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
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