On Sat, Apr 17, 2010 at 1:43 PM, Stelios Bounanos <[email protected]> wrote:
> > 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. > > True, and I seriously doubt that anyone has bothered to do that. > Well, Chen has taken a first pass using HF channel simulator software which tells us something. The CCIR evidently did a fair amount of work to quantify the characteristics of an HF channel. Chen used that. (I am finding that I am coming at this from behind several people who have done significant work but it doesn't seem to me that anyone has taken it beyond building a couple of modems.) > > > 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. > > This is somewhat more complex than it sounds... Oh, I figured that. > what would work right > now would be to generate two audio files (with signals at suitable > frequencies of course) and mix them using another program. I'm sure sox > can do that in one short command line. > <dope slap to the forehead> Duh! Of course! And that would be very simple. Of course, that is what Chen did for his testing too. > > In fact if fldigi had xmlrpc methods to start/stop audio file > generation, it would be easy to write a script to do an audio file for > each mode (for a particular input text), and then you could mix them as > needed for your tests. I guess I'll need to write those xmlrpc methods > first :-) > > [snip] > > > Time to take the next step and build on Chen's work by getting real, > > controlled data from live propagation. > > Some comparisons have been posted to the digitalradio yahoo group. > There is one by K2MO, and another by WZ7I. URL for the latter: > > http://mysite.verizon.net/wz7i/digitalmodes.html > > The tests done by K2MO are in the files area of digitalradio, and he has > posted them to the group a few times too. > > I can't say how scientific those experiments were, but I know that the > path simulator used (pathsim) is GPL'ed so at least that aspect of the > tests can be verified by anyone so inclined. > The problem as I see it is that we have these similar-but-different fixed modes; e.g. olivia, contestia, MFSKx, DominoEX, and then the various flavors of PSK; that make multiple changes but don't allow us to evaluate the protocols while only changing one parameter at a time. That is certainly useful for hams wondering which mode to try now but isn't helpful in selecting which mode is likely to perform best. Where *is* the crossover between IFK, MFSK, and PSK? Hmm, wouldn't it be nice if the transmission header contained a descriptor of the MODEM/CODEC so that the receiver could put together the correct pieces. > ES1HJ also has published some test results but I don't recall if they > were on-air or simulation tests. > OK, there is a lot more research than I was aware of. I am trying to soak all this up (and feeling like I am drinking from a fire hose) with the idea of eventually building something to automate the process, i.e. evaluate the channel and then configure the best operating mode. Thank you for all the pointers Stelios. -- 73 de Brian, WB6RQN/J79BPL
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