Hey all, I'm looking at some schematic for an FM IC. There is a pin
that is labled MPX and it is said to be the multiplex output. Is this
the mono signal? I'm just curious.
The chip may have a built-in de-emphasis filter block (75uS?)
which also produces some gain to more easily drive a power
output stage. This "MPX" output comes in front of that (and
therefore lacks the gain) but also has the full bandwidth to
carry the added stereo sub-carrier centered at 38KHz.
So, if you want to build a simple, cheap monaural table radio,
you ignore the "MPX" output (or maybe attach the 75uS de-emphasis
network there) and connect the gain block output to a "power"
amplifier chip through a volume controll and you're done.
If you want to go full stereo, you ignore the output gain block,
connect the "MPX" output pin to a stereo decoder chip (usually
available from the same manufacturer), connect dual de-emphasis
networks to the outputs of the stereo demodulator chip then to
dual volume controls and dual "power" amplifier chips.
"Power" amplifier chips might be in the neighborhood of 2 Watts
with 10% distortion for a small table stereo rig!
HTH
Rick
