AFRICAM Version 5.0 - December 2004

Make sure you unzip everything into the same directory.  The AFRICAM.000
is an overlay module: I ran out of space (64K .com) hi!

I have added another mode.   It is called FSK.  Use your mouse to toggle
the "MODE" into "FSK".  To use this mode successfully, you will need
GPS-sync.  The bitsync is done by selecting "EXT" from the AUTOTRACK
choices, the framesync is done by using the command "SYNC -1" - once
the program is running.

Please note that AFRICAM does not generate audio directly, it only
outputs a bitstream on RTS - you have to work with that to make your
RF signal.  For PSK it's pretty easy: just invert the carrier according
to RTS.  For FSK it's a little more of a hassle.  You need to be able to
switch between two frequencies which straddle the nominal carrier freq.
For coherent MSK it's even tougher - you have to remove the jitter from
RTS and count clocks into a GPS-locked DDS to meet the tight phase
requirements of MSK.  Extra hardware will be needed.  It seems like a
lot of hassle, so we compromise with narrow-shift FSK.  It's easier
to generate and it will still have the narrow spectral characteristic
of regular MSK.

At the Rx end, you must tune for a nominal 800-Hz audio signal in the
center (halfway between the two transmitted frequencies, f1, f2) - if
you are using very narrow shift keying, it can be hard to tune.

AFRICAM will demodulate FSK with any shift; the only rule is that the
higher of the two transmitted frequencies represents a "1" (when RTS
is asserted), the lower of the two frequencies represents a "0".  If
you get it upside down at the Tx end you can use the "FLIP" switch on
the Rx to flip it back.

MSK is a special case of FSK where the shift (from the nominal 800-Hz
carrier frequency) is plus or minus X Hz, where X = bitrate/4.  So,
for example, if you're using MS100, that's 10 bits per second, so the
shift would be +/- 2.5 Hz.

Please note that at this point the AUTOTRACK=ON does not work with FSK,
you will need to use GPS-disciplined bit timing.  Grab seems to be OK.

The FSK demodulator in AFRICAM isn't very robust - if you can't hear the
signal you probably won't get good copy.  On the other hand, it chugs
along nicely at MS25 (40 bits per second) using very narrow shifts if
the signal is strong and in the clear.

Good luck!
Bill VE2IQ