RE: noise blanking comparisons

[Leif Asbrink <leif.asbrink@xxxxxxxxxxxxxxxxxxxx]

Hi Ron,

> Greetings all. I am interested in weak signal HF CW and have been 
> drawn to investigate Linrad because of Lief's reputation and my 
> desire to try to be just behind the bleeding edge. I haven't got 
> Linrad running yet so I was especially interested in the WAVE files 
> W3SZ provided. I listened to them carefully.
Please note that Linrad still has only weak signal CW mode, the
SSB mode is actually still the weak cw mode but you can put an
alternative set of parameters that fit SSB bandwidth.

As a consequence the blanker does not work well on strong signals
unless one makes adjustments to the blanker levels which in turn will
not make the blanker optimal for weak signals. You can hear this
malfunctioning as a strong distortion on the loud SSB signal while
the weaker station is not distorted.

The mechanism is that the frequency range that you selected, the
frequency range of a strong SSB signal in Rogers recording, will
be routed together with the noise floor and the weak signals through 
the noise blanker. In weak CW mode the blanker "knows" that the 
total power of the desired signal is much smaller than the total
power of the noise floor, something that is incorrect in this case.

> I would like to hear any preliminary conclusions about the 
> performance of the K2, Linrad and SDR -1000 on both CW and SSB
> 
> With regard to noise reduction am I right in my understanding that 
> the noise cancellation/blanking in Linrad and SDR-1000 is focused on 
> impulse noise and not on reduction of background noise associated 
> with lowband HF propagation ?
I can not say anything about SDR-1000 but the presently implemented
routines of Linrad are for wideband impulse noise only. The Linrad
blanker will be extremely efficient for powerline noise, electrical
fences, car ignition noise and most of your local QRN.

On HF bands there is noise from distant thunderstorms and those
pulses are distorted by multipath propagation in such a way 
that Linrad can not resolve the individual pulses and therefore
the pulse removal does not work. Lirad has to rely on conventional
blanking and is very efficient for short "noise bursts". Linad should
be far better than K2 or any other receiver in case there are 
strong signals near the desired signal for short "noise bursts".
 
> Has anyone in this community investigated de-noising functions 
> provided by Wavelet techniques ?
I do not know what "Wavelet techniques" means, what the fundamental
theory is. To do something that is more clever than to attenuate
the signal, possibly down to zero (blank it out) when the S/N ratio
is lower due to an increased noise level one has to have some
information about the noise source. I have very little experience
with HF signals, but I have not been able to find any information
in the HF "noise bursts" so I do not think anything better is possible.

Linrad does not yet have the procedure to remove longer "noise bursts"
because I do not have suitable test signals. I do not have any 
reasonable HF antennas and I do not know what will be typical in 
"real life" so I am waiting for good recordings of difficult 
situations on HF bands. There is a line "Reserved for blanker" 
in the baseband graph. This is the area for the control functions 
that will be needed for the procedure that will take care of 
longer "noise bursts". I guess one could use Wavelet techniques, 
but since I do not know what it is I may use some simple procedure 
in the time domain. CPU load is not any problem because the baseband 
data rate is low.

73

Leif  /  SM5BSZ




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