MPX filter
The so-called MPX filter is an existing notch filter on some cassette recorders and tape recorders that can be switched on as required.
The filter removes the 19- kHz - pilot tone , the receiver these frequencies but not as well as the 38 kHz subcarrier of an FM stereo multiplex signal and is required if you want to do with the recorder a recording of a FM transmitter to an antenna or cable sufficiently suppressed.
The abbreviation “MPX” stands for “Multiplex” and refers to the underlying pilot tone multiplex method .
The filtering of these frequencies is important for cassette recorders, although many people can no longer hear the 19 kHz sound either. If this signal were recorded on magnetic tape, however, this constant pilot tone would cause unnecessary magnetization of the tape at 19 kHz. In addition, these two signals or their harmonics often intermodulate with the erasure frequency of the recorder, which is noticeable in the form of very annoying whistling tones.
Without sufficient filtering of the pilot tone, the function of a noise suppression system such as Dolby can also be impaired, as this interprets the pilot tone as a high (useful) tone.
When recording from other sources, e.g. B. CDs , this filter should remain switched off, it is only relevant for the recording of FM stereo sources, and only if the receiver is not already doing such a filtering. This filter is irrelevant for playing back cassettes.