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[aprssig] "Best" packet decoder solution

Stephen H. Smith wa8lmf2 at aol.com
Mon Dec 27 23:10:53 UTC 2010


On 12/27/2010 12:02 PM, Lynn W. Deffenbaugh (Mr) wrote:
> Experimenting with the CD would hopefully get you reproducible results, but 
> IMHO the actual performance of a set of TNCs in your "airspace" as it were, 
> is really what you need to know if you're trying to choose a TNC.  One thing 
> I have learned in my experiences with APRS over RF is that YMMV (Your Mileage 
> May Vary) is an absolute truism.
>

This is what I created the test CD to address!

One of the tracks is a high-quality live off-the-air recording of the Los 
Angeles-area afternoon rush-hour mayhem on 144.39 MHz (when the _LOCAL_ APRS 
traffic approachs 500% of the Aloha limit!) taken directly from the mini-DIN 
discriminator connection of a Yaesu FT-1500.    This includes over- and under- 
deviated signals,  pre-emphasized signals from TNC-into-mic-jack setups, and 
flat non-preemphasized signals from Kenwood D7xx radios.  (The internal TNC on 
Kenwoods goes directly into the TX modulator with NO pre-emphasis) .  The 
recording includes numerous packet collisions and idiots running CW ID on their 
TNCs(!).     Forty minutes of realtime transmissions have been compressed into 
about 20 minutes by editing out all dead air over a half second in the Cool 
Edit audio editing program. This non-stop flow of traffic really challenges the 
buffer overflow problems of TNCs prone to this issue.

This CD is a normal audio CD with lossless standard PCM CD audio tracks (not a 
CD-ROM with lossy phase-damaged MP3 recordings).  Playing this back on a good 
audio CD player (or from the earphone jack of a CD-ROM drive) avoids all the 
issues of inaccurate and phase-jittery sound card clock rates and cheap noisy 
sound card analog audio circuitry.

Played back from a CD player directly into a TNC under test simulates a flat 
raw discriminator audio connection to a radio (i.e. connected via the mini-DIN 
data jack).   Add a simple RC network between the CD player and the unit under 
test, to simulate a de-emphasized  audio-from-speaker-jack  connection.

Another track is a recording of a single Mic-E burst from a Kenwood D700 
captured from the low-distortion wideband discriminator of my IFR-1500 service 
monitor.  I then copied/pasted this burst 99 times in Cool Edit to create a 
track of 100 absolutely identical bursts.  The idea is to use this track to 
modulate an RF signal generator.  One would play this track back while 
gradually reducing the RF level fed into a radio/TNC combination under test to 
determine the decode performance as the signal falls out of full quieting.  
(The IF bandwidth and linearity of the FM discriminator varies widely between 
radios and has major effects on weak signal TNC performance.)

There are several other tracks, with and without precise pre-emphasis created 
in Cool Edit, intended to evaluate the tolerance of TNCs to various levels of 
pre-emphasis.

More details on the test CD, and a download link for the image file you can use 
to burn a copy of this CD are here on my website:

<http://wa8lmf.net/TNCtest>


-------------------------------------------------------------------------------

--

Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

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