MPX filter
Encyclopedia
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.
Some cassette deck
s have an MPX filter, which is effectively a notch filter that will block the 19 kHz guard tone. You should be able to hear the difference by recording from an FM stereo source and engaging and disengaging the MPX filter switch (on a three-head deck with monitoring, you will be able to hear this 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.
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...
and General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
). The normal monaural
Monaural
Monaural or monophonic sound reproduction is single-channel. Typically there is only one microphone, one loudspeaker, or channels are fed from a common signal path...
audio, the pilot tone (which serves as a phase reference when decoding the stereographic information) and the double sideband stereographic information, are all multiplexed
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
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
Compact Cassette
The Compact Cassette, often referred to as audio cassette, cassette tape, cassette, or simply tape, is a magnetic tape sound recording format. It was designed originally for dictation, but improvements in fidelity led the Compact Cassette to supplant the Stereo 8-track cartridge and reel-to-reel...
. As such, some of the tape will be saturated by the pilot tone, and recording and playback of lower frequencies may be adversely affected.
Some cassette deck
Cassette deck
A cassette deck is a type of tape recorder for playing or recording audio compact cassettes. A deck was formerly distinguished from a recorder as being part of a stereo component system, while a recorder had a self-contained power amplifier...
s have an MPX filter, which is effectively a notch filter that will block the 19 kHz guard tone. You should be able to hear the difference by recording from an FM stereo source and engaging and disengaging the MPX filter switch (on a three-head deck with monitoring, you will be able to hear this while recording). The filter should not be engaged when recording from other sources, such as Compact Disc
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...
. The setting of the switch should have no effect during playback.