MPX and de-emphasis
- Thread starter morseguy
- Start date
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The MPX filter is to remove the 19 KHz stereo pilot from the audio outputs. The 19 KHz stereo pilot has a nasty effect of mixing with a tape recorder's bias signal and producing an audible tone in the recording. You might not notice the pilot signal if you're not tape recording, depends on how good your hearing is. A raw demodulated stereo FM signal actually has a bunch of signals above 15 KHz that you don't want in your audio. (The 19 KHz stereo pilot is just the first one above 15 KHz.) In the old days mono FM stations may have had usable audio beyond 15 KHz, so turning off the MPX filter could extend the audio frequency response. All station are stereo now a days, so you should leave the MPX filter on all the time.
I've never seen a switch to turn off the de-emphasis. If you turn off the de-emphasis the audio will be insanely bright (like turning your treble control all the way up). But, if you like it that way, go ahead.
All modern FM tuners have MPX and de-emphasis filters that can't be turned off.
Bobby D.
Bob@FM
Well-Known Member
De-emphasis - seen on vintage solid state from the 70's and 80's. Two reasons for this switch
1) was to allow the same tuner to be sold or used in different markets. Europe uses 50us, USA and Canada 75us. Usually these tuners also had voltage selection switches.
2) Dolby FM - Dolby decoding on FM made a brief appearance in the mid to late 70's, and so tuners had a switch for 25us de-emphasis for Dolby. Dolby FM done correctly required a Dolby decoder, either built in or outboard.
MPX (noise) filters - a term loosely used to describe circuits that would "lessen the noise" of weak signal FM stereo reception, yet still retain some stereo separation. Stereo FM has about 23dB more noise at the lowest FM signal strength than mono FM. So these circuits were included to make stereo less noisy when the signal was weak. They used blending of the L and R channels, usually only at high frequencies, or high frequency roll-off (low pass filters) to decrease the noise.
Note that later FM tuners incorporated circuits to detect noise and automatically engage these noise reduction techniques, going to mono if needed, with no capability for user intervention. And without informing the user they were being used or engaged. This is how all modern car radios, and most new FM tuners work, including the Sony XDR-F1HD.
When seen on tape decks, these were 19kHz pilot filters. Most solid state vintage had 19kHz pilot filters built into the audio section, and were not switchable. I can't recall ever seeing a tuner that had a switchable 19kHz pilot filter, although one I recall had 2 outputs, one unfiltered and one filtered (Technics ST-9130)
Bob
