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Re: FM vs SSB



On Thu, 31 Jul 2003, John Stephensen wrote:

> When the audio bandwidth is reduced to 200-3000 Hz the maximum
> Articulation index is 75%.

Now I am beginning to wonder how we communicate on narrowband FM at all if
we only understand 75% of what we hear on a dead full quieting signal...
I guess that we do it in "context".  and the AI numbers are based on
random syllables?  But this table is perfect:

> Average RF Power & Processed Speech
>
>         DSB     12kHz
> AI      SSB     FM     DIfference in dB
> 20%     31      40     9
> 30%     35      42     7
> 40%     39      44     5
> 50%     43      45     2
> 60%     47      46     1
> 70%     52      51     1

Now that is what I was looking for.  For the same power, it shows that if
we want good Comms, then use FM.  If we want weak signal multi-user DX,
use SSB.  But then I guess we all knew that all along...

So it all boils down to "what we want"... sigh...

Bob
> ----- Original Message -----
> From: "Bob Bruninga" <bruninga@usna.edu>
>
> > 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
>
>
>
> ----
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de WB4APR@amsat.org, Bob

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