[Elecraft] K3 Noise Reduction

[Bill W4ZV]

Roy Morris-6 wrote:
> 
> If noise reduction is not recommended with reduced bandwidth in CW, and
> unpleasant artifacts occur in SSB; then I find NR to be useless.
> 

You may want to read the following by Lyle KK7P:

http://www.zerobeat.net/mediawiki/index.php/K3_DSP

####################################################
NR explained by Lyle Johnson

I think there is a fundamental misunderstanding of what NR is and how it
works, at least in the context of the K3.

1) NR is better named SE (signal enhancer).

2) It is a "short" FIR filter whose coefficients are continuously being
recalculated.

3) It defaults to suppressing everything.

4) When it senses that there is something that correlates (i.e., has a
pattern that doesn't seem to be entirely random), it attempts to build a
filter around those frequencies that seem less random.

5) Because the FIR filter that is being implemented is short, the filter
being built is less selective than the normal DSP filters in the radio.

6) NR is rarely useful if the bandwidth is narrow. If you set your CW width
to 400 Hz or less, for example, there is no point in running NR *unless* you
want to use it as a sort-of "smart squelch."

7) The narrower you set your WIDTH, the more that noise appears like a
signal, and the worse the NR will perform. And the less noise there is
anyway, assuming there is a signal present. NR cannot compete with a narrow
filter, and was not designed to.

8) I find NR most useful during CW operation with the 2.8 kHz roofing filter
NORM'ed so the Rx bandwidth is wide. Assuming band activity is low, the Rx
is quiet. If a CW station comes on within the Rx passband, a filter will be
built around the station and I can hear it.

9) Similarly, I find NR in SSB is mostly useful as a sort of squelch when
tuning around, or monitoring a frequency you are expecting a call on
(probably a net or a sked). I use NR1-1 or NR1-2 for this. Mild suppression,
not too much impact on fidelity, and lets me hear weak signals, too.

I hope this is helpful in better understanding how NR is implemented in the
K3. 
#####################################################

Unfortunately the term "Noise Reduction" implies that NR should yield some
S/N improvement.  In my experience (with both Orion's and K3's NR) it does
not.  If you're adept at adjusting the K3's WIDTH and SHIFT controls, I
don't believe there's much to be gained by using NR...except possibly
operator confusion.  A filter is a filter, whether it's built automatically
(by NR) or whether the operator determines it manually by adjusting WIDTH
and SHIFT.  Noise bandwidth reduction follows the same laws of physics no
matter how the filter characteristics are set up.  

73,  Bill



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