Hi all,
My MF RX-experiments are continuing and i discovered SpecLab's
frequency selective limiter:
Today it is a quite noisy night.
As reported previously i'm running two SpecLab instances from the same
signal source (VAC1) feeding two WSPR instances, so i can directly see
the effect of the NB or other techniques.
This image shows the experiment:
https://dl.dropboxusercontent.com/u/19882028/MF/Frequence%20selective%20limiter.png
The upper WSPR+SpecLab instance is using the NB, the lower one just
makes the necessary frequency conversion and SSB filter setting (474.2
kHz "dial").
Using SpecLab's noise blanker (NB) makes an improvement of 3...7 dB
tonite, as reported by WSPR-2, so it is worth to run it! Actually there
were many decodes by that instance which is using the NB that didn't
decode on the other one, for example G3WCB at 22:32 UTC (see text
fields). Without the NB there was no trace at all!
However, when running the NB, strong signals will cause the NB the rise
the noise in the spectrum. DK2DB is a strong signal here, showing 40 dB
above the noise (in 1 Hz), (yes i'm such a strong signal too, i
know!!!). You can see the effect on the upper spectrogram at 21:50 or
22:00 UTC. A small signal like G3WCB would be lost in that background
noise which is caused by the NB. However without the NB it wouldn't
decode as well!
So what is the solution? Maybe something like an auto notch filter but
if this is very sharp, it also causes a higher noise arround. Actually
it must be something which just lowers the strong signals a bit, maybe
25 dB but leaves the rest as it is. In a phone call with Markus we
discussed about this and discovered the the frequence selective
limiter and it's performance. It can be found in the options
register card of the filter control window and must be enabled there.
I've the configured a band pass filter including this limiter. After
that filter and limiter, the NB is running in the blackbox between L4
and L5.
Now, when enabling the filter and defining a limit just about 10 dB
above the average noise floor, the signal of DK2DB is reduced to a
level which does not cause the NB to rise the noise floor! You can see
it at 22:08 UTC. In the list you can see that the displayed SNR dropped
from +3 to -15 dB at 22:16 UTC where is reduced the limiter threshold a
bit more. So decodes of the strong local stations do still happen
while the NB is not acting on the strong signal, and QRN is
significantly blanked, allowing to decode weak signals! For
example, see G3XIZ at 22:44 UTC, showing -17 dB instead of -25 dB, i.e.
an improvement of 8 dB! DK2DB has been active to that time.
73, Stefan
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