O
information	O
into	O
the	O
carrier	O
wave	O
by	O
varying	O
its	O
amplitude	B-Application
in	O
direct	O
sympathy	O
with	O
the	O
analogue	O
signal	O
to	O
be	O
sent	O
.	O
</s>
<s>
There	O
are	O
two	O
methods	O
used	O
to	O
demodulate	B-Algorithm
AM	I-Algorithm
signals	I-Algorithm
:	O
</s>
<s>
The	O
envelope	B-Algorithm
detector	I-Algorithm
is	O
a	O
very	O
simple	O
method	O
of	O
demodulation	B-Device
that	O
does	O
not	O
require	O
a	O
coherent	O
demodulator	B-Device
.	O
</s>
<s>
It	O
consists	O
of	O
an	O
envelope	B-Algorithm
detector	I-Algorithm
that	O
can	O
be	O
a	O
rectifier	B-Algorithm
(	O
anything	O
that	O
will	O
pass	O
current	O
in	O
one	O
direction	O
only	O
)	O
or	O
other	O
non-linear	O
component	O
that	O
enhances	O
one	O
half	O
of	O
the	O
received	O
signal	O
over	O
the	O
other	O
and	O
a	O
low-pass	B-Algorithm
filter	I-Algorithm
.	O
</s>
<s>
The	O
rectifier	B-Algorithm
may	O
be	O
in	O
the	O
form	O
of	O
a	O
single	O
diode	O
or	O
may	O
be	O
more	O
complex	O
.	O
</s>
<s>
Many	O
natural	O
substances	O
exhibit	O
this	O
rectification	O
behaviour	O
,	O
which	O
is	O
why	O
it	O
was	O
the	O
earliest	O
modulation	B-Application
and	O
demodulation	B-Device
technique	O
used	O
in	O
radio	O
.	O
</s>
<s>
The	O
filter	O
is	O
usually	O
an	O
RC	B-Algorithm
low-pass	B-Algorithm
type	O
but	O
the	O
filter	O
function	O
can	O
sometimes	O
be	O
achieved	O
by	O
relying	O
on	O
the	O
limited	O
frequency	O
response	O
of	O
the	O
circuitry	O
following	O
the	O
rectifier	B-Algorithm
.	O
</s>
<s>
The	O
crystal	O
set	O
exploits	O
the	O
simplicity	O
of	O
AM	B-Device
modulation	B-Application
to	O
produce	O
a	O
receiver	O
with	O
very	O
few	O
parts	O
,	O
using	O
the	O
crystal	O
as	O
the	O
rectifier	B-Algorithm
and	O
the	O
limited	O
frequency	O
response	O
of	O
the	O
headphones	O
as	O
the	O
filter	O
.	O
</s>
<s>
The	O
product	O
detector	B-Algorithm
multiplies	O
the	O
incoming	O
signal	O
by	O
the	O
signal	O
of	O
a	O
local	O
oscillator	O
with	O
the	O
same	O
frequency	O
and	O
phase	O
as	O
the	O
carrier	O
of	O
the	O
incoming	O
signal	O
.	O
</s>
<s>
SSB	B-Device
is	O
a	O
form	O
of	O
AM	B-Device
in	O
which	O
the	O
carrier	B-Device
is	I-Device
reduced	I-Device
or	I-Device
suppressed	I-Device
entirely	I-Device
,	O
which	O
require	O
coherent	O
demodulation	B-Device
.	O
</s>
<s>
Frequency	B-Application
modulation	I-Application
(	O
FM	B-Application
)	O
has	O
numerous	O
advantages	O
over	O
AM	B-Device
such	O
as	O
better	O
fidelity	O
and	O
noise	O
immunity	O
.	O
</s>
<s>
However	O
,	O
it	O
is	O
much	O
more	O
complex	O
to	O
both	O
modulate	O
and	O
demodulate	B-Device
a	O
carrier	O
wave	O
with	O
FM	B-Application
,	O
and	O
AM	B-Device
predates	O
it	O
by	O
several	O
decades	O
.	O
</s>
<s>
There	O
are	O
several	O
common	O
types	O
of	O
FM	B-Application
demodulators	B-Device
:	O
</s>
<s>
The	O
quadrature	B-Algorithm
detector	I-Algorithm
,	O
which	O
phase	O
shifts	O
the	O
signal	O
by	O
90	O
degrees	O
and	O
multiplies	O
it	O
with	O
the	O
unshifted	O
version	O
.	O
</s>
<s>
The	O
most	O
common	O
is	O
a	O
Foster	B-Device
–	I-Device
Seeley	I-Device
discriminator	I-Device
.	O
</s>
<s>
This	O
is	O
composed	O
of	O
an	O
electronic	B-Architecture
filter	I-Architecture
which	O
decreases	O
the	O
amplitude	B-Application
of	O
some	O
frequencies	O
relative	O
to	O
others	O
,	O
followed	O
by	O
an	O
AM	B-Device
demodulator	B-Device
.	O
</s>
<s>
Another	O
method	O
uses	O
two	O
AM	B-Device
demodulators	B-Device
,	O
one	O
tuned	O
to	O
the	O
high	O
end	O
of	O
the	O
band	O
and	O
the	O
other	O
to	O
the	O
low	O
end	O
,	O
and	O
feed	O
the	O
outputs	O
into	O
a	O
difference	O
amplifier	O
.	O
</s>
<s>
Using	O
a	O
digital	B-Architecture
signal	I-Architecture
processor	I-Architecture
,	O
as	O
used	O
in	O
software-defined	B-Architecture
radio	I-Architecture
.	O
</s>
<s>
QAM	O
demodulation	B-Device
requires	O
a	O
coherent	O
receiver	O
.	O
</s>
