MPX filter is a function found in cassette decks.
FM stereo broadcasts contain a pilot tone at 19 kHz that indicates stereo information is available at higher frequencies (this system was developed jointly by Zenith and General Electric). The normal monaural audio, the pilot tone (which serves as a phase reference when decoding the stereographic information) and the double sideband stereographic information, are all multiplexed together.
The high-frequency components of the resulting multiplexed signal can be problematic when recording to analog magnetic media. While the higher frequencies are unlikely to survive filtering in the recording deck, the 19 kHz tone is within the 20 to 20,000 Hz frequency range for many tape formats, including compact cassette. As such, some of the tape will be saturated by the pilot tone, and recording and playback of lower frequencies may be adversely affected.
MPX filter is effectively a notch filter that will block the 19 kHz guard tone. The difference should be heard by recording from an FM stereo source and engaging and disengaging the MPX filter switch. On a three-head deck with monitoring, this can be heard while recording. The filter should not be engaged when recording from other sources, such as Compact Disc. The setting of the switch should have no effect during playback.