[aprssig] Kenwood D700 Internal TNC Amazingly Mediocre!

Tom Hayward esarfl at gmail.com
Sun Aug 7 11:22:13 CDT 2016


I did a similar test a year or two ago. I tuned one of my D710
receivers to 144.39 and set up two igates. The first was connected to
the serial output of the D710 TNC. The second used the "9600 data"
(flat rx audio) pin on the DATA connector into DireWolf on a Raspberry
Pi (slow CPU). I tracked how many unique packets each gated to
APRS-IS. This makes it a little different from your test, because I
was testing TNC latency as well as decode ability--only the fastest
decode counted.

What I found was that it was basically a wash. Each TNC gated a
similar number of unique packets. I was surprised to see how many
packets could be decoded by one TNC and not the other (by watching the
log--the APRS-IS test did not include this). It went both ways. My
takeaway from this is that it could be valuable to run different TNCs
in parallel for maximum decode. I believe this concept is reflected in
the algorithms of the modern soft TNCs.

And with the pace at which the soft TNCs are improving, my test
results are probably invalid now :-)

I developed this tool to help with the test:
https://dl.dropboxusercontent.com/u/8174/igate-stats.html
By default, it sorts by Unique Rate to show which igates are most
effective (or are sending delayed data to APRS-IS!).

Tom KD7LXL

On Fri, Aug 5, 2016 at 8:53 PM, Stephen H. Smith via aprssig
<aprssig at tapr.org> wrote:
> I had always assumed that the built-in TNC of the D700 was fairly optimized,
> due to being mated directly to the radio's discriminator, avoiding audio
> channel distortion, audio level issues, squelch rise times, de-emphasis
> issues, etc.
>
> I just got back from a road trip to the Colorado Rockies for the annual
> Evergreen Jazz Festival.  While there, I did some coverage testing in the
> mountains around Evergreen of a potential portable digipeater and tracker
> for the festival shuttle bus, that may be used next year.
>
> This is very rugged mountain terrain with few line-of-sight paths and lots
> of multipath.  I used the D700's internal TNC with one copy of UIview on my
> Panasonic CF-51 mobile Toughbook.
>
> I also connected the 6-pin mini-DIN "data port" of the D700 through one of
> my homebrew sound card interfaces to a Behringer UCA-202 external USB
> "soundcard". In turn, the UCA-202 was used by the UZ7HO Soundmodem software
> TNC.  The Soundmodem was linked to a second copy of UIview running on the
> same computer. (The Soundmodem faithfully emulates the AGW Packet Engine
> interface supported by most current APRS clients.)
>
> The tests consisted of placing my FT-1500/TinyTrack 3 ammo-box tracker at
> the hillside location I picked for the digipeater, based on radials I drew
> on Delorme TopoUSA. The tracker was set to beacon every 20 seconds on 144.37
> MHz. I then drove the 4-mile loop route taken by the shuttle bus during the
> event.
>
> [The original intent of using the soft TNC on the mini-DIN port at all was
> to try using 300-baud HF-style packet on VHF/FM to gain 3-4 dB of noise
> margin compared to 1200 baud.]
>
> I was completely shocked at the difference between the D700's hardware TNC
> and the software TNC, both at 1200 baud.  When the signals were clean and
> fully-quieted, there was essentially no difference between the two TNCs.
> When the signals fell out of full quieting, or were "fuzzed up" with
> multipath distortion, the Soundmodem massively outperformed the D700
> internal TNC.
>
> Driving the 4-mile winding loop around Evergreen, only about a third of the
> loop showed on the D700 hardware TNC, while 100% of the beacon transmissions
> showed on the instance of UIview linked to the Soundmodem soft TNC.
>
> When I saw how well the Soundmodem worked at 1200 baud, I abandoned the idea
> of going to 300 baud for this project.  (Staying at 1200 means I can
> intermix off-the-shelf 1200-baud TNCs, radios and soft TNCs, while 300 baud
> would require that every node involve a PC of some sort running the 300-baud
> software Soundmodem.)
>
> ____________________________________________________________________
>
>  Stephen H. Smith    wa8lmf (at) aol.com
> Skype:        WA8LMF
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