If anybody's interested in trying my version of the python FT8 decoder with
a more sensible SNR
implementation just go here: https://github.com/mcogoni/weakmon
I modified the code to look for the lowest power bin within the input
bandwidth and each
individual signal is compared to this.
To obtain better SNR values avoid multiple passes for decoding since it
implies
subtracting the strongest signals, bin by bin, from the waterfall so the
relative
powers get all modified for the "hidden" signals.
marco / IS0KYB
Il giorno gio 1 ago 2019 alle ore 21:46 Richard (Rick) Karlquist <
richard@karlquist.com> ha scritto:
> It is instructive to calculate the Shannon
> maximum theoretical data rate (power limited case)
> (refer to wikipedia page for Shannon-Hartley theorem).
>
> If S/N ratio (BW=2,500 Hz) = -24 dB,
> then S/N ratio (BW=1Hz) = -24 + 10 log 2,500 = -24 +34
> = +10 dB. 10 dB converted to a dimensionless ratio is 10.
>
> Now, channel capacity = C <= 1.44 X 10 = 14.4 bits/second.
>
> This rate is a little more than twice the FT-8 rate.
>
> Now a days, achieving 1/2 of the Shannon limit is
> possible for AWGN. If your noise isn't AWGN, well
> then that is another source of error.
>
> Thus the claim of -24 dB sensitivity seems plausible,
> where the S/N is the true signal vs AWGN, as opposed
> to whatever random number FT-8 reports.
>
> So I think the beef with FT-8 is in the way it
> calculates the displayed S/N. We used to call
> those "marketing specs".
>
> It is also notable that FT-8 uses at least twice
> the average power compared to CW. If you compared
> them on an average power basis (vs PEP) the FT-8
> advantage, if any, would drop 3 dB.
>
> On CW, you could send your call many times
> in 15 seconds for "error correction" and take advantage
> of QSB peaks. That tends to level the playing field.
> More playing field leveling is using Super Check Partial
> analogous to what FT-8 does.
>
> Rick N6RK
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> Reflector
>
--
marco cogoni
CRS4
http://sibamanna DOT duckdns DOT org
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