'Negative squelch' software

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Cuatro

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Jun 21, 2008
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Hi all,

I have recently picked up some broadcasts on the low end of the FM spectrum and rather than sit at my PC all day listening to a few clean signals between mostly noisy static i was hoping to record them.

The issue is, given that it's FM, the actual broadcasts are much quieter than the static, and as a result, any VOX/Squelch software i try, made to filter out weaker signals, just records this louder noise.

Does anyone know of any software/way i can record only BELOW a certain volume threshold?

Thank you
 

zz0468

QRT
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Feb 6, 2007
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I know of no software that will do what you're asking, but an easy fix could be had in hardware, if you're able to fabricate simple circuits. You could build a simple VOX circuit, adjusted to respond to the static, and invert the switched output so that it mutes the static, but not the program audio. It would function as a squelch, of sorts. Better still would be to modify the receiver and put in a real noise squelch.

I presume it's real broadcast FM... is it stereo? Use the 19 KHz stereo pilot signal, which usually drives a "stereo" indicator light to switch your audio. The drawback to that is that it would be non-responsive to a mono broadcast. But it would be cheap and easy.
 

Cuatro

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Jun 21, 2008
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Hey, thanks for the reply.

Hmm, I don't think i can be bothered making any hardware for this, it's not that important, i was just hoping someone knew of a simple program that say, had a volume filter which allowed recording only between certain levels.

I imagine my 'setup' is very different to most here. These broadcasts are at about 76MHz, and i pick them up on my tv/fm tuner card in my PC. Nice idea about the stereo pilot signal, but in any case i think these transmissions are mono...

Never mind, and thanks anyway.
 

Shortwavewave

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Feb 20, 2007
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I have a small mp3 player that has an FM radio that has a 'japan' setting for the broadcast and somewhere in the 76mhz is a local tv stations for me.
 

Cuatro

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Jun 21, 2008
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Haha, for me they are slightly more interesting. They are police/fire comms transmissions.
 
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