<s>
An	O
envelope	B-Algorithm
detector	I-Algorithm
(	O
sometimes	O
called	O
a	O
peak	O
detector	O
)	O
is	O
an	O
electronic	O
circuit	O
that	O
takes	O
a	O
(	O
relatively	O
)	O
high-frequency	O
amplitude	B-Device
modulated	I-Device
signal	O
as	O
input	O
and	O
provides	O
an	O
output	O
,	O
which	O
is	O
the	O
demodulated	B-Device
envelope	B-Algorithm
of	O
the	O
original	O
signal	O
.	O
</s>
<s>
The	O
capacitor	O
in	O
the	O
circuit	O
above	O
stores	O
charge	O
on	O
the	O
rising	O
edge	O
and	O
releases	O
it	O
slowly	O
through	O
the	O
resistor	O
when	O
the	O
input	O
signal	O
amplitude	B-Application
falls	O
.	O
</s>
<s>
The	O
diode	O
in	O
series	O
rectifies	B-Algorithm
the	O
incoming	O
signal	O
,	O
allowing	O
current	O
flow	O
only	O
when	O
the	O
positive	O
input	O
terminal	O
is	O
at	O
a	O
higher	O
potential	O
than	O
the	O
negative	O
input	O
terminal	O
.	O
</s>
<s>
Most	O
practical	O
envelope	B-Algorithm
detectors	I-Algorithm
use	O
either	O
half-wave	O
or	O
full-wave	O
rectification	B-Algorithm
of	O
the	O
signal	O
to	O
convert	O
the	O
AC	O
audio	O
input	O
into	O
a	O
pulsed	O
DC	O
signal	O
.	O
</s>
<s>
Filtering	B-Architecture
is	O
then	O
used	O
to	O
smooth	O
the	O
final	O
result	O
.	O
</s>
<s>
This	O
filtering	B-Architecture
is	O
rarely	O
perfect	O
and	O
some	O
"	O
ripple	O
"	O
is	O
likely	O
to	O
remain	O
on	O
the	O
envelope	B-Algorithm
follower	I-Algorithm
output	O
,	O
particularly	O
for	O
low	O
frequency	O
inputs	O
such	O
as	O
notes	O
from	O
a	O
bass	O
instrument	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
AM	B-Device
,	O
φ(t )	O
(	O
the	O
phase	O
component	O
of	O
the	O
signal	O
)	O
is	O
constant	O
and	O
can	O
be	O
ignored	O
.	O
</s>
<s>
In	O
AM	B-Device
,	O
the	O
carrier	O
frequency	O
is	O
also	O
constant	O
.	O
</s>
<s>
Thus	O
,	O
all	O
the	O
information	O
in	O
the	O
AM	B-Device
signal	O
is	O
in	O
R(t )	O
.	O
</s>
<s>
R(t )	O
is	O
called	O
the	O
envelope	B-Algorithm
of	O
the	O
signal	O
.	O
</s>
<s>
with	O
m(t )	O
representing	O
the	O
original	O
audio	O
frequency	O
message	O
,	O
C	O
the	O
carrier	O
amplitude	B-Application
and	O
R(t )	O
equal	O
to	O
C	O
+	O
m(t )	O
.	O
</s>
<s>
So	O
,	O
if	O
the	O
envelope	B-Algorithm
of	O
the	O
AM	B-Device
signal	O
can	O
be	O
extracted	O
,	O
the	O
original	O
message	O
can	O
be	O
recovered	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
FM	B-Application
,	O
the	O
transmitted	O
has	O
a	O
constant	O
envelope	B-Algorithm
R(t )	O
=	O
R	O
and	O
can	O
be	O
ignored	O
.	O
</s>
<s>
However	O
,	O
many	O
FM	B-Application
receivers	O
measure	O
the	O
envelope	B-Algorithm
anyway	O
for	O
received	O
signal	O
strength	O
indication	O
.	O
</s>
<s>
The	O
simplest	O
form	O
of	O
envelope	B-Algorithm
detector	I-Algorithm
is	O
the	O
diode	O
detector	O
which	O
is	O
shown	O
above	O
.	O
</s>
<s>
If	O
the	O
resistor	O
and	O
capacitor	O
are	O
correctly	O
chosen	O
,	O
the	O
output	O
of	O
this	O
circuit	O
should	O
approximate	O
a	O
voltage-shifted	O
version	O
of	O
the	O
original	O
(	O
baseband	B-Algorithm
)	O
signal	O
.	O
</s>
<s>
An	O
envelope	B-Algorithm
detector	I-Algorithm
can	O
also	O
be	O
constructed	O
using	O
a	O
precision	O
rectifier	B-Algorithm
feeding	O
into	O
a	O
low-pass	B-Algorithm
filter	I-Algorithm
.	O
</s>
<s>
The	O
envelope	B-Algorithm
detector	I-Algorithm
has	O
several	O
drawbacks	O
:	O
</s>
<s>
The	O
input	O
to	O
the	O
detector	O
must	O
be	O
band-pass	O
filtered	O
around	O
the	O
desired	O
signal	O
,	O
or	O
else	O
the	O
detector	O
will	O
simultaneously	O
demodulate	B-Device
several	O
signals	O
.	O
</s>
<s>
Most	O
of	O
these	O
drawbacks	O
are	O
relatively	O
minor	O
and	O
are	O
usually	O
acceptable	O
tradeoffs	O
for	O
the	O
simplicity	O
and	O
low	O
cost	O
of	O
using	O
an	O
envelope	B-Algorithm
detector	I-Algorithm
.	O
</s>
<s>
An	O
envelope	B-Algorithm
detector	I-Algorithm
can	O
be	O
used	O
to	O
demodulate	B-Device
a	O
previously	O
modulated	O
signal	O
by	O
removing	O
all	O
high	O
frequency	O
components	O
of	O
the	O
signal	O
.	O
</s>
<s>
The	O
capacitor	O
and	O
resistor	O
form	O
a	O
low-pass	B-Algorithm
filter	I-Algorithm
to	O
filter	O
out	O
the	O
carrier	O
frequency	O
.	O
</s>
<s>
Such	O
a	O
device	O
is	O
often	O
used	O
to	O
demodulate	B-Device
AM	B-Device
radio	O
signals	O
because	O
the	O
envelope	B-Algorithm
of	O
the	O
modulated	O
signal	O
is	O
equivalent	O
to	O
the	O
baseband	B-Algorithm
signal	I-Algorithm
.	O
</s>
<s>
An	O
envelope	B-Algorithm
detector	I-Algorithm
is	O
sometimes	O
referred	O
to	O
as	O
an	O
envelope	B-Algorithm
follower	I-Algorithm
in	O
musical	O
environments	O
.	O
</s>
<s>
It	O
is	O
still	O
used	O
to	O
detect	O
the	O
amplitude	B-Application
variations	O
of	O
an	O
incoming	O
signal	O
to	O
produce	O
a	O
control	O
signal	O
that	O
resembles	O
those	O
variations	O
.	O
</s>
<s>
Envelope	B-Algorithm
detectors	I-Algorithm
are	O
often	O
a	O
component	O
of	O
other	O
circuits	O
,	O
such	O
as	O
a	O
compressor	O
or	O
an	O
auto-wah	O
or	O
envelope-followed	O
filter	O
.	O
</s>
<s>
In	O
these	O
circuits	O
,	O
the	O
envelope	B-Algorithm
follower	I-Algorithm
is	O
part	O
of	O
what	O
is	O
known	O
as	O
the	O
"	O
side	O
chain	O
"	O
,	O
a	O
circuit	O
which	O
describes	O
some	O
characteristic	O
of	O
the	O
input	O
,	O
in	O
this	O
case	O
its	O
volume	O
.	O
</s>
<s>
Both	O
expanders	O
and	O
compressors	O
use	O
the	O
envelope	B-Algorithm
's	O
output	O
voltage	O
to	O
control	O
the	O
gain	O
of	O
an	O
amplifier	O
.	O
</s>
<s>
Modern	O
envelope	B-Algorithm
followers	I-Algorithm
can	O
be	O
implemented	O
:	O
</s>
