Yes, the chopping of the SSB voice is very similar to what the second
half of the MT63 signal sounds like. Next week, we'll try MT63 and just
compare the sounds to SSB phone over the air.
Thanks for the simulation!
If you get a chance, please run Contestia against Olivia and see if you
get any difference in print.
73 - Skip KH6TY
Tony wrote:
[Attachment(s) <#TopText> from Tony included below]
On 4/21/2010 3:25 PM, KH6TY wrote:
This morning, SSB phone was very badly chopped up, but signals varied
from S1 to S4, so we had another opportunity to test digital modes.
We tried DominiEx 11, DominoEx 11 with FEC, Thor 11, and Contestia
16-500. In each case, Contestia produced about 90% copy (there were a
few words with errors), whereas DominoEX 11, DomimoEx 11 with FEC ,
and Thor 11 had over 50% errors.
Skip,
Your results seem to agree with the Doppler tests I ran with the path
simulator. I found that there's an obvious difference in how much
Doppler spread each mode can handle and Olivia tends to be the most
tolerant. Frequency spreading does cause the rapid fade effect we
spoke about and in this test, the fades are faster than the 2 to 3Hz
you mentioned.
There's a sample of the Doppler spread audio attached to this mssage.
The first half is a normal MT63 signal without distortion; the second
half shows the effect of frequency spreading (7 Hertz).
Tony -K2MO
Path Simulation:
Frequency Spread 7 Hz
SNR -3db
THOR11
tiq Rck brown fox juc ekver the la0 nr e;5>yd G
to lsGa tmps over the lazy dog
taAHk brown fox jumpoOireCoer
DominoEX11
riefox zukpl over theeizydqtT
theepuick brocrfak Iuksl ower te layy dty
the quidT ßtwn xox jpsovtr the lazj hoz
DominoEX11 FEC
e quick bÄwn fox jumps over the laonithe q¸?yeXºe
ecteips oveords oo¯he quixoôroc ávs over the lazy d o
Aquick bmt ª?ox jumps over the lazy dog
Olivia 8-500
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.
Olivia 16-500
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.
PSK31 (no print)
PSK63
the /ui btown fox 6smps om r laoty dogt
he qutc^(TM)I own fo jumps over tme la_ogl
the |-ipk yrown fox j om on er hlazb dog
Tony wrote:
On 4/20/2010 3:32 AM, KH6TY wrote:
Hi Tony, When both stations are within the same ducting level, the
only audible Doppler effect is usually reflections from airplanes,
and sounds much like your recording. When there is no propagation
enhancement showing on the Hepburn maps, there is usually a fast,
constant, "chopping up" of the SSB phone signal, and when we switch
to a relatively wide digital mode - print is perfect.
It sounds like there are two different propagation modes in play
Skip. The steadier signals that tend to coincide with the Hepburn
maps would appear to be coming from real tropospheric ducting (which
says a lot for those maps) while the other mode may be tropospheric
scatter.
For what it's worth, the path simulator can emulate the rapid fade
characteristics you mentioned by introducing low-frequency Doppler
spread. This seems to coincide with the 2 to 3 fades per-second you
mentioned (see profiles jpg). The fade frequency tends to become
more rapid as the Doppler spread frequency is increased.
It's difficult to say what's really going on, but the digital modes
themselves may tell us something. We know for a fact that
narrow-band PSK modes cannot tolerate Doppler spread while MFSK
modes have little or no trouble coping. This seems to be the
situation with your tests on 432 and suggests that the throughput
failures are Doppler induced.
I think you can determine if Doppler spread is present, but it's not
going to show up in the waterfall with most digital modes; it needs
to be fairly intense for that to happen. I've found that the best
approach is to measure the spread of a carrier signal using Spectran
or SBSpectrum. The frequency-spread carrier will appear broad
compared to a normal signal; the software "magnifies" the effect --
see SBspectrum images 1 and 2.
As you can see in the waterfall images (1 and 2) it's difficult to
tell the difference between mild Doppler spreading at 0.25Hz and
more intense Doppler spread at 5Hz, yet the difference is night and
day in terms of throughput with narrow modes. Of course you can use
the tuning indicator with PSK31, but it's not as precise.
A few more questions:
Are there times when the fading frequency increases beyond 2 or 3
Hz? Are the "choppy" signals generally weaker than those that
coincide with the Hepburn maps? What are the distances between your
QTH and the stations you work on VHF/UHF? Have the narrow modes like
PSK31 worked at all on what seems to be tropo-scatter mode?
Looking forward to hearing more about the VHF/UHF digital tests Skip.
Thanks,
Tony -K2MO
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