...Since we now have shown that is technical possible to copy our signals over
10000km and more the time might be right to develop a mode that will allow
us to make QSO's over these distances.
73, Rik ON7YD
Dear LF Group,
I think this is a good idea for what could be a useful and easily
implemented mode. Consider the advantages:
-Potential for significant speed increase, to facillitate 2-way QSOs
-"Backwards compatible" with QRSS/DFCW modes - could be copied using
existing "visual"spectrogram techniques. At the same time, machine decoding
could presumably be applied, perhaps enhancing reception.
-Bandwidth not much greater than existing QRSS/DFCW, so easy to fit lots of
signals into the band, dodge QRM etc.
-Simple TX hardware; The 7 tone multi-FSK signal could be generated using a
VCXO circuit keyed by an extremely simple interface from an 8 bit printer
port; for example, an extension of G3AQC's DFCW generator. Since only one
tone would be transmitted at a time, there would be no need for linear
amplifiers or envelope shaping other than the normal click suppression.
Then again, more sophisticated DSP techniques could be used if desired, or
both tones transmitted simultaneously, at the expense of greater complexity.
For visual copy, the frequency stability requirement would be comparable to
current QRSS; provided the centre frequency of the signal drifted less than
1/2 of the tone spacing during one over there would be no ambiguity. So
0.1Hz tone spacing should be practical for most stations, judging by the
signals currently seen on LF. Errors of a similar order in the individual
frequency shifts could also be tolerated - for visual interpretation, the
exact dot length and tone spacings would not be important, provided that
care was taken to design the code so that the maximum and minimum
frequencies could be identified, eg. by using the space symbol as a
reference, as Rik suggests. A machine decoding system might require better
accuracy for optimum performance - but could still be copied by someone
using the more basic visual method.
I think VK2ZTO has done work on this type of system before, including using
a larger number of tones with an individual tone for each character. I
guess this might give a 3dB advantage, but would make the transmitter more
difficult, and I imagine trying to pick out one of 40 or 50 possible tones
on the screen might be tricky for visual reception.
Cheers, Jim Moritz
73 de M0BMU
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