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Re: 1200/9600 Baud REAL test



> on 7/19/00 10:13 PM, Jon Ogden wrote about Bob's 1200 AFSK vs 9600 baud
> FSK test comparison:
 
> There is less difference between AFSK and FSK than one would think.
> First off, BOTH signals are analog signals.

I dont think I would agree with that.  AFSK is  pair of analog modem tones
on an analog linear FM voice system.  FSK is a two level bi-polar
(digital) direct FM modulation.

Thus you must think of the FM limiter/threshold effect of gaining
additional S/N ratio in the analog regime before the tones are
demodulated in AFSK.  I attribute this factor as what is giving us the 2
dB better results than  one would expect from the 9 dB bandwidth ratio.

ANother way of looking at it is that we have a 16 KHz FM bandwidth radio.
Yes, 9600 baud effectively fills that noise bandwith for optimum use.  If 
you were to use the same radio (same bandwidth) at 1200 baud FSK you
would not gain ANY of the theoretical 9 dB because you didnt change the
noise bandwidth.  But by using AFSK, you are effectively EXPANDING the
bandwidth of the 1200 baud signal to more effectively fill the available
bandwidth of the given voice radio and therefore on receipt, the
AFSK demodulation process is recovering the "spread" energy effeciently
and we are getting a 7 dB or so improvement.

> I think your tests show very clearly that the link budget with a 9600 baud
> data link is far less than with a 1200 baud link.  The fact that with a
> crude measurement setup you got very close to the theoretical value tells me
> you did a good job.

Thanks.  Its been something I have wanted to do for years!  Now with an
off the shelf data radio with both modes built into the same RF system, I
was glad to be finally able to do it.  Again, it is still an apples and
oranges comparison, but shows what we are dealing with.  Next week we will
do it again with two D700''s...

de WB4APR, Bob

P.S.  It also shows HOW TERRIBLE MOST ON AIR 1200 BAUD SIGNALS ARE.  Many
digipeaters and most all users stations I see take DEAD FULL QUIETING
signals before they can be doecoded, simply because they were never
adjsuted properly...  This is NOT the fault of AFSK.  It is the fault of
applicance operators not paying attention to modulation levels and tone
skew.

With a properly adjusted 1200 baud AFSK signal I am AMAZED at how noisy
the packet can be and still be decoded 100% every time.  Again, 95% of
what you hear on the air is a MESS!

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