[aprssig] 1200 vs 9600 baud

Robert Bruninga bruninga at usna.edu
Fri Dec 20 08:33:38 CST 2013


> 1200 vs 9600 baud for APRS.  I think I understand this:
> 1.  9600 lets you send more data per time interval.
> 2.  9600 requires tighter radio integration (can't just plug into the
microphone jack).
> 3.  9600 seems to work best (only?) on UHF.

As implemented, with typical TX delays, 9600 is only about twice as fast
as 1200

Fortunately 1200/9600 are fully factory integrated in all commercial APRS
radios so it is trivial to compare.

9600 baud is 7 dB or so worse than 1200 as measured with fully integrated
factory adjusted radios.  UHF is worse for 9600 baud due to 3 times more
rapid mobile flutter.  UHF has almost ten times more path loss than VHF
but usually pick up about 3 dB in easier to achieve antenna gain.

> So how would you answer the question, "When should I use 9600 baud for
APRS and when should I use 1200 baud?"

Use 9600 when you have dedicated built hardware with extremely short TXD
and no transceiver T/R switching delays between fixed stations (no mobile
flutter).  .  All modern antenna switghing is electronic and instantaneous
so the biggest delay is the transceivers PLL synthesizer T/R switching.
This is why off the shelf Amateur Transceivers simply are too slow for any
benefit for bursty 9600.

Amateur 9600 wins when doing bulk file transfer with long packets and
continuous data, not short bursty data where the delays are longer than
the data burst.

That said, the best way to improve 1200 baud VHF performance is to shorten
the packet and get rid of all the redundant text.  Classic wasteful
packets are digipeaters with Beacons like "Green Mountain digipeater -
Green Mountain Radio Club, GMRC"

Bob, WB4aPR


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