MPX Filter
MPX Filter indicates a filter intended to reduce the 19 kHz pilot signal when recording from an FM stereo source.
The pilot is part of FM stereo signalling rather than the wanted programme audio. Residual pilot content at the tuner output can interfere with parts of a recording system, particularly certain noise-reduction circuits.
MPX Filter — Tape Deck
Use the filter according to the deck’s instructions when recording from an applicable FM source. It is not a general cure for tape hiss, hum or poor radio reception. Whether it is needed depends on the tuner output and the recording path.
Check whether the filter is switchable and which modes engage it. Because it acts near the top of the audio range, its effect is different from changing the tape formulation or recording bias. Its presence alone does not establish the recording’s frequency response or noise-reduction compatibility.