[OZAPRS] FT8 for HF APRS ?
Glen English VK1XX
glenlist at pacificmedia.com.au
Sat Dec 2 11:24:39 AEDT 2017
Yeah no need to preserve AX25 frame precisely agreed, just need to be
interface compatible. I've got a few HF modems up my sleeve, most
require *some* linearity in the PA, so maybe just move to nFSK /single
tone this tends to be more tolerant to bad setup...especially when the
aim is to share the nearby spectrum. With the single tone modes, if they
are kept >1400Hz in the microphone input, the overdriving distortions
usually don't get through the SSB filter...
And Andrew, most microcontrollers can execute reasonable modems these
days, there is enough grunt, there is a blur between general purpose
microcontrollers and dedicated DSPs. Where help is required is on the
decode side for the FEC, especially with large frame sizes (which yields
greater "coding gain") , For a signal sense mode, it only has to detect
the presence of a signal, not fully decode it. For decoders, things like
Odroids and other small linux boxes make excellent demods.
On 2/12/2017 10:51 AM, Mark Jessop wrote:
> FT8, JT65 and similar are far too restrictive for something like HF
> APRS. They are quite specifically designed for making short QSOs, not
> for passing of arbitrary data (though is is possible, in a limited
> sense). Take a look at the JT65 spec and you will see what I mean.
> The concept of a MFSK-based mode with *good* FEC is still a good one
> though, but you'd probably be better off either rolling your own or
> starting from something like THOR or Olivia.
> Would the AX25 packet format still need to stay? If you could drop
> that it would give a lot more flexibility...
> Mark VK5QI/VK4
> (Currently SW of Longreach...)
> On 2 Dec. 2017 08:15, "Glen English VK1XX"
> <glenlist at pacificmedia.com.au <mailto:glenlist at pacificmedia.com.au>>
> I have my own low bit rate HF capable position/ aprs reporting
> modem etc but I was thinking at FT8, given the widespread use,
> would make a good replacement for FSK300 (which is dumb arse
> poorly suited with no FEC on HF), and that the decoders allow for
> multiple simultaneous overlapping transmissions in the same BW.
> doing a port for dire wolf would be fairly straightforward.
> glen Vk1XX
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