<s>
Frequency	B-Application
modulation	I-Application
(	O
FM	B-Application
)	O
is	O
the	O
encoding	O
of	O
information	O
in	O
a	O
carrier	O
wave	O
by	O
varying	O
the	O
instantaneous	O
frequency	O
of	O
the	O
wave	O
.	O
</s>
<s>
The	O
technology	O
is	O
used	O
in	O
telecommunications	O
,	O
radio	O
broadcasting	B-Operating_System
,	O
signal	O
processing	O
,	O
and	O
computing	O
.	O
</s>
<s>
In	O
analog	B-Application
frequency	I-Application
modulation	I-Application
,	O
such	O
as	O
radio	O
broadcasting	B-Operating_System
,	O
of	O
an	O
audio	O
signal	O
representing	O
voice	O
or	O
music	O
,	O
the	O
instantaneous	O
frequency	B-Device
deviation	I-Device
,	O
i.e.	O
</s>
<s>
the	O
difference	O
between	O
the	O
frequency	O
of	O
the	O
carrier	O
and	O
its	O
center	O
frequency	O
,	O
has	O
a	O
functional	O
relation	O
to	O
the	O
modulating	B-Application
signal	I-Application
amplitude	B-Application
.	O
</s>
<s>
Digital	B-General_Concept
data	I-General_Concept
can	O
be	O
encoded	O
and	O
transmitted	O
with	O
a	O
type	O
of	O
frequency	B-Application
modulation	I-Application
known	O
as	O
frequency-shift	O
keying	O
(	O
FSK	O
)	O
,	O
in	O
which	O
the	O
instantaneous	O
frequency	O
of	O
the	O
carrier	O
is	O
shifted	O
among	O
a	O
set	O
of	O
frequencies	O
.	O
</s>
<s>
FSK	O
is	O
widely	O
used	O
in	O
computer	B-Application
modems	I-Application
,	O
such	O
as	O
fax	B-Device
modems	I-Device
,	O
telephone	O
caller	O
ID	O
systems	O
,	O
garage	O
door	O
openers	O
,	O
and	O
other	O
low-frequency	O
transmissions	O
.	O
</s>
<s>
Frequency	B-Application
modulation	I-Application
is	O
widely	O
used	O
for	O
FM	B-Application
radio	O
broadcasting	B-Operating_System
.	O
</s>
<s>
It	O
is	O
also	O
used	O
in	O
telemetry	O
,	O
radar	B-Application
,	O
seismic	O
prospecting	O
,	O
and	O
monitoring	O
newborns	O
for	O
seizures	O
via	O
EEG	B-Application
,	O
two-way	O
radio	O
systems	O
,	O
sound	O
synthesis	O
,	O
magnetic	O
tape-recording	O
systems	O
and	O
some	O
video-transmission	O
systems	O
.	O
</s>
<s>
In	O
radio	O
transmission	O
,	O
an	O
advantage	O
of	O
frequency	B-Application
modulation	I-Application
is	O
that	O
it	O
has	O
a	O
larger	O
signal-to-noise	O
ratio	O
and	O
therefore	O
rejects	O
radio	O
frequency	O
interference	O
better	O
than	O
an	O
equal	O
power	O
amplitude	B-Device
modulation	I-Device
(	O
AM	B-Device
)	O
signal	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
most	O
music	O
is	O
broadcast	B-Operating_System
over	O
FM	B-Application
radio	O
.	O
</s>
<s>
However	O
,	O
under	O
severe	O
enough	O
multipath	O
conditions	O
it	O
performs	O
much	O
more	O
poorly	O
than	O
AM	B-Device
,	O
with	O
distinct	O
high	O
frequency	O
noise	O
artifacts	O
that	O
are	O
audible	O
with	O
lower	O
volumes	O
and	O
less	O
complex	O
tones	O
.	O
</s>
<s>
With	O
high	O
enough	O
volume	O
and	O
carrier	O
deviation	O
audio	O
distortion	O
starts	O
to	O
occur	O
that	O
otherwise	O
would	O
n't	O
be	O
present	O
without	O
multipath	O
or	O
with	O
an	O
AM	B-Device
signal	O
.	O
</s>
<s>
Frequency	B-Application
modulation	I-Application
and	O
phase	B-Device
modulation	I-Device
are	O
the	O
two	O
complementary	O
principal	O
methods	O
of	O
angle	B-Device
modulation	I-Device
;	O
phase	B-Device
modulation	I-Device
is	O
often	O
used	O
as	O
an	O
intermediate	O
step	O
to	O
achieve	O
frequency	B-Application
modulation	I-Application
.	O
</s>
<s>
These	O
methods	O
contrast	O
with	O
amplitude	B-Device
modulation	I-Device
,	O
in	O
which	O
the	O
amplitude	B-Application
of	O
the	O
carrier	O
wave	O
varies	O
,	O
while	O
the	O
frequency	O
and	O
phase	O
remain	O
constant	O
.	O
</s>
<s>
If	O
the	O
information	O
to	O
be	O
transmitted	O
(	O
i.e.	O
,	O
the	O
baseband	B-Algorithm
signal	I-Algorithm
)	O
is	O
and	O
the	O
sinusoidal	O
carrier	O
is	O
,	O
where	O
fc	O
is	O
the	O
carrier	O
's	O
base	O
frequency	O
,	O
and	O
Ac	O
is	O
the	O
carrier	O
's	O
amplitude	B-Application
,	O
the	O
modulator	B-Application
combines	O
the	O
carrier	O
with	O
the	O
baseband	B-Algorithm
data	O
signal	O
to	O
get	O
the	O
transmitted	O
signal	O
:	O
</s>
<s>
where	O
,	O
being	O
the	O
sensitivity	O
of	O
the	O
frequency	B-Application
modulator	I-Application
and	O
being	O
the	O
amplitude	B-Application
of	O
the	O
modulating	B-Application
signal	I-Application
or	O
baseband	B-Algorithm
signal	I-Algorithm
.	O
</s>
<s>
In	O
this	O
equation	O
,	O
is	O
the	O
instantaneous	O
frequency	O
of	O
the	O
oscillator	O
and	O
is	O
the	O
frequency	B-Device
deviation	I-Device
,	O
which	O
represents	O
the	O
maximum	O
shift	O
away	O
from	O
fc	O
in	O
one	O
direction	O
,	O
assuming	O
xm(t )	O
is	O
limited	O
to	O
the	O
range	O
±1	O
.	O
</s>
<s>
While	O
most	O
of	O
the	O
energy	O
of	O
the	O
signal	O
is	O
contained	O
within	O
fc	O
±	O
fΔ	O
,	O
it	O
can	O
be	O
shown	O
by	O
Fourier	O
analysis	O
that	O
a	O
wider	O
range	O
of	O
frequencies	O
is	O
required	O
to	O
precisely	O
represent	O
an	O
FM	B-Application
signal	O
.	O
</s>
<s>
The	O
frequency	O
spectrum	O
of	O
an	O
actual	O
FM	B-Application
signal	O
has	O
components	O
extending	O
infinitely	O
,	O
although	O
their	O
amplitude	B-Application
decreases	O
and	O
higher-order	O
components	O
are	O
often	O
neglected	O
in	O
practical	O
design	O
problems	O
.	O
</s>
<s>
Mathematically	O
,	O
a	O
baseband	B-Algorithm
modulating	B-Application
signal	I-Application
may	O
be	O
approximated	O
by	O
a	O
sinusoidal	O
continuous	O
wave	O
signal	O
with	O
a	O
frequency	O
fm	B-Application
.	O
</s>
<s>
This	O
method	O
is	O
also	O
named	O
as	O
single-tone	O
modulation	B-Application
.	O
</s>
<s>
where	O
the	O
amplitude	B-Application
of	O
the	O
modulating	O
sinusoid	O
is	O
represented	O
in	O
the	O
peak	O
deviation	O
(	O
see	O
frequency	B-Device
deviation	I-Device
)	O
.	O
</s>
<s>
The	O
harmonic	O
distribution	O
of	O
a	O
sine	O
wave	O
carrier	O
modulated	B-Application
by	O
such	O
a	O
sinusoidal	O
signal	O
can	O
be	O
represented	O
with	O
Bessel	O
functions	O
;	O
this	O
provides	O
the	O
basis	O
for	O
a	O
mathematical	O
understanding	O
of	O
frequency	B-Application
modulation	I-Application
in	O
the	O
frequency	O
domain	O
.	O
</s>
<s>
As	O
in	O
other	O
modulation	B-Application
systems	O
,	O
the	O
modulation	B-Application
index	O
indicates	O
by	O
how	O
much	O
the	O
modulated	B-Application
variable	O
varies	O
around	O
its	O
unmodulated	O
level	O
.	O
</s>
<s>
where	O
is	O
the	O
highest	O
frequency	O
component	O
present	O
in	O
the	O
modulating	B-Application
signal	I-Application
xm(t )	O
,	O
and	O
is	O
the	O
peak	O
frequency-deviation	O
—	O
i.e.	O
</s>
<s>
For	O
a	O
sine	O
wave	O
modulation	B-Application
,	O
the	O
modulation	B-Application
index	O
is	O
seen	O
to	O
be	O
the	O
ratio	O
of	O
the	O
peak	O
frequency	B-Device
deviation	I-Device
of	O
the	O
carrier	O
wave	O
to	O
the	O
frequency	O
of	O
the	O
modulating	O
sine	O
wave	O
.	O
</s>
<s>
If	O
,	O
the	O
modulation	B-Application
is	O
called	O
narrowband	O
FM	B-Application
(	O
NFM	O
)	O
,	O
and	O
its	O
bandwidth	B-Algorithm
is	O
approximately	O
.	O
</s>
<s>
Sometimes	O
modulation	B-Application
index	O
is	O
considered	O
as	O
NFM	O
,	O
otherwise	O
wideband	B-Application
FM	I-Application
(	O
WFM	O
or	O
FM	B-Application
)	O
.	O
</s>
<s>
For	O
digital	O
modulation	B-Application
systems	O
,	O
for	O
example	O
binary	O
frequency	O
shift	O
keying	O
(	O
BFSK	O
)	O
,	O
where	O
a	O
binary	O
signal	O
modulates	O
the	O
carrier	O
,	O
the	O
modulation	B-Application
index	O
is	O
given	O
by	O
:	O
</s>
<s>
In	O
the	O
case	O
of	O
digital	O
modulation	B-Application
,	O
the	O
carrier	O
is	O
never	O
transmitted	O
.	O
</s>
<s>
Rather	O
,	O
one	O
of	O
two	O
frequencies	O
is	O
transmitted	O
,	O
either	O
or	O
,	O
depending	O
on	O
the	O
binary	O
state	O
0	O
or	O
1	O
of	O
the	O
modulation	B-Application
signal	O
.	O
</s>
<s>
If	O
,	O
the	O
modulation	B-Application
is	O
called	O
wideband	B-Application
FM	I-Application
and	O
its	O
bandwidth	B-Algorithm
is	O
approximately	O
.	O
</s>
<s>
While	O
wideband	B-Application
FM	I-Application
uses	O
more	O
bandwidth	B-Algorithm
,	O
it	O
can	O
improve	O
the	O
signal-to-noise	O
ratio	O
significantly	O
;	O
for	O
example	O
,	O
doubling	O
the	O
value	O
of	O
,	O
while	O
keeping	O
constant	O
,	O
results	O
in	O
an	O
eight-fold	O
improvement	O
in	O
the	O
signal-to-noise	O
ratio	O
.	O
</s>
<s>
(	O
Compare	O
this	O
with	O
chirp	B-Protocol
spread	I-Protocol
spectrum	I-Protocol
,	O
which	O
uses	O
extremely	O
wide	O
frequency	B-Device
deviations	I-Device
to	O
achieve	O
processing	O
gains	O
comparable	O
to	O
traditional	O
,	O
better-known	O
spread-spectrum	O
modes	O
)	O
.	O
</s>
<s>
With	O
a	O
tone-modulated	O
FMwave	O
,	O
if	O
the	O
modulation	B-Application
frequency	O
is	O
held	O
constant	O
and	O
the	O
modulation	B-Application
index	O
is	O
increased	O
,	O
the	O
(	O
non-negligible	O
)	O
bandwidth	B-Algorithm
of	O
the	O
FM	B-Application
signal	O
increases	O
but	O
the	O
spacing	O
between	O
spectra	O
remains	O
the	O
same	O
;	O
some	O
spectral	O
components	O
decrease	O
in	O
strength	O
as	O
others	O
increase	O
.	O
</s>
<s>
If	O
the	O
frequency	B-Device
deviation	I-Device
is	O
held	O
constant	O
and	O
the	O
modulation	B-Application
frequency	O
increased	O
,	O
the	O
spacing	O
between	O
spectra	O
increases	O
.	O
</s>
<s>
Frequency	B-Application
modulation	I-Application
can	O
be	O
classified	O
as	O
narrowband	O
if	O
the	O
change	O
in	O
the	O
carrier	O
frequency	O
is	O
about	O
the	O
same	O
as	O
the	O
signal	O
frequency	O
,	O
or	O
as	O
wideband	O
if	O
the	O
change	O
in	O
the	O
carrier	O
frequency	O
is	O
much	O
higher	O
(	O
modulation	B-Application
index>1	O
)	O
than	O
the	O
signal	O
frequency	O
.	O
</s>
<s>
For	O
example	O
,	O
narrowband	O
FM	B-Application
(	O
NFM	O
)	O
is	O
used	O
for	O
two-way	O
radio	O
systems	O
such	O
as	O
Family	O
Radio	O
Service	O
,	O
in	O
which	O
the	O
carrier	O
is	O
allowed	O
to	O
deviate	O
only	O
2.5kHz	O
above	O
and	O
below	O
the	O
center	O
frequency	O
with	O
speech	O
signals	O
of	O
no	O
more	O
than	O
3.5kHz	O
bandwidth	B-Algorithm
.	O
</s>
<s>
Wideband	B-Application
FM	I-Application
is	O
used	O
for	O
FM	B-Application
broadcasting	B-Operating_System
,	O
in	O
which	O
music	O
and	O
speech	O
are	O
transmitted	O
with	O
up	O
to	O
75kHz	O
deviation	O
from	O
the	O
center	O
frequency	O
and	O
carry	O
audio	O
with	O
up	O
to	O
a	O
20kHz	O
bandwidth	B-Algorithm
and	O
subcarriers	O
up	O
to	O
92kHz	O
.	O
</s>
<s>
For	O
the	O
case	O
of	O
a	O
carrier	O
modulated	B-Application
by	O
a	O
single	O
sine	O
wave	O
,	O
the	O
resulting	O
frequency	O
spectrum	O
can	O
be	O
calculated	O
using	O
Bessel	O
functions	O
of	O
the	O
first	O
kind	O
,	O
as	O
a	O
function	O
of	O
the	O
sideband	O
number	O
and	O
the	O
modulation	B-Application
index	O
.	O
</s>
<s>
The	O
carrier	O
and	O
sideband	O
amplitudes	B-Application
are	O
illustrated	O
for	O
different	O
modulation	B-Application
indices	O
of	O
FM	B-Application
signals	O
.	O
</s>
<s>
For	O
particular	O
values	O
of	O
the	O
modulation	B-Application
index	O
,	O
the	O
carrier	O
amplitude	B-Application
becomes	O
zero	O
and	O
all	O
the	O
signal	O
power	O
is	O
in	O
the	O
sidebands	O
.	O
</s>
<s>
Since	O
the	O
sidebands	O
are	O
on	O
both	O
sides	O
of	O
the	O
carrier	O
,	O
their	O
count	O
is	O
doubled	O
,	O
and	O
then	O
multiplied	O
by	O
the	O
modulating	O
frequency	O
to	O
find	O
the	O
bandwidth	B-Algorithm
.	O
</s>
<s>
For	O
example	O
,	O
3kHz	O
deviation	O
modulated	B-Application
by	O
a	O
2.2kHz	O
audio	O
tone	O
produces	O
a	O
modulation	B-Application
index	O
of	O
1.36	O
.	O
</s>
<s>
Suppose	O
that	O
we	O
limit	O
ourselves	O
to	O
only	O
those	O
sidebands	O
that	O
have	O
a	O
relative	O
amplitude	B-Application
of	O
at	O
least	O
0.01	O
.	O
</s>
<s>
Then	O
,	O
examining	O
the	O
chart	O
shows	O
this	O
modulation	B-Application
index	O
will	O
produce	O
three	O
sidebands	O
.	O
</s>
<s>
These	O
three	O
sidebands	O
,	O
when	O
doubled	O
,	O
gives	O
us	O
(	O
6	O
×	O
2.2kHz	O
)	O
or	O
a	O
13.2kHz	O
required	O
bandwidth	B-Algorithm
.	O
</s>
<s>
A	O
rule	O
of	O
thumb	O
,	O
Carson	O
's	O
rule	O
states	O
that	O
nearly	O
all	O
( ≈98	O
percent	O
)	O
of	O
the	O
power	O
of	O
a	O
frequency-modulated	B-Application
signal	O
lies	O
within	O
a	O
bandwidth	B-Algorithm
of	O
:	O
</s>
<s>
where	O
,	O
as	O
defined	O
above	O
,	O
is	O
the	O
peak	O
deviation	O
of	O
the	O
instantaneous	O
frequency	O
from	O
the	O
center	O
carrier	O
frequency	O
,	O
is	O
the	O
Modulation	B-Application
index	O
which	O
is	O
the	O
ratio	O
of	O
frequency	B-Device
deviation	I-Device
to	O
highest	O
frequency	O
in	O
the	O
modulating	B-Application
signal	I-Application
and	O
is	O
the	O
highest	O
frequency	O
in	O
the	O
modulating	B-Application
signal	I-Application
.	O
</s>
<s>
where	O
W	O
is	O
the	O
highest	O
frequency	O
in	O
the	O
modulating	B-Application
signal	I-Application
but	O
non-sinusoidal	O
in	O
nature	O
and	O
D	O
is	O
the	O
Deviation	O
ratio	O
which	O
the	O
ratio	O
of	O
frequency	B-Device
deviation	I-Device
to	O
highest	O
frequency	O
of	O
modulating	O
non-sinusoidal	O
signal	O
.	O
</s>
<s>
FM	B-Application
provides	O
improved	O
signal-to-noise	O
ratio	O
(	O
SNR	O
)	O
,	O
as	O
compared	O
for	O
example	O
with	O
AM	B-Device
.	O
</s>
<s>
Compared	O
with	O
an	O
optimum	O
AM	B-Device
scheme	O
,	O
FM	B-Application
typically	O
has	O
poorer	O
SNR	O
below	O
a	O
certain	O
signal	O
level	O
called	O
the	O
noise	O
threshold	O
,	O
but	O
above	O
a	O
higher	O
level	O
–	O
the	O
full	O
improvement	O
or	O
full	O
quieting	O
threshold	O
–	O
the	O
SNR	O
is	O
much	O
improved	O
over	O
AM	B-Device
.	O
</s>
<s>
The	O
improvement	O
depends	O
on	O
modulation	B-Application
level	O
and	O
deviation	O
.	O
</s>
<s>
FM	B-Application
broadcasting	B-Operating_System
using	O
wider	O
deviation	O
can	O
achieve	O
even	O
greater	O
improvements	O
.	O
</s>
<s>
Additional	O
techniques	O
,	O
such	O
as	O
pre-emphasis	O
of	O
higher	O
audio	O
frequencies	O
with	O
corresponding	O
de-emphasis	O
in	O
the	O
receiver	B-Device
,	O
are	O
generally	O
used	O
to	O
improve	O
overall	O
SNR	O
in	O
FM	B-Application
circuits	O
.	O
</s>
<s>
Since	O
FM	B-Application
signals	O
have	O
constant	O
amplitude	B-Application
,	O
FM	B-Application
receivers	B-Device
normally	O
have	O
limiters	O
that	O
remove	O
AM	B-Device
noise	O
,	O
further	O
improving	O
SNR	O
.	O
</s>
<s>
FM	B-Application
signals	O
can	O
be	O
generated	O
using	O
either	O
direct	O
or	O
indirect	O
frequency	B-Application
modulation	I-Application
:	O
</s>
<s>
Direct	O
FM	B-Application
modulation	B-Application
can	O
be	O
achieved	O
by	O
directly	O
feeding	O
the	O
message	O
into	O
the	O
input	O
of	O
a	O
voltage-controlled	O
oscillator	O
.	O
</s>
<s>
For	O
indirect	O
FM	B-Application
modulation	B-Application
,	O
the	O
message	O
signal	O
is	O
integrated	O
to	O
generate	O
a	O
phase-modulated	B-Device
signal	I-Device
.	O
</s>
<s>
This	O
is	O
used	O
to	O
modulate	O
a	O
crystal-controlled	O
oscillator	O
,	O
and	O
the	O
result	O
is	O
passed	O
through	O
a	O
frequency	O
multiplier	O
to	O
produce	O
an	O
FM	B-Application
signal	O
.	O
</s>
<s>
In	O
this	O
modulation	B-Application
,	O
narrowband	O
FM	B-Application
is	O
generated	O
leading	O
to	O
wideband	B-Application
FM	I-Application
later	O
and	O
hence	O
the	O
modulation	B-Application
is	O
known	O
as	O
indirect	O
FM	B-Application
modulation	B-Application
.	O
</s>
<s>
Many	O
FM	B-Application
detector	B-Algorithm
circuits	O
exist	O
.	O
</s>
<s>
A	O
common	O
method	O
for	O
recovering	O
the	O
information	O
signal	O
is	O
through	O
a	O
Foster	B-Device
–	I-Device
Seeley	I-Device
discriminator	I-Device
or	O
ratio	B-Algorithm
detector	I-Algorithm
.	O
</s>
<s>
A	O
phase-locked	O
loop	O
can	O
be	O
used	O
as	O
an	O
FM	B-Application
demodulator	O
.	O
</s>
<s>
Slope	O
detection	B-Algorithm
demodulates	O
an	O
FM	B-Application
signal	O
by	O
using	O
a	O
tuned	O
circuit	O
which	O
has	O
its	O
resonant	O
frequency	O
slightly	O
offset	O
from	O
the	O
carrier	O
.	O
</s>
<s>
As	O
the	O
frequency	O
rises	O
and	O
falls	O
the	O
tuned	O
circuit	O
provides	O
a	O
changing	O
amplitude	B-Application
of	O
response	O
,	O
converting	O
FM	B-Application
to	O
AM	B-Device
.	O
</s>
<s>
AM	B-Device
receivers	B-Device
may	O
detect	O
some	O
FM	B-Application
transmissions	O
by	O
this	O
means	O
,	O
although	O
it	O
does	O
not	O
provide	O
an	O
efficient	O
means	O
of	O
detection	B-Algorithm
for	O
FM	B-Application
broadcasts	B-Operating_System
.	O
</s>
<s>
When	O
an	O
echolocating	O
bat	B-Application
approaches	O
a	O
target	O
,	O
its	O
outgoing	O
sounds	O
return	O
as	O
echoes	O
,	O
which	O
are	O
Doppler-shifted	O
upward	O
in	O
frequency	O
.	O
</s>
<s>
In	O
certain	O
species	O
of	O
bats	B-Application
,	O
which	O
produce	O
constant	O
frequency	O
(	O
CF	O
)	O
echolocation	O
calls	O
,	O
the	O
bats	B-Application
compensate	O
for	O
the	O
Doppler	O
shift	O
by	O
lowering	O
their	O
call	O
frequency	O
as	O
they	O
approach	O
a	O
target	O
.	O
</s>
<s>
FM	B-Application
is	O
also	O
used	O
at	O
intermediate	O
frequencies	O
by	O
analog	O
VCR	O
systems	O
(	O
including	O
VHS	O
)	O
to	O
record	O
the	O
luminance	O
(	O
black	O
and	O
white	O
)	O
portions	O
of	O
the	O
video	O
signal	O
.	O
</s>
<s>
Commonly	O
,	O
the	O
chrominance	O
component	O
is	O
recorded	O
as	O
a	O
conventional	O
AM	B-Device
signal	O
,	O
using	O
the	O
higher-frequency	O
FM	B-Application
signal	O
as	O
bias	O
.	O
</s>
<s>
FM	B-Application
is	O
the	O
only	O
feasible	O
method	O
of	O
recording	O
the	O
luminance	O
(	O
"	O
black-and-white	O
"	O
)	O
component	O
of	O
video	O
to	O
(	O
and	O
retrieving	O
video	O
from	O
)	O
magnetic	O
tape	O
without	O
distortion	O
;	O
video	O
signals	O
have	O
a	O
large	O
range	O
of	O
frequency	O
components	O
–	O
from	O
a	O
few	O
hertz	O
to	O
several	O
megahertz	O
,	O
too	O
wide	O
for	O
equalizers	O
to	O
work	O
with	O
due	O
to	O
electronic	O
noise	O
below	O
−60dB	O
.	O
</s>
<s>
FM	B-Application
also	O
keeps	O
the	O
tape	O
at	O
saturation	O
level	O
,	O
acting	O
as	O
a	O
form	O
of	O
noise	O
reduction	O
;	O
a	O
limiter	O
can	O
mask	O
variations	O
in	O
playback	O
output	O
,	O
and	O
the	O
FM	B-Application
capture	O
effect	O
removes	O
print-through	O
and	O
pre-echo	O
.	O
</s>
<s>
These	O
FM	B-Application
systems	O
are	O
unusual	O
,	O
in	O
that	O
they	O
have	O
a	O
ratio	O
of	O
carrier	O
to	O
maximum	O
modulation	B-Application
frequency	O
of	O
less	O
than	O
two	O
;	O
contrast	O
this	O
with	O
FM	B-Application
audio	O
broadcasting	B-Operating_System
,	O
where	O
the	O
ratio	O
is	O
around	O
10,000	O
.	O
</s>
<s>
Consider	O
,	O
for	O
example	O
,	O
a	O
6-MHz	O
carrier	O
modulated	B-Application
at	O
a	O
3.5-MHz	O
rate	O
;	O
by	O
Bessel	O
analysis	O
,	O
the	O
first	O
sidebands	O
are	O
on	O
9.5	O
and	O
2.5MHz	O
and	O
the	O
second	O
sidebands	O
are	O
on	O
13MHz	O
and	O
−1MHz	O
.	O
</s>
<s>
FM	B-Application
is	O
also	O
used	O
at	O
audio	O
frequencies	O
to	O
synthesize	O
sound	O
.	O
</s>
<s>
This	O
technique	O
,	O
known	O
as	O
FM	B-Application
synthesis	O
,	O
was	O
popularized	O
by	O
early	O
digital	O
synthesizers	O
and	O
became	O
a	O
standard	O
feature	O
in	O
several	O
generations	O
of	O
personal	B-Device
computer	I-Device
sound	B-Device
cards	I-Device
.	O
</s>
<s>
Edwin	O
Howard	O
Armstrong	O
(	O
1890	O
–	O
1954	O
)	O
was	O
an	O
American	O
electrical	O
engineer	O
who	O
invented	O
wideband	O
frequency	B-Application
modulation	I-Application
(	O
FM	B-Application
)	O
radio	O
.	O
</s>
<s>
He	O
patented	O
the	O
regenerative	O
circuit	O
in	O
1914	O
,	O
the	O
superheterodyne	O
receiver	B-Device
in	O
1918	O
and	O
the	O
super-regenerative	O
circuit	O
in	O
1922	O
.	O
</s>
<s>
Armstrong	O
presented	O
his	O
paper	O
,	O
"	O
A	O
Method	O
of	O
Reducing	O
Disturbances	O
in	O
Radio	O
Signaling	O
by	O
a	O
System	O
of	O
Frequency	B-Application
Modulation	I-Application
"	O
,	O
(	O
which	O
first	O
described	O
FM	B-Application
radio	O
)	O
before	O
the	O
New	O
York	O
section	O
of	O
the	O
Institute	O
of	O
Radio	O
Engineers	O
on	O
November	O
6	O
,	O
1935	O
.	O
</s>
<s>
As	O
the	O
name	O
implies	O
,	O
wideband	B-Application
FM	I-Application
(	O
WFM	O
)	O
requires	O
a	O
wider	O
signal	B-Algorithm
bandwidth	I-Algorithm
than	O
amplitude	B-Device
modulation	I-Device
by	O
an	O
equivalent	O
modulating	B-Application
signal	I-Application
;	O
this	O
also	O
makes	O
the	O
signal	O
more	O
robust	O
against	O
noise	O
and	O
interference	O
.	O
</s>
<s>
Frequency	B-Application
modulation	I-Application
is	O
also	O
more	O
robust	O
against	O
signal-amplitude-fading	O
phenomena	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
FM	B-Application
was	O
chosen	O
as	O
the	O
modulation	B-Application
standard	O
for	O
high	O
frequency	O
,	O
high	O
fidelity	O
radio	O
transmission	O
,	O
hence	O
the	O
term	O
"	O
FM	B-Application
radio	O
"	O
(	O
although	O
for	O
many	O
years	O
the	O
BBC	O
called	O
it	O
"	O
VHF	B-Algorithm
radio	O
"	O
because	O
commercial	O
FM	B-Application
broadcasting	B-Operating_System
uses	O
part	O
of	O
the	O
VHF	B-Algorithm
band	I-Algorithm
—	O
the	O
FM	B-Algorithm
broadcast	I-Algorithm
band	I-Algorithm
)	O
.	O
</s>
<s>
FM	B-Application
receivers	B-Device
employ	O
a	O
special	O
detector	B-Algorithm
for	O
FM	B-Application
signals	O
and	O
exhibit	O
a	O
phenomenon	O
known	O
as	O
the	O
capture	O
effect	O
,	O
in	O
which	O
the	O
tuner	B-Device
"	O
captures	O
"	O
the	O
stronger	O
of	O
two	O
stations	O
on	O
the	O
same	O
frequency	O
while	O
rejecting	O
the	O
other	O
(	O
compare	O
this	O
with	O
a	O
similar	O
situation	O
on	O
an	O
AM	B-Device
receiver	B-Device
,	O
where	O
both	O
stations	O
can	O
be	O
heard	O
simultaneously	O
)	O
.	O
</s>
<s>
Frequency	O
drift	O
was	O
a	O
problem	O
in	O
early	O
(	O
or	O
inexpensive	O
)	O
receivers	B-Device
;	O
inadequate	O
selectivity	O
may	O
affect	O
any	O
tuner	B-Device
.	O
</s>
<s>
An	O
FM	B-Application
signal	O
can	O
also	O
be	O
used	O
to	O
carry	O
a	O
stereo	O
signal	O
;	O
this	O
is	O
done	O
with	O
multiplexing	B-Architecture
and	O
demultiplexing	B-Architecture
before	O
and	O
after	O
the	O
FM	B-Application
process	O
.	O
</s>
<s>
The	O
FM	B-Application
modulation	B-Application
and	O
demodulation	O
process	O
is	O
identical	O
in	O
stereo	O
and	O
monaural	O
processes	O
.	O
</s>
<s>
A	O
high-efficiency	O
radio-frequency	O
switching	O
amplifier	O
can	O
be	O
used	O
to	O
transmit	O
FM	B-Application
signals	O
(	O
and	O
other	O
constant-amplitude	O
signals	O
)	O
.	O
</s>
<s>
For	O
a	O
given	O
signal	O
strength	O
(	O
measured	O
at	O
the	O
receiver	B-Device
antenna	O
)	O
,	O
switching	O
amplifiers	O
use	O
less	O
battery	O
power	O
and	O
typically	O
cost	O
less	O
than	O
a	O
linear	O
amplifier	O
.	O
</s>
<s>
This	O
gives	O
FM	B-Application
another	O
advantage	O
over	O
other	O
modulation	B-Application
methods	O
requiring	O
linear	O
amplifiers	O
,	O
such	O
as	O
AM	B-Device
and	O
QAM	B-Device
.	O
</s>
<s>
FM	B-Application
is	O
commonly	O
used	O
at	O
VHF	B-Algorithm
radio	O
frequencies	O
for	O
high-fidelity	O
broadcasts	B-Operating_System
of	O
music	O
and	O
speech	O
.	O
</s>
<s>
Analog	O
TV	O
sound	O
is	O
also	O
broadcast	B-Operating_System
using	O
FM	B-Application
.	O
</s>
<s>
Narrowband	O
FM	B-Application
is	O
used	O
for	O
voice	O
communications	O
in	O
commercial	O
and	O
amateur	O
radio	O
settings	O
.	O
</s>
<s>
In	O
broadcast	B-Operating_System
services	O
,	O
where	O
audio	O
fidelity	O
is	O
important	O
,	O
wideband	B-Application
FM	I-Application
is	O
generally	O
used	O
.	O
</s>
<s>
In	O
two-way	O
radio	O
,	O
narrowband	O
FM	B-Application
(	O
NBFM	O
)	O
is	O
used	O
to	O
conserve	O
bandwidth	B-Algorithm
for	O
land	O
mobile	O
,	O
marine	O
mobile	O
and	O
other	O
radio	O
services	O
.	O
</s>
<s>
Demonstration	O
of	O
frequency	B-Application
modulation	I-Application
was	O
carried	O
out	O
on	O
the	O
laboratory	O
model	O
.	O
</s>
