Hello John, >RFSM8000 has come as close as anything so far, but it's weakness was operating >well under poor conditions (read >normally poor these days) As far as I know, RFSM8000 uses a protocol close to 110A. This protocol is fine as it permits a great dynamic of data speed and moreover it has an excellent "Mean power/Peak power" ratio, very close to 1.
However, it has a problem as the symbol rate being equal to 2400 bauds, it is required an equalization (indispensable above 100 bauds). This equalization is efficient only for good (or not too bad) signal to noise ratio (with too much noise, the equalizer does not converge or lead to false solutions). The other solution is to be at 100 bauds or less and to multiply the number of carriers (as in MT63). However, in that case the "Mean power/Peak power" ratio is not good (as in MT63) and it can be a problem if for example, you are limited to 10 watts (to keep linear) with a 100 watts transceiver. To be able to have a very good S/N ratio (as good as the one supposed for P3), it is necessary to have a sequence of symbols (carrying the specific transmission parameters) at slow speed that can be recognized easily without error (i.e with a strong autocorrelation function) as for the RS ID. Once recognized, all is more simple as this sequence acts as a pivot: you can add your message at a given speed, repeat it if necessary (an "ARQ memory" is of course indispensable), increase or decrease the speed according to the ionospheric conditions... 73 Patrick ----- Original Message ----- From: John Bradley To: [email protected] Cc: [email protected] Sent: Tuesday, February 24, 2009 11:48 PM Subject: RE: [digitalradio] on another note "I don't think it would be technically very difficult to do something equivalent to P3 with sound cards. I think that it would even possible to do much better with, for example, multi-users protocol." RFSM8000 has come as close as anything so far, but it's weakness was operating well under poor conditions (read normally poor these days) "The problem is the time necessary to do this. An amateur can work (with pleasure and passion of course), let's say a day per week, when a company can make work a small team 5 days a week. Now an amateur project must be short in time (several months at a maximum) as it is done for fun. Working on the same project during years seems very difficult (and surely boring) for amateurs." No kidding.. it is amazing and very appreciated with what you and other software authors have done, and is especially important when it is done as a hobby. "Another point is that the number of Hams really interested in ARQ modes is very weak..." I can't answer that , outside of the fact there are a pile of winlink users. Think you might underestimate the number of folks interested in ARQ soundcard modes, and maybe this is an opportunity for Andy to do a survey on his website? "Note: have tests on the minimum S/N versus the data rate done, for pactot P2 and P3? (as it is difficult to trust commercial data) " don't know, but think so . but my ear tells me it works better than OLIVIA, which is about the top of the heap right now. I cannot hear the other station and yet the connect runs at fast speed. John VE5MU
