Hi Gerhard,
You should first read page 45 of the
Orion II Operating Manual:
http://radio.tentec.com/cms-files/566_manual_release2_0106.pdf
*********************************************
"DSP noise reduction in the ORION II
interacts somewhat with the digital AGC
system. Here is why: when turning on the
noise reduction, without changing digital
AGC, the receiver will get very quiet, very
quickly. This is not the desired effect of
noise reduction! The desired effect is to
identify what is signal and what is noise and
improve the ratio between the two, rather
than making the whole receiver quieter,
signals and noise. When DSP NR is
selected, digital AGC artificially reduces the
threshold setting of the digital AGC, and
you will notice that the overall noise level
can increase, but signal-to-noise ratio
improves and that is the ultimate goal of the
NR system.
In practical terms, how does it work? There
are 9 different settings, and each of the 9 is
used to determine only how aggressively
(quickly) the NR adapts and identifies
what is signal and what is noise. Here is
the rub (and it's logical): with weak signals, it
is harder for any DSP NR system to
determine what is noise and what is signal.
When turning DSP NR on with a setting of
"1" with a signal that is very weak, it's going
to take a very long time for the algorithm to
figure out what is signal and what is noise.
For a somewhat louder signal (20 dB or
more above the ambient noise level, still
fairly weak), a setting of "1" will adapt very
fast! What is needed for weak signals is
more aggression. By starting the NR at "3"
or "4", the NR will more quickly adapt to
what is signal and what is noise for a weak
signal. Once the NR has adapted, for the
same signal, no change in NR will be
made when adjusting the value! If you
turn the DSP NR on, and just cycle through
the values looking for "best", nothing will
happen. Because the DSP NR builds a
bandpass filter to automatically reduce
noise, it produces the same effect as
manually selecting a very narrow DSP
passband filter."
***********************************************
I'm no DSP expert (maybe Sinisa
or others are) but I'll make an attempt at
your questions:
>Why "with weak signals it is harder for the DSP NR to determine
what is noise and what is signal"?
For the same reasons your ears have
difficulty. Imagine the signals being in
a spectrum display. If the desired signal
is barely visible above the noise level,
your ear or the DSP algorithm has trouble
distinguishing the signal from noise. DSP
has the advantage that it can average over
a long time, so eventually the signal may
become "visible" to it, but the disadvantage
is that averaging builds a filter so narrow
that it may "ring" for keyed CW signals which
typically have bandwidths of 40-50 Hz even
at slow speeds (~15 WPM). There is simply
no advantage to building a very narrow filter
that may work fine for detecting a carrier
(~0 Hz BW), but cannot pass the bandwidth
required by keyed CW. Note that this is why
DSP "Waterfall Displays" can detect the
presence of a CW signal that may not be readable
by ear. *Detecting* a signal is very different
than *decoding* a signal, mainly because the
information bandwidth required is wider for the
latter.
>Why does a broader band of noise make the DSP NR more effective?
Will a roofing filter reduce the effectiveness of DSP noise
reduction, do wideband I-Q signals improve it?
For the simple reason that DSP NR is
really a bandwidth-narrowing device. If the
bandwidth is already narrow, DSP cannot
improve upon the situation. This is exactly
what the manual is referring to in the last
sentence quoted above. If BW is already
narrowed (either by roofing or DSP filters),
NR cannot make much, if any, improvement.
>Why shouldn't we use a different NR techniques for different modes?
I believe the technique is the
same but the minimum bandwidths are
very different depending on the mode.
Narrowing bandwidth improves S/N because
you filter noise out of the signal plus
noise. For a given signal plus noise,
every time you reduce the bandwidth by
a factor of two you reduce noise by 3 dB
and therefore improve S/N by 3 dB. When
you reach the BW limit determined by the
fundamental information bandwidth of the
specific mode, there is no more that DSP
can do. I am not sure what lower BW limit
Orion's NR assumes for each mode, but I
suspect it is similar to the basic DSP
BW limit for CW (i.e. 100 Hz). SSB may
be in the 1700-1800 Hz range but I have
really not played with it enough to know.
1500 Hz is about the limit I find manually
but it requires some major adjustment of
PBT which I suspect the DSP NR does not do.
In summary, I've said this before
but will repeat it here again:
***NR will not do anything that adjusting DSP
BW and Gain controls will not do faster and
more directly.***
If anyone still questions my statement, please
again read the last paragraph from the Orion
manual quoted above. The only reason the
above might not hold true would be if Oiron's
NR actually creates a filter of less bandwidth
than can be done by reducing the BW control.
Some of us want to find a way to
circumvent the basic relationship between
information, bandwidth and noise described
by Nyquist, Shannon and Hartley at Bell Labs
in the late 1940's, but there is simply no free
lunch, DSP or no DSP, advertising hype (hello
Icom!) or not. If you want to learn more, there
are plenty of good references available on the
Internet by searching "information bandwidth
noise" and adding one or more of the above
three names.
73, Bill W4ZV
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