MPX filter is a function found in analogue stereo FM
FM broadcasting is a method of radio broadcasting using frequency modulation (FM). Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to provide high fidelity sound over broadcast radio. FM broadcasting is capa ...
broadcasting and personal monitor equipment, FM tuners and cassette deck
A cassette deck is a type of tape machine for playing and recording audio cassettes that does not have a built-in power amplifier or speakers, and serves primarily as a transport. It can be a part of an automotive entertainment system, a part of a ...
s. An MPX filter is, at least, a notch filter
In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a band-pass filter. A notch filter is a ...
blocking the 19 kHz pilot tone, and possibly higher frequencies in the 23-53kHz and 63-75kHz bands.
Broadcasting and personal monitors
FM stereo
FM broadcasting is a method of radio broadcasting using frequency modulation (FM). Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to provide high fidelity sound over broadcast radio. FM broadcasting is cap ...
broadcasts contain a ''pilot tone
An aircraft pilot or aviator is a person who controls the flight of an aircraft by operating its directional flight controls. Some other aircrew members, such as navigators or flight engineers, are also considered aviators, because they ar ...
'' - a 19 kHz sinewave serving as a phase reference for decoding the stereophonic information. The system was developed jointly by Zenith
The zenith (, ) is an imaginary point directly "above" a particular location, on the celestial sphere. "Above" means in the vertical direction ( plumb line) opposite to the gravity direction at that location ( nadir). The zenith is the "high ...
and General Electric
General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable energ ...
, and approved by the FCC
The Federal Communications Commission (FCC) is an independent agency of the United States federal government that regulates communications by radio, television, wire, satellite, and cable across the United States. The FCC maintains jurisdictio ...
in 1961. Normal monaural
Monaural or monophonic sound reproduction (often shortened to mono) is sound intended to be heard as if it were emanating from one position. This contrasts with stereophonic sound or ''stereo'', which uses two separate audio channels to reproduc ...
audio, the pilot tone and the double sideband stereophonic difference information are all mixed together into composite FM baseband signal extending to 53 kHz (stereo audio only) or 99 kHz (stereo audio plus an auxiliary subchannel, so-called ''SCA''). The process of encoding the difference signal into the 23-53kHz band via double-sideband carrier-suppressed amplitude modulation is an instance of multiplexing
In telecommunications and computer networking, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource - a ...
(hence the name ''MPX filter'').
The pilot tone resides inside the audio band (although beyond the range of many adult listeners), and can be compromised by high-energy treble components of the source. Any energy at frequencies above 19 kHz, which is the Nyquist frequency
In signal processing, the Nyquist frequency (or folding frequency), named after Harry Nyquist, is a characteristic of a sampler, which converts a continuous function or signal into a discrete sequence. In units of cycles per second ( Hz), i ...
of FM stereo, may cause offensive audible aliasing
In signal processing and related disciplines, aliasing is an effect that causes different signals to become indistinguishable (or ''aliases'' of one another) when sampled. It also often refers to the distortion or artifact that results when ...
described as "monkey chatter". Any energy between 18.5 and 19.5 kHz may disrupt stereo decoding, causing sudden rotation of the soundfield. For this reasons, source programs for commercial FM broadcasting are limited to 50 Hz – 15 kHz bandwidth with very steep 15 kHz low-pass filter
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filt ...
s.
Source programs transmitted to personal monitors on stage or in the recording studio
A recording studio is a specialized facility for sound recording, mixing, and audio production of instrumental or vocal musical performances, spoken words, and other sounds. They range in size from a small in-home project studio large en ...
do not need to follow the broadcast 50 Hz – 15 kHz standard, and may have substantial upper treble content. Sources that were subjected to excessive treble boost, or excessive stereo panning are particularly capable of degrading stereo FM reception. For this reason, they must pass through a brickwall MPX filter to clean up space for the pilot tone.
Magnetic recording
Residual high-frequency components of the signal remaining after de-multiplexing can be problematic when recording to analog magnetic media. The pilot tone is transmitted at 10% of maximum modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the '' carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informat ...
level and further reduced by 15 db in the receiver to compensate for the pre-emphasis
Typically, prior to some process, such as transmission over cable, or recording to phonograph record or tape, the input frequency range most susceptible to noise is boosted. This is referred to as "pre-emphasis"before the process the signal will u ...
on the transmitting side. High quality FM tuners have built-in MPX filters, which must be very sharp (at least -60 dB rejection at precisely 19 kHz) to be effective.
If, however, no MPX filtering takes place in the tuner, the pilot tone passes through at -35 dB below theoretical maximum. This is sufficient to cause intermodulation distortion with source treble content, and to cause audible beat with the bias oscillator during recording. More importantly, the pilot tone interferes with the proper functioning of reciprocal noise reduction
Noise reduction is the process of removing noise from a signal. Noise reduction techniques exist for audio and images. Noise reduction algorithms may distort the signal to some degree. Noise rejection is the ability of a circuit to isolate an un ...
systems, causing audible artefacts such as breathing
Breathing (or ventilation) is the process of moving air into and from the lungs to facilitate gas exchange with the internal environment, mostly to flush out carbon dioxide and bring in oxygen.
All aerobic creatures need oxygen for cell ...
and pumping
Pumping may refer to:
* The operation of a pump, for moving a liquid from one location to another
**The use of a breast pump for extraction of milk
* Pumping (audio), a creative misuse of dynamic range compression
* Pumping (computer systems), ...
For this reason, the MPX filter is mandatory for all cassette recorders equipped with Dolby B
A Dolby noise-reduction system, or Dolby NR, is one of a series of noise reduction systems developed by Dolby Laboratories for use in analog audio tape recording. The first was '' Dolby A'', a professional broadband noise reduction sy ...
and Dolby C systems. On some decks (those capable of recording to 19 kHz) is usually defeatable, and should be engaged only for recording from FM stereo (but not other sources such as Compact Disc
The compact disc (CD) is a digital optical disc data storage format that was co-developed by Philips and Sony to store and play digital audio recordings. In August 1982, the first compact disc was manufactured. It was then released in Octo ...
). Sometimes, defeatable MPX filter engages only when noise reduction is enabled. 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.
Cheaper decks may have MPX filtering performed by a low-pass filter
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filt ...
that rolls off everything above 15 kHz. A proper MPX filter for quality recording is, at least, a notch filter
In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a band-pass filter. A notch filter is a ...
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 setting of the switch has no effect during playback.
Notes
{{reflist Multiplexing Linear filters Wireless tuning and filtering Tone, EQ and filter