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Hi Stefan,
Thanks for the interesting test. I seem to have little luck with
the dropbox links (they almost always result in some sort of
file-not-found error) but I can imagine the combination. Maybe
there's another link to your dropbox storage, or maybe the space
characters in the filename make it unaccessable ?
I'm curious about what actually causes the spectral spreading of
the strong carrier (caused by the normal noiseblanker), and why
the frequency-selective limiter can achieve a steeper roll-off
(over frequency) than the carefully shaped noiseblanker response.
Maybe the 'window function' of the noiseblanker itself can be
improved (by something more elaborate than the
raised-cosine-shaped edges, and the flat 'window ledge' in
between) ?
All the best,
Wolf DL4YHF
(QRN season and extraordinary temperatures up to 40 °C on their
way ... yucc...)
Am 01.07.2015 01:36, schrieb DK7FC:
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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