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Re: FM vs SSB



On Fri, 25 Jul 2003, John Stephensen wrote:

> ...This table compares the modulation types by the received signal level
> required for an articulation index of 30%.
>
> Unprocessed     Processed       RF
> PEP     Avg.    PEP    Avg.    PEP     BW     Mod.
> 39      53      35      42       3      SSB
> 46      46      42      42      12     FM
>
> The articulation index is the percentage of spoken words received correctly.
> The audio bandwidth in all cases is 300-3000 Hz.

Ah, this is really weak signal stuff if only 30% of the words are
understood at the receive end.  And of course FM is much worse at the weak
signal part of the knee..  THus this presentation ONLY applies to digging
out the weak ones over a very poor very noisey link...

For serious two-way non-contest communicatinos link from GEO, I dont want
to listen to weak-signal very noisy painful SSB.  There are many
applications in the Amateur Satellite Service that could use such a clean
signal as FM.

In comparing SSB to FM, remember, that above the FM threshold affect, the
dB improvement for FM is far greater than 1:1 where as for SSB, it always
remains 1:1.  THus, I'd like to see the numbers after only a 3 dB increase
in power for both.  My guess is that FM would improve 10 dB and SSB on ly
3.  FM in this case would have a much higer articulation index in this
case...  So, is there a table in that book for an articulation level of
97%?  If we want to pass meaningful traffic via satellite (not just chase
DX) then we need a better signal...

Thanks
Bob, WB4APR

> Note that narrowband FM requires the lowest peak power level. SSB requires
> the lowest average power level.

Yes, For the absolute minimum weak signal contact.  But for say 20 dB SNR,
doesnt FM win?  Not arguing, just trying to figure this all out..

Bob
de WB4APR@amsat.org, Bob

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