Message-ID: <581B83FC.6060600@posteo.de>
Date: Thu, 03 Nov 2016 19:37:48 +0100
From: DK7FC <selberdenken@posteo.de>
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Subject: Re: LF: Smart noise cancelling?!?
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Hi Peter,

That sounds interesting and looks convincing. But it only works for 
'compressed' spectra covering several kHz and hours, right?

As far as i understand this method can't be done for weak signal 
detections because you 'need' all the energy available from the weak 
signal to fill the bin (one pixel) and just throw away what causes a 
reduction of the SNR, so just the stronger QRN bursts are blanked and 
most of the signal is coming through whereas your method selects just a 
small fraction of the incoming energy (?).

My idea/question came from the consideration that different kind of QRN 
has different optimum blanker settings.

Am 02.11.2016 21:25, schrieb Paul Nicholson:
>
> Even more aggressive sferic blanking raises
> the decode to Eb/N0 +1.7 dB BER 38.2%
> S/N 16.10 dB in 25.4 uHz.

So, if propagation changes, the optimal blanker settings will change. So 
they vary all the time. If a post-processing of a transmission/recording 
taking several hours and night/day changes, it could be useful to 
dynamically vary the blanker settings. So these blanker setting levels 
would have to be determined/calculated by the incoming signal and then 
applied in a next step.
This may be CPU-load intensive, i don't know, it's behind my current 
skills. But the idea is there...


73, Stefan


Am 03.11.2016 18:55, schrieb Peter:
> Hi Stefan et al.,
>
> On 03.11.2016 14:22, DK7FC wrote:
>> ...
>> Last night i thought a bit about noise cancelling on LF/VLF.
> > ...
>
> Running an experimental receiver at VLF/LF for SID-detection (on RPi 
> 3) I chose an almost non-parametric procedure running in frequency 
> domain. It works as follows:
> Do a windowed FFT, compute the median (by sorting) from the power 
> spectrum. Store all spectra and corresponding median values. Next 
> choose a time period (let's say 100ms), pick the spectrum with the 
> lowest median, plot it, drop all the others. The key is that "Median 
> values" are more robust to outliers compared to other averaging 
> procedures.
>
> See what happened when switching from simple averaging to median 
> selection algorithm (~16:50 utc):
> http://lf-radio.de/cgi-bin/test/show_wf.cgi?date=16-10-02
>
> I know that this won't work in case of searching for coherent signal 
> detection, or would be hard to implement. But using this method I'm 
> detecting such very weak signals from far east like NDI or RTZ on a 
> regular basis.
>
> Drawbacks? Yes; it's throwing away a lot of information which may be 
> useful. Another pitfall has to be mentioned: using 1 sec. as a 
> selection window strong time service transmitters nearly vanished 
> since the algorithm will unerringly choose the gaps [^_^]. Therefore 
> I'm using only the spectral part between 15 and 50 kHz for computing 
> the median values.
>
> Peter, df3lp
>