(I forgot to reply to all.) On Sat, Apr 17, 2010 at 9:49 AM, Brian Lloyd <[email protected]> wrote:
> On Sat, Apr 17, 2010 at 9:01 AM, hteller <[email protected]> wrote: > >> Brian, >> >> We again tested DominoEx against Contestia on UHF today with the customary >> Doppler disturbance and DominoEx 8 did poorly, but Contestia had perfect >> copy. >> > > Now, did you do a fair and accurate comparison? Did you turn on FEC? > DominoEX without FEC is, frankly, not useful in a path dominated by flutter > and doppler. Turn on FEC on DominoEX or, better still, compare Contestia to > Thor, which is DominoEX with FEC done properly. I think you will find the > difference less apparent. > > If you are interested in *why* you are seeing the results you are, see > this: > > http://homepage.mac.com/chen/Technical/DominoEX/Measurements/ > > And specifically this graph: > > http://homepage.mac.com/chen/Technical/DominoEX/ITU/ITU/flutter.html > > Notice the very high error rate of DominoEX in the presence of flutter with > FEC turned off and the HUGE improvement with it turned on. This is a real > analysis of the characteristics of the protocols. This is the kind of thing > that needs to be done in order to find what works best. > > It is really Really *REALLY* important to understand what is going on with > these protocols in the presence of propagation anomalies in order to be able > to make decisions about which modes work best under what conditions. My own > recent experiments in monitoring and graphing the phase/frequency changes of > the WWV carrier over time (20-30 minute samples typically) have convinced me > that sequential testing of protocols, i.e. transmitting a message and then > immediately transmitting the same message using the other protocol, is not > likely to produce valid results unless repeated numerous times and then the > results averaged with outliers discarded. I have been pestering Dave to do a > test version of fldigi that would let me transmit the same data in two modes > at the same time in the same passband so that both experience *exactly* the > same propagation conditions concurrently. *THAT* will let us see which one > works better, especially if it can report the channel conditions for fading > (slow amplitude change), flutter (rapid amplitude change), multipath (rapid > phase change and selective fading), and doppler (slow steady phase/frequency > change). > > Digital data transmission over HF and NLOS VHF/UHF is an area ripe for > analysis and experimentation. Right now in this area hams have the ability > to be out in front again as they were through the 1950's. The only person I > know of who is doing meaningful work in this area is Kok Chen, W7AY. He > assures me that there are a few others but, frankly, I haven't met them. And > I say meaningful in that he has come up with a means of testing in a > controlled, simulated environment as a first pass in determining what we > think we should see if we truly understand what is going on. The next step > is to repeat that testing in the real world over live paths but in a > strictly controlled manner so that we can extract real performance data. > > So building a bunch of digital modes is nice and very useful but now we > need to go the next step and use the tools in a scientific way to extract > better understanding. (Don't get me wrong, I wouldn't be here pondering > this if it weren't for Dave's and Stelios' work in producing fldigi with all > these modes with which to experiment.) Time to take the next step and build > on Chen's work by getting real, controlled data from live propagation. > > -- > 73 de Brian, WB6RQN/J79BPL > > -- 73 de Brian, WB6RQN/J79BPL
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