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Noise cancelling question...

brandon7861

Loose Wire
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Nov 28, 2018
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Just like we have time stations for time and frequency reference, why can't we have noise reference frequencies every 5MHz where nobody transmits or even gets close to intentionally? Would't it then be possible to incorporate a crystal filter into every radio that samples the noise on whatever "noise reference frequency" is closest or similar in RFI content to combine with the channel audio?
 

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I'll explain it by example: You have something like power line hardware arcing, you own an Everready smart battery charger, or your neighbor is growing pot with those terrible LED lights. However it happens, you have noise on the channel you are trying to talk on. There would also be a nearby frequency with nobody talking on it, and that nearby frequency also has the interference, but not your friend (or anyone else). The audio from the frequency only containing noise has its audio signal inverted and combined with the audio from the frequency you are listening to. The noise is common to both signals, and being inverted copies, the noise cancels out. The person talking is only on one frequency, so no voice cancels out.
 
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The reason for a "quiet" channel is because if skip were to roll in and it fill with audio, that audio would be unique to only one audio signal and also not cancel. So to use a frequency to sample audio interference from RFI that affects a broadband range, you need a place to sample that noise without other modulation.
 
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Giving up one channel out of our 40 to allow for the creation of a bulletproof noise blanker is what I mean (theoretically). I'm not saying we should, I'm just curious if it would work.
 
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I can almost envision a little VFO and mixer on the noise blanker board so that if the noise you are trying to cancel isn't on the predetermined noise sampling frequency,, it can be shifted from another quiet frequency that does have the same noise.
 
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I just figured that if it were done internally on the same antenna using filters with matched passband characteristics, that the amplitudes of the signals would be a far better match and more efficiently cancel.. Im not sure how the MFJ works, but if the antenna is different, the noise profile probably is too.
 
Had that idea fifty-plus years ago. Figured all you need is a quiet channel, convert it up (or down) to the frequency you're listening to and do the invert/add that way.

Then I discovered Lamb's "silencer" idea. It's the basis for the noise blanker in any analog receiver. Took care of power line buzz, so that seemed good enough. But that technique just creates "holes" in the analog audio where the noise pulses used to be. This has drawbacks for digital modulation and scrambles data.

So yeah, when someone works out this trick inside the radio, I'll be interested.

73
 
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Here's what I don't know, and I would find out if I currently had any noise on the air. The sound of power line interference always sounds like 120Hz regardless of the frequency you are listening to, so if I were to look at some of these noise sources on a spectrum display connected to the headphone jack, would the average intensity of the individual peaks remain the same as I turned the VFO dial to another channel (like a quiet one)? Or would some but not all peaks stay similar?
 
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