Skip,
> Based on your simulations, Doppler spread may not be the major problem on
> VHF/UHF, but Doppler "shift" is
A spectrogram would show both Doppler spread and Doppler shift. If you have a
recording (steady carrier preferred) we can analyze the signal. I've attached a
spectrogram that shows 4 separate Doppler induced signals on 2 meter CW.
The 130 mile QSO took place this year between WA2FGK and yours truly. The top 3
traces certainly show aircraft Doppler, especially the one in the middle that's
dropping 3Hz per second. Notice that there's no Doppler spread to speak of.
PathSim is capable of both Doppler spread and shift. The two can be combined to
work simultaneously or used separately. Unfortunately, it's not capable of
sweeping Doppler across the channel to simulate aircraft scatter for VHF/UHF
work.
> We are now starting tests comparing DominoEx 4 to DominoEx 8
Since propagation can change in a instant, it might be useful if you could
transmit both modes simultaneously so that each is exposed to the same
propagation conditions. It saves time and the results should be useful. I had
done this some time ago with a fellow in the U.K using HF (experimental
purposes of course).
My Vista OS allows two instances of Fldigi to run simultaneously and it will
transmit and receive as well. You'll have to leave some room between the two
modes. It's worth a shot.
Tony -K2MO
----- Original Message -----
From: KH6TY
To: digitalradio@yahoogroups.com
Sent: Saturday, January 09, 2010 7:08 AM
Subject: Re: [digitalradio] Re: DominoEX, PSK63F Path Simulations
Tony, thanks for the simulations.
Based on your simulations, Doppler spread may not be the major problem on
VHF/UHF, but Doppler "shift" (i.e. total displacement of the signal in
frequency, and not speading between symbols or inter-symbol interference),
which basically "mistunes" the signal may be. MFSK16 has to be tuned within 4
Hz, and the AFC on PSK63F can capture maybe within 15 Hz (with weak signal) ,
but using DominoEx 4, shifts of 50 Hz still produce good copy, even just at the
noise threshold. DominoEx 8 usually produces poorer copy than DominoEx 4, but
we are talking about signals so weak that the better minimum S/N of DominoEx 4
comes into play.
We are now starting tests comparing DominoEx 4 to DominoEx 8, and maybe we will
find DominoEx 8 outperforming DominoEx 4 under some circumstances. However, it
is seldom possible to do much mode switching fast enough to maintain copy, so
the need is to find a single mode that works the best most of the time and use
that. Our conclusion so far is that nothing outperforms DominoEx 4, but it is
too slow for messaging, so the next best choice for emcomm is DominoEx 8 for
VHF/UHF.
As you say, your simulations set the signal level far enough above the noise so
PSK63F copies 100%, while DominoEx 8 has a few more errors. In this case, it
looks like the combination of Doppler spread and path delay shows PSK63F
outperforming DominoEx 4 or DominoEx 8, but the problem is that QSB is almost
always present, and strong enough that the differences in minimum S/N affect
the copy. I don't know how QSB can be added to the simulations, and the period
of the QSB fade varies between a few seconds and maybe three per second.
Unfortunately, our real-world tests have to include both QSB and Doppler
effects, and under those conditions, it has been rare that DominoEx 8 can
outperform DominoEx 4, and PSK63F doesn't work at all, probably due to enough
Doppler shift to cause mistuning that the AFC cannot handle. I think Moe's
psk31 program has a "satellite" mode with AFC that can track Doppler better,
but I think it does not include PSK63F, so no way to use that program for
testing.
73
Skip KH6TY
Tony wrote:
> Tony, a comparison of PSK63F to DominoEX 8 and DominoEX4 on your simulator
> would be very appreciated! Skip
Skip,
According to PathSim, DominoEX4 seems to have a relatively low tolerance to
Doppler spread. As you can see, DominoEX8 and some of the others seem to be the
better choice. The question is whether the UHF path will allow you to test
these modes since they may be less sensitive compared to DominoEX4. They
obviously need to stay well above minimum decode threshold in order to make
sure the Doppler effect is causing the throughput loss and not the QSB. I know
that can be difficult to accomplish with weak signals.
I set the simulator SNR to -3db to keep signals well above minimum SNR for each
mode tested. I used a relatively small path delay for the #2 path which will
hopefully represent the multipath delay found on VHF/UHF DX. Of course there's
a large number of multipath possibilities so you can only expect so much from
the path simulator.
Path #1 Path#2
Doppler Spread: 5Hz Doppler Spread 5Hz
Path Delay: 0 Path delay: 0.1ms
It will be interesting to see your tests results Skip. Please record a few
signals so we can compare them on the spectrogram. Thanks for all...
Tony -K2MO
PSK31
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DominoEX4
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THMnUIC6RO7N FOXJKGPtO7arN D3ar LA'3 DCG
KqeiUILK BD,WN FOX JUGAe LVAR DKq LAZ3 DCi
DominoEX8
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PSK63F
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the quick brown fox jumps over the lazy dog
the quick brown fox jumps over the lazy dog
PSK63
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THOR8
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THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG
THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG
OLIVIA 500/8
THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG
THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG
THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG
THROBX-2
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