[aprssig] Interesting Findings - 300 Baud AX.25 on VHF-FM APRS

Ev Tupis w2ev at yahoo.com
Sat Apr 2 07:30:18 EDT 2016

In regard to the findings below...you said that you were surprised that the difference played out on FM as it does on SSB but didn't go on to say why you thought that it would not.
It's going to be tough for anyone to offer comments until they know what you are unclear about, exactly. ;-)
As an aside, during the early years of the PropNET Project (www.PropNET.org) we used 1200 baud AX.25 on 10 meter SSB specifically because it was easier to tune (and keep tuned, using the 10 meter transceivers of the era) and found that the Minimum Discerable Signal threshold difference was outweighed by both that and QSB; which, if one takes the time to instrument plot will find will vary by 6+ dB within moments on an HF path.

      From: Stephen H. Smith via aprssig <aprssig at tapr.org>
In the 1200-baud mode, -109 dBm (.795 uVolts) was required for "perfect copy". 
At this point, the un-modulated generator carrier was yielding soft 
"fine-grained" hiss in the FT-1500's receiver, but no "pop corn" noise. I.e. 
something resembling 20 dB quieting in a classic SINAD test.

In the 300-baud mode, only -113 dBM (.501 uVolts) was required for "perfect 
copy".  At this RF level, the unmodulated generator "dead carrier" was 
yielding rather severe "pop corn" noise similar to about 10 dB quieting on a 
classic SINAD test.

In other words, just switching from 1200 baud to 300 baud bought a 4 dB 
improvement in receive performance. I would expect something like this on 
HF/SSB, especially with a DSP variable-bandwidth IF system that can be screwed 
down around the width of the transmitted signal.

I was surprised that it worked in such a similar manner on FM.  The 
Soundmodem's internal audio DSP incorporates does incorporate 
variable-bandwidth AUDIO filters keyed to the selected baud rate.

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