The answer is here:  http://en.wikipedia.org/wiki/MPX_filter

As a practical matter, you should just leave it on all the time.

It is essential to have it on when recording directly from an FM tuner to avoid problems in the recording process.

hcsunshine wrote:

there's a button on my nak tape deck that says "MPX filter" (on/off). one of my old dealers told me it was for recording but i don't really know what it does. does anybody? 

But hardly ever used it.  It would cut down on a very small amount of high frequency warble and crosstalk that are probably a result of de-multiplexing a weak signal.  If I couldn't get a nice strong signal I just wouldn't record it.  I felt MPX noticibly rolled off the high frequency response of the recording.

It seems like it was on almost as a novelty on many decks that aspired to be "high-end" but only a handful of decks could reliably record to the 19k frequency that the filter supposes to take out.  Anyhow, be it as it may, most "notch" filters are anything but and either start rolling off well before the notch and can cause ringing just before the notch frequency sets in.  It's not all that unlike the brick wall filter that CD's use.  Try it with a high bias setting with a good metal tape and you'll hear exactly what I'm talking about.