Indeed yes, all correct. This is one of the "other" methods I had in mind
when I wrote one of my earlier posts. And narrow bandwidth makes the job of
separating "good" signals from "bad" signals more difficult when using this
particular method. Of course there are ways to mitigate that also. Takes
more processing though.
Another point...the signal is highly correlated. Hence little if any
reduction takes place really close to the signal. The noise further from
the signal is, we hope, highly uncorrelated. Hence it gets reduced a ton.
Result ? The signal level away from the "good" signal drops precipitously,
while that close in does not - giving the appearance of a band pass filter
having been dropped in place over the signal. Can be kinda misleading huh ?
73 de Gary, AA2IZ
----- Original Message -----
From: "Dr. Gerald N. Johnson" <geraldj@storm.weather.net>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Friday, December 08, 2006 12:32 PM
Subject: Re: [TenTec] Noise Reduction Setting
> On Fri, 2006-12-08 at 09:36 -0600, Grant Youngman wrote:
> > > Likewise, the phrase "building a filter around the signal"
> > > which has been used in several posts in various forms does
> > > not mean that there is an actual pass band being established.
> > > I've seen that language used elsewhere as a simplified
> > > explanation of DSP, referencing the more familiar analog of a
> > > filter, instead of trying to explain the math, etc, of true
> > > DSP noise reduction.
> >
> > In the v1 Orion it is most definitely the case. It's clearly observable
on
> > a spectrum analyzer, and isn't just the "analog thinking" of a newbie.
This
> > is as much "true DSP noise reduction" as any other DSP-based process
> > targeting the task.
> >
> >
> >
> > Grant/NQ5T
> >
> While the effect on the output spectrum is that of a bandpass filter, I
> don't think NR works that way. I think it works on correlation of the
> pass band with a time delayed copy of the pass band. When the time delay
> is a integral number of cycles of the tone signal, that tone adds up
> while the noise doesn't because the noise isn't coherent. That can also
> work for multiple tones, it gets a lot more difficult to create multiple
> tracking filters for those multiple tones, yet NR should work for those.
> At least every noise reduction book I've read in the library depends on
> autocorrelation. The differences is how they choose the time delay.
>
> Its a major problem in "noise" reduction when the roofing filter has a
> narrow bandwidth. That tends to make the noise more correlated and so
> makes NR less effective. And some things that would be considered noise
> to listeners, such as computer hash, may not be random at all but be
> well correlated and so the autocorrelation process won't eliminate it
> but may enhance it. That can also apply to power line noise and possibly
> lightning static from isolated events.
> --
> 73, Jerry, K0CQ,
> All content copyright Dr. Gerald N. Johnson, electrical engineer
>
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>
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