[linrad] Re: Noise Reduction

["Rein A. Smit" <ix.netcom.com; rein0zn@xxxxxxxxxxxxxxxx>]

Hi Leif,

Thanks for the message. I will have to go back to your documentation again
in order to ask you the right questions. In partcular how to determine 
whether
my front-end  the TS850 is a limiting factor in my environment or not.

My concern is not so much the blanking of pulses but the suppression of 
noise
I mean the blanking of pulses as in lightning, line noise and the like. 
As this is not
much of a problem here, I never paid much attention to that aspect of  
Linrad,
probably the real strong side of linrad.

It is really the rumbling noise what bothers all of us in the end.

I guess there is on the one hand the ability to do weak signal in a
highly polluted RF environment with strong undesired signals, possibly
overloading our receivers and on the other hand, the rumbling  noise
that seems to be  inherent with the processing of  noise and what
prevents us from hearing the dots and dashes as dots and dashes.

Then there is also of course selectivity for desired sigals and birdies.


More to follow if I may.... .

73 Rein


leif@xxxxxxxxxx wrote:

> Hi Rein,
>
>  
>
> > So far I have always used linrad and winrad with output from a 
> > ssb receiver ( 1.8 kHZ filter ) not realizing that this might 
> > not be the optimum conditions for the noise reducer and noise 
> > blanking to perform..
> >    
>
> It depends. The Linrad blanker may work perfectly well if the
> noise pulses are separated by more than 0.5 ms or so. (In contrast
> to simple blankers that do not compensate for the phase response
> of the SSB filter.)
>
> The Linrad calibration procedure sets up a filter that corrects
> for sub-optimal amplitude/phase response of an analog receiver.
> Some pretty old info is available here:
> http://www.sm5bsz.com/pcdsp/blnini.htm
>
> With Linrad one can set the sampling speed at just about twice 
> the upper audio frequency. For 300-2100 Hz, sample at 5kHz
> (if your soundcard can go that low) The calibration routine
> will make noise pulses extend one or two 2 samples, typically
> 5 times shorter than in the original audio.
> (see http://www.sm5bsz.com/linuxdsp/pulresp/pulresp.htm )
> Energy is conserved so the pulses also become much higher 
> and easier to locate. Linrad will subtract the weak 
> oscillations that belong to the short pulse. This is a 
> linear operation that does not introduce intermodulation 
> or loss of information.
>
> The process works ONLY if the radio has adequate dynamic range
> within the passband. Many receivers do not, AGC must be off
> and RF volume low enough for the interference pulses to stay 
> within the linear range.
>
> More bandwidth is absolutely necessary when pulses have higher
> repetition frequencies. If the pulses can not be separated 
> from each other, the process fails.
>
> With a bandwidth of 5 MHz on an analog system it is possible 
> to remove static rain almost completely even though it
> sounds like white noise and lifts the S-meter to S9+20 with
> the blanker switched off.
> A description in Swedish is linked to from this page:
> http://www.sm5bsz.com/blanker.htm
>
> I would think that a bandwidth in the order of 1 MHz would be 
> enough for Linrad to remove static rain.
>
> 73
>
> Leif
>
>
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