I agree that Direwolf is probably a better performer. Can we get an
audio stream out of an AX5043?
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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