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technical question , right channel audio , FM station

A

alok

My local AM has an FM translator. I noted that one day the right channel only was missing, other FM's in the area had both channels. What I found was every time I tuned to this particular station, the right channel would be there for afew seconds then fade down to nothing . . . left channel unaffected,it was OK.

If I tuned off and back on to this station with buttons or knob ,it did the same. Again other stations were OK, tuning off and back on them made no difference left & right channels were there all the time.
This problem occurred on just this one station.

I did call station and told them what I was hearing, ,they thanked me and said they'd check it out.

My question is . . . what would cause this ,I never heard this happen before ?


Al
 
Did the stereo indicator light come on about the time the audio faded out? They might not be feeding both channels to their fm translator. When you first tune to that station, your tuner is probably going to mono mode, blending both channels (the one with audio and the one without). When the tuner gets a good signal (with the stereo pilot) it fades to stereo, and you get two channels (but one is silent).
 
kenglish said:
... When you first tune to that station, your tuner is probably going to mono mode, blending both channels (the one with audio and the one without). ...
Click to expand...

To clarify, analog FM stereo broadcasting does not transmit the left and right audio channels separately. To retain compatibility of the stereo broadcast signal with monaural receivers, the sum of L&R audio modulates the main carrier, and the L-R difference signal modulates a 38 kHz subcarrier. An analog FM stereo receiver has the circuitry to demodulate the 38 kHz subcarrier, and recover (de-matrix) the separate L&R audio waveforms from the main and subchannel modulation.
 
For the first few seconds did you recognize just the left channel content? Or did you just notice sound versus silence with both speakers live at first and right speaker dead later?

Hmmmm. (just imagining; speculating)

If the station had monaural L+R accidentally plugged into the stereo multiplex sub channel that wants L-R then ...

... the receiver demultiplexing any weak stereo station might drop into mono mode at first, making both speakers live. Then settle in on stereo, adding main (L+R as normal) and sub (now actually L+R) and getting monaural on the left speaker. But normally subtracting, taking main (L+R) and applying sub (unexpected L+R) and coming up with "nothing" (instead of the expected 2R that the human ear probably can't distinguish from normal strength R) for the right speaker.
 
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