I looked a bit last night at the ax5043; I didn't realize (I should have
remembered) that it doesn't just convert to baseband, it actually
demodulates the signal.  Are current designs just using that for FM
demodulation and doing the AFSK modem in the TMS570?  I don't see a way
it could do both.

It does appear to do GMSK.  The only concern there is if it can support
the polynomial used by G3RUH for randomization, I think.  I'd be
surprised if it didn't.  But I don't have the programmer's guide.
And it doesn't matter, I guess, if it can't do FM and GMSK at the same
time.

For AFSK, you can do a lot better than what a hardware decoder can do.
See https://github.com/wb2osz/direwolf/blob/master/doc/A-Better-APRS-Packet-Demodulator-Part-1-1200-baud.pdf
for details.  direwolf can pull signals out of the noise in a way that a
hardware decoder can't.  The difference is significant.  I've done some
thing in my modem that can improve things even more.

There is a similar situation for 9600:
https://github.com/wb2osz/direwolf/blob/master/doc/A-Better-APRS-Packet-Demodulator-Part-2-9600-baud.pdf
But that's only the receive side, since this would only be transmitting
it doesn't matter.

You could probably do the AFSK demodulation on a TMS570.  Modulation of
9600 can probably be table driven, so that should be doable.

Note that there are far better modulation techniques than these.  Almost
anything being done now is using OFDM of some type.  VARA is taking over
in the packet world.  All modern modulation for cell phones is OFDM.  I
think digital TV is, too.  OFDM is certainly better for fading and
multipath and since it's using low-baud subcarriers I'd guess it's
better for doppler, too, but that's just a guess.  It would affect the
orthogonality (?) of the subcarriers though.  Not sure.

You probably couldn't do OFDM on the TMS570.  Certainly not 4 channels.
You would probably need one of the TI chips that has a separate DSP.

On the ground side, anyone with a sound card modem and a reasonably
modern PC could handle it.  It would provide better performance, I'd
guess singificantly better, than using AFSK and G3RUH.  (It would be
even better if you could get rid of putting it inside an FM carrier and
directly modulate, but that's probably not a practical option.)

Also, on the satellite, if you converted to I and Q directly from RF,
and you had a DSP or a fast enough processor, you could get rid of the
AX5043s and do the FM and modem in the DSP.  I remember seeing single
chips that could do this, but I would have to hunt to find them.

Anyway, since we are just getting started, I wanted to point out that
options are available that are better from a pure technical point of
view than what is currently being proposed.  I know there are other
concerns like the availability of current working circuits, power budget,
timeframe, etc.

-corey - AE5KM
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