Yes, the unscrambling is for whitening.   The FEC is a Viterbi algorithm which would be best if we could give it
some guess as to the likelihood that a particular bit is correct.  Normally it might be, for example, how close an
amplitude shift keying signal was to the 1/0 line.  Perhaps the RSSI would help on the 5043, except that in that
case it would be the same over quite a long period--not much help.



73,

Burns Fisher, WB1FJ
AMSAT(R) Engineering -- Flight Software


On Tue, Sep 27, 2022 at 3:42 PM Corey Minyard <minyard@acm.org> wrote:
On Tue, Sep 27, 2022 at 10:03:27AM -0500, Bill Reed wrote:
> I agree that Direwolf is probably a better performer.  Can we get an audio
> stream out of an AX5043?

It's not so much that we use direwolf, which we can't do.  It's if we
are going to consider other options.

It sounds like, from Burn's description, that the AX5043 does FM
demodulation then AFSK (and I assume HDLC).  The packets from AFSK are
then sent to the main processor where unscrambling (is this used to
pseudo-ramdomize the data, or whiten it?) and FEC is done.  That's not
standard, as he said.

The FEC is going to go a long way to improving the performance.
You could still do better with OFDM, but the current design is probably
good enough.

-corey

>
> On 9/23/2022 2:06 PM, Corey Minyard wrote:
> > 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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