Re: LF: SpecLab's frequence selective limiter: Better performance for your WSPR decoder!

[Markus Vester <markusvester@aol.com>]

Hi Wolf, Stefan,

 

> Maybe the 'window function' of the noiseblanker itself can be improved

 

In the presence of strong in-band signals, there's probably a fundamental tradeoff regarding the choice of noiseblanker ramp timing: We want fast ramps to achieve effective blanking with little signal loss, but slow ramps to have narrow spectral skirts around a strong chopped-up carrier. If the desired signal is only a few Hz away both can't be had at the same time, no matter how nicely shaped the ramps would be.

 

One approach would be to do a FFT first, and route channels inhabited by strong signals around the noiseblanker. If I recall right this strategy has been implemented by SM5BSZ in the Linrad noise blanker (which I haven't tried yet). Up to now, my usual workaround has been to manually draw a -25 dB FFT notch around strong signals, but this is obviously not very flexible. The SpecLab selective limiter solves the problem because it automatically reduces strong carriers already in the linear domain, before they get widened up by the noiseblanker modulation.

 

Best 73,
Markus (DF6NM)

-----Ursprüngliche Mitteilung----- 
Von: DK7FC <selberdenken@posteo.de>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Mi, 1 Jul 2015 2:06 pm
Betreff: Re: LF: SpecLab's frequence selective limiter: Better performance for your WSPR decoder!

Hi Wolf,

Am 01.07.2015 11:13, schrieb wolf_dl4yhf:

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 ?

Hm, here the link works when clicking on it into the mail. I'm using the thunderbird browser..

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) ?

Ehm, yes. If you think so, then it must be possible! :-)

73, Stefan (sitting in a air-conditioned office ;-) )

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