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Dear Chris, Laurence, LF,
I have tried to look at the Alaska pulsegroups
in more detail. On http://www.alice-dsl.net/df6nm/LoranView/LORANALASKA_5990.jpg (183
kB) there is a graphical "SSTV-type" plot of Laurence's complete wav, with 11.10
kHz sampling and "line rate" set to 59.90 ms. The appended clip shows
the area of the first crossover.
The four different GRI 5990 pulsegroups can be seen
as a vertical set of lines from top to bottom. GRI 5980 advances 100 us to
the left in each row, with the first two of eight pulses blinked on
only for about 10% of the time.
I think the selective fading effect described by
Chris can be understood by the varying delay during the pulsegroup overlap
period. Whenever the pulses coincide, they will add up coherently,
depending on the phase of the RF path and the phasecoding state. About five GRI
periods (0.3 s) later they will be separated 500us, with minimum
1 kHz and maximum 2 kHz content, and no dependence on RF phase.
Looking in detail at the first time of
coincidence (yellow arrow), you can see that alternating "knots" appear brighter
and darker. This indicates that we are experiencing the
superposition of an "A" and a "B" type phasecoded pulsegroup, with the
relative phases alternating for subsequent pulses. The second coincidence
near the end of the file has equal phasecode states (A-A and B-B) in both
rates, and all knots are equally bright. This may also be contributing
to the more pronounced phasing sound on this one.
Another interesting detail unrelated to the phasing
is that some of the 5990 pulses from Shoal Cove appear to be missing.
Plotting it against its former other rate 7960 revealed that the
alternate blanking scheme (ie. omitting a pulse for one rate when one has
to be transmitted for the other at the same time) is still active, even though
the actual transmittission for the second rate has been turned off on Feb
8th.
Last not least there are many single pulses
"floating around". These are occuring approximately every 8.33
ms, most likely an artifact from a 60 Hz switching power
supply.
Kind regards,
MArkus
Sent: Saturday, February 20, 2010 12:21 PM
Subject: Re: LF: Loran A and C
Dear Markus,
Lawrence and others,
Thanks for your interesting comments on this
theme.
> The slow phasing effect is caused by pulsegroups of these
very similar rates, walking through one another in time.
I think I can
hear two effects which might be described as phasing.
One is due to the
different GRIs (Group Repetition Intervals, i.e. pulse group rates) which Markus
describes. This appears a a change in the chittering sound. When the groups
coincide you hear it as "chit chit chit" and the
pulsiness is more pronounced: you can hear this about 1/5 to 1/4 of the way
through the recording. When they fit in each others' gaps you hear
"chitchitchit" and the sound seems smoother: you can hear this just before half
way through the recording.
The other effect is a change in the relative
strengths of higher and lower audio frequencies. You can hear this from about
4/5 of the way through to nearly the end, where the sound cycles between treble
and bass. Presumably this is cancellation in some part of the 90-110 kHz
spectrum, though the treble will be represented by both the parts just above
90kHz and those just below 110 kHz, so presumably it is subtler than that*.
Maybe this is caused by frequency-selective fading, presumably what Lawrence
described as the "musical sounds they made as sky waves did their thing". Could
someone more knowledgable than me comment on that?
* Unless Lawrence was
demodulating as SSB, in which case he would see only one half of the
spectrum.
For those who have deleted the original message, the
recording is at
> http://kl1x.com/loranalaska.wav
73,
Chris
G4OKW
-----------------------
Dr Chris Trayner
School of Electronic
& Electrical Engineering,
The University of Leeds,
Leeds LS2 9JT,
United Kingdom
Tel: +44 113 34 32053
Fax: +44 113 34
32032
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