[aprssig] Interesting Preliminary Findings on Soundcard "Soft TNC" Shootout

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Mon Dec 26 11:42:42 EST 2011

On 12/26/2011 11:32 AM, Stephen H. Smith wrote:
> I.e. the "cliff" between "always" and "mostly failing" is surprisingly 
> abrupt, not at all like the perceived "gradually getting noisier" one 
> experiences on voice as the signal weakens.

Not surprising to me if you think about it.   Drop one bit in an AX.25 
packet, and the entire packet is not decoded.  There's no such thing as 
"almost" with AX.25 decoding.  And that's not a fault of AX.25 
exclusively.  Any digital mode that uses checksums or CRCs or other 
error detection mechanisms without any form of error correction will 
suffer the same fate.

The human ear, when "decoding" voice or even CW, has lots of error 
correction capability.  We have a list of expected words to match 
against the detected phonemes and even context sensitivity to replace 
entire words in many QSOs.  Hence the estimates bandied about that 
digital modes require approximately twice the signal strength of other 

Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32

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