I have just uploaded AFRICAM 5.0 to my website.
Essentially we have added another mode. I decided to compromise and
call it FSK, not MSK :-) Since the demodulator can handle very narrow
frequency shifts, I didn't want to get drawn into the VMSK controversy.
MSK is just CPFSK with a frequency shift of plus/minus bitrate/4. It
offers very narrow bandwidth (compared, e.g. to BPSK) - and a reasonable
error rate when you have good knowledge at the Rx end of everything except
the actual datastream. To do it right pretty much means GPS-disciplined
DDS with some extra hardware. It's way much hassle. We can get the same
bandwidth advantage by using CPFSK at a deviation ratio of 0.5, ignoring
the tight phase requirements at the Rx end required for real MSK. There
is a price to be paid for not fully using the phase information: higher
error rates. But if the signal is loud and in the clear, we can chug
along quite nicely with a very narrow shift (it works on the bench, hi!)
at MS25 using AFRICAM. I just tried it at MS100 (local, but through a
radio) and it was still printing correctly with a shift of +/- 0.05 Hz,
some 50 times narrower than standard MSK. I can't hear that shift.
Should fit nicely in the watering hole. Move over VMSK, here we come!
Suggested calibration trick: first use BPSK transmitted on your nominal
carrier freq, measure the received audio freq using AFRICAM, then set it
in as your exact carrier frequency (fcar=xxx.xx) before you switch over
to FSK. Do not use the AUTOFREQ mode, it's way too jittery, and make
your audio sampling device is well warmed up beforehand.
At this point I still haven't figured out a good way to do the bit-sync
without using GPS, but AFRICAM already supports GPS-disciplined bit
and frame sync, so I just used it. The ET1, etc error-correcting codes
are also supported, which helps a lot with FSK. If you already have
AFRICAM set up for GBPSK, to try the new mode you'll just need a DDS
synthesizer that can generate either of two frequencies according to
an input signal (RTS from AFRICAM).
One extra note: When you use the AFRICAM "SEND" command to send a message,
it sends the message in whatever mode you have selected at the time. If
you then change modes, it will *continue* sending in its original mode.
So remember, if you change modes, issue the SEND command again so it will
start up in the new mode.
If you want to play with it, AFRICAM 5.0 is available from my website at
http://www.magma.ca/~ve2iq
Hope we all have a little fun with this,
Bill VE2IQ
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