Hi John and the group:
I've just been looking at this MSK thing again and it doesn't look
too bad. Yes, I know, you've heard that before, hi! I think I can
put it into AFRICAM with some effort, as an extra transmission mode,
then we'd still be able to use ET1, etc for error correction.
There are lots of potential problems for appliance operators who like to
just run the software and see perfect copy rolling across the screen right
away. For one thing, the decoder is even more sensitive to mistuning than
BPSK. And the problem gets worse at very slow data rates where the
frequency shift becomes progressively less. For example, at MS1000 the
total frequency shift is only 0.50 Hz (+ or - 0.25 Hz away from the nominal
800 Hz audio carrier freq) - and any mistuning seriously eats into our
noise margins. That sort of tuning accuracy is awful hard for most folks
to achieve. Which probably means we will have to run it at a pretty good
clip, say at least MS100, which in turn means we lose our ability to pull
the weak signals out with nice long integration times like we can do with
BPSK, especially GPS-sync'd BPSK. At this point I can't see any straight-
forward way to search over a wide frequency range for an MSK signal, find
it and lock onto it. Maybe with a lot of thought that will come, but
initially we'll have to rely on absolute tuning. It's clear this isn't
going to be a weak signal mode (the lower the SNR, the worse MSK looks
compared to BPSK), but at least the signal is confined to a narrow band,
so we might be able to run it in the "watering hole" :-)
When/if I'm ever ready (?) I'd love to do some on-air testing. What I need
to know is this: if I put it into AFRICAM the same way we do BPSK, who out
there has the capability to generate an MSK signal using just the RTS output
from AFRICAM? What this entails is transmitting one of two possible
frequencies (based on the value of RTS) - with continuous phase at the
transitions. The frequencies ideally should be exactly (not approximately)
5.0 Hz apart, e.g. for MS100, but in all likelihood a millihz or so isn't
going to make or break us, so a DDS-type synthesizer should be able to
handle it if we are sure the signal stays continuous when the frequency
changes.
An alternative would be to generate the MSK audio output with a sound card
(or S-D DAC board), and square it up to drive a transmitter, but that can
get rather messy. I did it in the old COHERENT program but dropped the
scheme in AFRICAM because there was just too much variation in sound card
sampling rates. I believe there is still one of my old programs out there
that can generate MSK that way, but unless you know the sampling rate of
your sound card very precisely it's likely to be more problem-causing than
problem-solving. And there aren't a whole lot of SD-DAC boards out there,
and going that route means a SSB-type LF transmitter.
So for running some tests this winter, who out there has GPS hooked up to
AFRICAM with a DDS synthesizer that can switch between two frequencies
based on the RTS value, and might be willing to spend some time testing
this?
Thanks,
Bill VE2IQ
At 09:12 AM 11/23/2004 -0500, John Andrews wrote:
Bill, et al:
I gave up on it.
But if you do go back to it, I'll be happy to participate. There are
probably enough of us who have the necessary hardware to do some real world
tests.
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