[aprssig] 1200 vs 9600 baud

PE1RDW aprs at pe1rdw.demon.nl
Fri Dec 20 10:53:37 CST 2013


On 20-12-13 15:33, Robert Bruninga wrote:
>> 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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>
that offcourse does not mean 9600 is useless for aprs, it can be 
perfectly used for backbone trafic where 1k2 trafic is collected from 
more then one source, probably a number of alt inputs for short period 
trackers and maybe even a stream from aprs-is covering several hunderd 
kilometer and unlike 1k2 dwait does not have to be 0 because there is 
more then enough bandwith for these types of digipeaters to wait for 
eachother and still have relevant data in fact putting dwait to a 
resonable level will reduce the poor txd problem.
Personaly I do not see the poor preformance of 9k6 for mobiles as a 
problem because aprs is not just for mobile use, it is a 2 way medium of 
local assets, not just trackers.

73 Andre PE1RDW


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