Re: PSK Squelch Function and another "effect"


Dave Kjellquist, WB5NHL
 

Steve;
There is another "effect" relating to the color
waterfall that I have become aware of as a developer
of PSK31 sofware. Whether the waterfall colors are RGB
or CMY based unlike a greyscale the increments of
signal strength are continuous but may large steps
depending on the number of colors. In most waterfalls
the signal strength is logarthimic.

Let me illustrate the effects, suppose there are 2
stations. The first a QRP station has display strength
of 10 and the second a display strength 20. Thus, 10x
relative difference (remember logarithms). Assume the
noise floor is 3. BTW, 100x or more differences are
very common. This gives the weak station about 7db
above the noise (without heavy math, just about
copyable). OBTW, don't even try for IMD on the weak
station it will be bogus and even the strong station
in this example is suspect, but this is a different
topic for another time.

With a greyscale (usually 256 values) waterfall
display, the signal differences show as intensity
differences just above. Now for color, if we assume, 8
colors for the entire display range say 0-80. First
color 0-10, second 10-20 etc. Ooops..... The lower
signal and noise have the same color....gone!

I have contrived this example a little...but only a
little. I fundamentally do not like a waterfall
particularly color for weak signal work. The solution
is a spectrum display (which I'm adding to our Linux
Kpsk program) available on most PSK programs.

Steve, maybe if everone turns off the squelch and uses
spectrum displays we'll get some of those big gun
scores ;-)

---- 070@yahoogroups.com wrote:
I received six responses to my question on whether
any of you use
the squelch function available in PSK. As expected,
the responses
varied. Two people never use it, while the other
four use it at least
occasionally. Three of those four specifically note
that they turn it
off when working with weak signals, while the fourth
carefully
watches visually for the weak signals.

My conclusion based on this non-scientific sampling
is that the
people who responded to this question understand
what the
squelch function is, when to use it, and most
importantly when
NOT to use it. But I also think that there are many
PSK operators
who do not have this same understanding.

As a dedicated QRP operator, I recognize that my
signal is usually
one of the weaker ones on the waterfall. And I know
that it is
difficult for me to stand out in a pileup. But I
find that there are
many times when I try to respond to the CQ of a very
strong station
and it seems he can't hear me. Even if the other
station is running
100 watts, he's only 13 dB stronger than I
am--that's about 2 S-
units. (Note that antenna gain works both ways,
transmit and
receive, so the only real variables are our
transmitter powers and
our receiver sensitivities and noise levels.)

Last weekend in Delaware there were a few stations
that didn't
respond to my call, even though they were plenty
strong in my
waterfall, and their power levels weren't
drastically more than mine.
Perhaps their noise levels were higher than mine,
but I also
speculate that they may have set their squelches in
a way that
"filtered out" the weaker stations.

From a technical standpoint, I'd like to point out
that PSK acts a
lot more like SSB than it does FM. With FM, there is
a real
threshhold effect--below a certain SNR, there is
virtually no copy,
and just a fraction of a dB higher, there is very
good copy. This
type of modulation is wonderful for squelch, and you
can eliminate
the annoying background noise when there's no signal.

But PSK, like SSB, has a more gradual relationship
between SNR
and performance. Of course there is a large range of
signals that
have virtually perfect copy. But we've all
experienced the ranges of
signals that have varied from 10% copy up to 98%
copy. This is the
tricky range, where a squelch setting may eliminate
a lot of random
letters, but it may also eliminate a weak signal
with only 65%
copy. This level (65% copy) admittedly won't make
for a pleasant
ragchew. But 65% is plenty good enough, with a few
repeats, for a
contest QSO or for DXing.

I don't know of anyone who uses a squelch during SSB
operation,
especially during a contest or while DXing. Yes,
there is static, and
sometimes it's hard to make out the other station.
But we train our
ears and mind to ignore the noise and copy the voice
on the other
end. I think the same principle applies to PSK
operation. By
opening up the squelch a little bit more (or turning
it off
completely), there will be more random or erroneous
characters on-
screen. But just as you can train yourself to HEAR
through the
noise, you can also train yourself to READ through
the noise. And
sometimes that can be the difference between making
the contact
and not.

Let me encourage you to try operating without your
squelch on, or
at least opening it up more than usual. Yes, you
will see some
more garbage characters, but you'll also find that
you will be able
to copy some signals much better. And with a little
practice, I think
you'll find (as I did) that the garbage characters
won't bother you as
much as you think they will.

73,
Steve
W3HF

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