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[aprssig] APRS digi performance and TONES!

Robert Bruninga bruninga at usna.edu
Thu Sep 6 16:14:48 UTC 2007

> Something else I've noticed, is that your mobile 
> will work just fine at home, but when you get 
> 1000+ miles away, something about the digis is 
> different, and your mobile might not "see" any APRS 
> activity.  

STUMP ON:  This is because sometimes packet is treated as
plug-n-play.  Just plug in a TNC and "it works", ignoring the
fact that there can be 60 dB differnce between "it works" for a
50W rig 2 miles away, versus, a 1 W signal at 30 miles away...

The tones have to be set for:
1) Proper modulation
2) Proper "skew" High tone versus low tone
3) Emphasis. De-emphasis, or none

There are LOTS of bad signals on the air (in fact the majority
are not properly set and sound bad!).  They "work" sometimes
with some rigs, but because they are so far from being optimum,
they don't work with some others.

The internal TNC in the kenwoods are connected directly to the
discriminator and give a flat rsesponse.  They can decode other
flat packet signals with as little as 3 tiny LCD marks showing.
The kenwoods will also work OK with a digi that has proper
emphasis and de-emphasis, but they and most other decoders will
fail even on a full scale signal if the tones are too skewed.
Most TNC's will choke if the low tone is more than a few dB
higher than the high tone. 

Listen to the signals on 144.39 and the wide variety in tone
levels, skew, and deviation tell you why performance varries so

PACKET is not plug-and-play.  Like anything else, it requires
proper settings for optimum performance.

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