Posted by
Guy, K2AV on
Dec 06, 2011; 8:17am
URL: http://elecraft.85.s1.nabble.com/Fw-CQ-WW-DX-Contest-CW-2011-Unofficial-claimed-scores-tp7060148p7065975.html
Not to argue but to move toward clarity...
Low level signals are by definition closer to the noise. That would
also mean that there would be more "stuff" between CW bauds. "Crisp"
to some means that the artifacts which define the beginning and the
ends of a baud are there. The extreme artifact is a key click. A
normal artifact is additional bandwidth occurring at the edges of the
baud, but reduced in level. These are down 6, 12, 18, 24 dB from the
pure sine wave. Without these, the baud sounds soft, and the
transitions on and off only excite the directly on-tone nerves in the
ear, instead of some number of tones.
Once the signal is close enough to the noise the baud start and stop
artifacts become obscured by the noise, but the signal is still
copyable by itself. If there are multiple signals in this state and
the tones are close, our ears lose all clue of baud on/off other than
slow (not "sharp") level transitions. We lose harmonic artifacts, and
now must rely only on very soft level changes. Ancient Bell Labs
studies on these issues for telephone would say that for most people,
a change from one CW signal to another at close to the same frequency,
less than three dB in amplitude, would not be a perceptible change.
On these issues in that QRM situation, one would EXPECT that kind of
"muddyness" in the S unit above the noise level. This would happen in
a perfect receiver. This is why someone's signal sending fast when
they go into the noise "blurs" the CW and the only way to overcome
that is to slow down signficantly.
**IF** the AGC was using settings that removed even those level
differences from hearing, the muddying would begin at higher levels
above noise. This would happen in a perfect receiver.
**IF** the hardware AGC was exposed to significant CW signals outside
the DSP skirts due to use of a much wider roofer than the DSP width
setting (such as one listing of 500-800 Hz DSP under a 1000 roofer)
the variable IF gain stage would be tracking the HAGC imposition on
the gain, and raising and lowering the signal of the collective IN-DSP
signals with foreign on/off information not related to any of the
in-band signals. Muddle would be a good word to describe that effect.
This is in a volume region where one is already losing clarity keying
artifacts to the noise floor.
***This effect would occur the worst in the range between where the
DSP AGC started to take up down to the noise***, and would otherwise
be flattened back by the DSP AGC responding only to the in-band
signals and treating the HAGC variations as "propagation changes".
Slowing down the hardware AGC would prevent in-roofer-out-of-DSP
baud-speed external signal gain modulation to weak, under AGC in-band
signals.
I should note for the record that in contests I am either running a
400 8 pole with DSP at 450 or a 250 8 pole with DSP at 350 or 150.
When the offsets of the filters are carefully adjusted, these produce
the sharpest composite skirts down 30 and 40 dB. There is never any
signal outside the DSP limits making it to the hardware AGC. I always
use slow AGC with the fastest possible decay. I always have my
PRE/ATT and RF gain pushed back to match the band.
There may be something else going on, but with my settings I have
never heard any of the reported muddiness, other than the expected
perfect RX conditions listed above.
I am going to try the cap modification to my KRX3 only and listen in
diversity to a single antenna. I will also set the main RX to keep
the 400 8 pole as roofer all the way down. Then at 350 bandwidth, I
will be listening to a wide roofer on one and an equal roofer on the
other.
Should be easy to put an XG3 + step attenuator tone IN band with a
loud CW signal between the skirts and see what happens. Anything
audio happening to one RX and not the other should be really obvious.
73, Guy.
On Mon, Dec 5, 2011 at 12:39 PM, David Gilbert <
[hidden email]> wrote:
>
> "The only time I heard mush on the K3 was when we had worked down the
> pileup and only had very low strength guys all calling on the same
> frequency (the packet/reverse beacon spotting phenomenon)."
>
>
> Multiple low level signals close in frequency is EXACTLY the situation
> that I and at least several others are concerned about. I don't notice
> the problem on stronger signals either, and my low level hearing is
> still pretty good. Operating from ZF-land where you almost always have
> somebody strong calling you is not the typical situation many of the
> rest of us experience from our home QTH's, and I would therefore expect
> that you are less likely to experience the problem. Great sensitivity
> and dynamic range are less useful if what you hear cannot be deciphered.
>
> By the way, it has been pointed out to me that "mush" may not be the
> best descriptor for what I hear, and I agree with that. I think "blur"
> might be a better term. When the problem shows up I can distinguish the
> individual tones (the signals are not dead zero beat) but the crispness
> of the keying disappears. It's as if something is filling in the spaces
> between the keying elements, and I tend to believe that the culprits are
> the sum/difference products of other nearby signals that happen to
> overlap the signal I'm trying to copy.
>
> 73,
> Dave AB7E
>
>
>
> On 12/5/2011 8:34 AM, K5WA wrote:
>> I really can't agree with the mush comment. I've just returned from ZF1A
>> where we put almost 8000 Qs in the log over the CQWW CW weekend. I took my
>> K3 and was able to evaluate it against the station's existing TS-850 since
>> we were M/S and rotated positions. In my opinion, the 850 WAS mushy and the
>> K3 was very clear however, K6AM, our host (who is a VERY serious contester),
>> prefers the 850 since he has been using it for 10 years. He also owns 4 K3s
>> but didn't bring them on the trip since the 850s "live" in Cayman and he
>> brought many pounds of other equipment. Personal preference plays a huge
>> part of this discussion. In this case, K6AM feels like his hearing is
>> limited to a frequency range of 6-7K while mine is still near 20K. Maybe
>> that is part of the puzzle.
>>
>> We regularly saw the 10 minute QSO rate meter in the 300 range and topped
>> 400 occasionally. I loved the way the K3 was able to plow through the
>> pileups which were calling us and work through them as fast as possible.
>> The only time I heard mush on the K3 was when we had worked down the pileup
>> and only had very low strength guys all calling on the same frequency (the
>> packet/reverse beacon spotting phenomenon). These must have been micro-watt
>> QRP'ers or stations with minimal/indoor antennas. I had my AGC on and
>> didn't have any time to try multiple settings but I am extremely happy with
>> the K3's receiver and think it is in the high end competition grade for me.
>>
>> I am sending Eric my configuration file as he requested in case he is able
>> to find an improvement but I will be amazed if one can be found. I've
>> always been extremely happy with Elecraft's commitment and motivation to
>> exceed expectations but they have already exceeded mine as it is. ;-)
>>
>> 73,
>> Bob K5WA
>>
>>
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