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MPX filter is a function found in cassette decks.
FM stereo broadcasts contain a pilot tone at 19 kHz when 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 stereophonic information) and the double sideband stereophonic difference information are all mixed and broadcast together. The process of encoding the difference signal into the 23-53kHz band via double-sideband carrier-suppressed amplitude modulation is an instance of multiplexing.
Residual high-frequency components of the signal remaining after de-multiplexing 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, and typically provides an audible beat (difference) tone in conjunction with the tape head bias signal used when recording. The recording may also be saturated by the pilot tone. More importantly, the pilot tone interferes with the proper functioning of noise reduction systems such as Dolby B, C and S. Because the pilot tone is relatively loud in comparison to the overall loudness of the FM program (typically -20dB), the source signal is not correctly processed by the noise reduction system, leading to audible artefacts such as breathing and pumping. For this reason many decks with defeatable MPX filters have been designed in such a way that the MPX filter can only be enabled when also a noise reduction system is enabled.
An MPX filter on a tape deck is, at least, a notch filter that will block the 19 kHz pilot tone, and possibly higher frequencies in the 23-53kHz and 63-75kHz bands. The difference can be heard when 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 has no effect during playback.
Decks with no MPX filter switch typically have a non-defeatable MPX filter incorporated in their design, which limits the overall (i.e. record to playback) frequency response to about 15-16 kHz.