<s>
MPX	B-Algorithm
filter	I-Algorithm
is	O
a	O
function	O
found	O
in	O
analogue	O
stereo	O
FM	O
broadcasting	O
and	O
personal	O
monitor	O
equipment	O
,	O
FM	B-Device
tuners	I-Device
and	O
cassette	O
decks	O
.	O
</s>
<s>
An	O
MPX	B-Algorithm
filter	I-Algorithm
is	O
,	O
at	O
least	O
,	O
a	O
notch	O
filter	O
blocking	O
the	O
19	O
kHz	O
pilot	O
tone	O
,	O
and	O
possibly	O
higher	O
frequencies	O
in	O
the	O
23-53kHz	O
and	O
63-75kHz	O
bands	O
.	O
</s>
<s>
Normal	O
monaural	O
audio	O
,	O
the	O
pilot	O
tone	O
and	O
the	O
double	O
sideband	O
stereophonic	O
difference	O
information	O
are	O
all	O
mixed	O
together	O
into	O
composite	B-Algorithm
FM	I-Algorithm
baseband	I-Algorithm
signal	I-Algorithm
extending	O
to	O
53	O
kHz	O
(	O
stereo	O
audio	O
only	O
)	O
or	O
99	O
kHz	O
(	O
stereo	O
audio	O
plus	O
an	O
auxiliary	O
subchannel	O
,	O
so-called	O
SCA	O
)	O
.	O
</s>
<s>
The	O
process	O
of	O
encoding	O
the	O
difference	O
signal	O
into	O
the	O
23-53kHz	O
band	O
via	O
double-sideband	O
carrier-suppressed	O
amplitude	O
modulation	B-Application
is	O
an	O
instance	O
of	O
multiplexing	B-Architecture
(	O
hence	O
the	O
name	O
MPX	B-Algorithm
filter	I-Algorithm
)	O
.	O
</s>
<s>
Any	O
energy	O
at	O
frequencies	O
above	O
19	O
kHz	O
,	O
which	O
is	O
the	O
Nyquist	O
frequency	O
of	O
FM	O
stereo	O
,	O
may	O
cause	O
offensive	O
audible	O
aliasing	B-Error_Name
described	O
as	O
"	O
monkey	O
chatter	O
"	O
.	O
</s>
<s>
For	O
this	O
reasons	O
,	O
source	O
programs	O
for	O
commercial	O
FM	O
broadcasting	O
are	O
limited	O
to	O
50	O
Hz	O
–	O
15	O
kHz	O
bandwidth	O
with	O
very	O
steep	O
15	O
kHz	O
low-pass	B-Algorithm
filters	I-Algorithm
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
they	O
must	O
pass	O
through	O
a	O
brickwall	O
MPX	B-Algorithm
filter	I-Algorithm
to	O
clean	O
up	O
space	O
for	O
the	O
pilot	O
tone	O
.	O
</s>
<s>
Residual	O
high-frequency	O
components	O
of	O
the	O
signal	O
remaining	O
after	O
de-multiplexing	O
can	O
be	O
problematic	O
when	O
recording	O
to	O
analog	O
magnetic	O
media	O
.	O
</s>
<s>
The	O
pilot	O
tone	O
is	O
transmitted	O
at	O
10%	O
of	O
maximum	O
modulation	B-Application
level	O
and	O
further	O
reduced	O
by	O
15	O
db	O
in	O
the	O
receiver	B-Device
to	O
compensate	O
for	O
the	O
pre-emphasis	B-Algorithm
on	O
the	O
transmitting	O
side	O
.	O
</s>
<s>
High	O
quality	O
FM	B-Device
tuners	I-Device
have	O
built-in	O
MPX	B-Algorithm
filters	I-Algorithm
,	O
which	O
must	O
be	O
very	O
sharp	O
(	O
at	O
least	O
-60	O
dB	O
rejection	O
at	O
precisely	O
19	O
kHz	O
)	O
to	O
be	O
effective	O
.	O
</s>
<s>
If	O
,	O
however	O
,	O
no	O
MPX	O
filtering	O
takes	O
place	O
in	O
the	O
tuner	B-Device
,	O
the	O
pilot	O
tone	O
passes	O
through	O
at	O
-35	O
dB	O
below	O
theoretical	O
maximum	O
.	O
</s>
<s>
More	O
importantly	O
,	O
the	O
pilot	O
tone	O
interferes	O
with	O
the	O
proper	O
functioning	O
of	O
reciprocal	O
noise	O
reduction	O
systems	O
,	O
causing	O
audible	O
artefacts	O
such	O
as	O
breathing	O
and	O
pumping	O
For	O
this	O
reason	O
,	O
the	O
MPX	B-Algorithm
filter	I-Algorithm
is	O
mandatory	O
for	O
all	O
cassette	O
recorders	O
equipped	O
with	O
Dolby	O
B	O
and	O
Dolby	O
C	O
systems	O
.	O
</s>
<s>
Sometimes	O
,	O
defeatable	O
MPX	B-Algorithm
filter	I-Algorithm
engages	O
only	O
when	O
noise	O
reduction	O
is	O
enabled	O
.	O
</s>
<s>
Decks	O
with	O
no	O
MPX	B-Algorithm
filter	I-Algorithm
switch	O
typically	O
have	O
a	O
non-defeatable	O
MPX	B-Algorithm
filter	I-Algorithm
incorporated	O
in	O
their	O
design	O
,	O
which	O
limits	O
the	O
overall	O
(	O
i.e.	O
</s>
<s>
Cheaper	O
decks	O
may	O
have	O
MPX	O
filtering	O
performed	O
by	O
a	O
low-pass	B-Algorithm
filter	I-Algorithm
that	O
rolls	O
off	O
everything	O
above	O
15	O
kHz	O
.	O
</s>
<s>
A	O
proper	O
MPX	B-Algorithm
filter	I-Algorithm
for	O
quality	O
recording	O
is	O
,	O
at	O
least	O
,	O
a	O
notch	O
filter	O
that	O
will	O
block	O
the	O
19	O
kHz	O
pilot	O
tone	O
,	O
and	O
possibly	O
higher	O
frequencies	O
in	O
the	O
23-53kHz	O
and	O
63-75kHz	O
bands	O
.	O
</s>
<s>
The	O
difference	O
can	O
be	O
heard	O
when	O
recording	O
from	O
an	O
FM	O
stereo	O
source	O
and	O
engaging	O
and	O
disengaging	O
the	O
MPX	B-Algorithm
filter	I-Algorithm
switch	O
.	O
</s>
