[Elecraft] DSP impressions

Sun Aug 10 15:23:01 2003

I think that someone has perhaps already posted on this, but as a general
fact of life with statistical noise reduction, the wider the bandwidth of
the signal plus noise, the better job the NR can do in improving the S/N.

It has to do with the coherency and stationarity of the noise process(es),
among several other aspects - but you knew that!   <:}

When noise is band-limited, it loses some of the randomness which is
required for the NR algorithms to work efficiently. As an absurd example,
consider a filter only a few Hz wide. The output of such a filter, fed with
wideband noise, would be almost a sinewave with little noise character to
it. Hence the NR would have nothing to work on.

Wolf has the right idea: use the widest bandwidth you can tolerate in order
to give the NR the most noisy noise to work with. Don't expect it to do much
when you narrow the bandwidth way down. That is another approach to
improving S/N but the two are almost mutually exclusive when one attempts to
use it with NR as well.

If you will fire up Spectrogram sometime and look at the noise output of
your receiver with the widest filter selected and then apply the NR, you can
see the changes in the noise spectrum that it makes. As you narrow the
bandwidth, you can see less and less change as NR is turned on and off.

It is equally interesting to watch the audio output on a scope and see how
the NR changes the waveform of the noise.

Narrow bandwidth or NR? Depends upon the noise and the signal and what you
can tolerate. A lot of folks balk at NR because of the change in the sound
of the signal. Two other aspects that are mutually exclusive are
preservation of the sound of the signal and reduction of the noise. I find
that I am willing to listen to a fair amount of distortion and watery sound
in order to hear the other station at all!  <:}

Interesting stuff!

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"
<mailto:[email protected]>

----- Original Message -----
From: "Wolf-Ruediger Juergens" <[email protected]>
To: "Elecraft List" <[email protected]>
Sent: Sunday, August 10, 2003 1:40 PM
Subject: Re: [Elecraft] DSP impressions

> On Sunday 10 August 2003 19:28, David A. Belsley wrote:
>
> > I would add that I don't find the DSP to be particularly good at
> > pulling a thoroughly marginal signal out of the noise.  This doesn't
>
> I find out a way to get a better reception of weak signals with the
> KDSP2.
> I set the CW-Filter to FL1, DSP Filter also to widest bandwidth or
> lowpass and than Noisereduktion to on and voila even very weak signals
> are coming out of the noise ;-).  In this setting the cw signal is not
> destroyed by the noisereduktion and sounds great. But this will only
> working if you have no nearby QRM.
>
> If I switch to FL2 or less bandwith the noisereduction makes a very ugly
> and harsh sound. I dont know why...
>
> 72 Wolf, DL2WRJ
> --
> Old programmers never die.  They just branch to a new address.
> (/usr/games/fortunes)
>
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