[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

*Subject*: Decoding morse*From*: Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>*Date*: Thu, 29 Nov 2007 00:23:50 +0100

Hi All, The problem of decoding CW in the computer for weak and unstable signals (as those we have in EME) is an interesting challenge that I have been looking at now and then over the years. Now, on vacation at a nice place in the Carribean I have plenty of time at the pool side so I am looking at it again:-) I have a reliable algorithm for establishing the keying rate of signals well below what I can reliably decode by ear. It is based on finding dashes, they last long and therefore have more energy in a narrower bandwidth compared to dots. The problem I face now is to fill in what is the most probable pattern of what I have between two reasonably close spaced dashes. To start with I have two cases: 1) length = 3 ___ _ ___ case 1 ___ ___ case 2 123 These two waveforms differ only in position 2. 2) length = 5 ___ _ ___ case 1 ___ _ ___ case 2 ___ _ ___ case 3 ___ ___ ___ case 4 ___ ___ case 5 12345 These five waveforms differ in positions 2, 3 and 4. I have the baseband as a complex pair I and Q and I compute the sum (integral) over the correct time intervals of a morse code unit (a dot.) I also compute the RMS power over each time unit. A typical result for a good EME signal (G4LOH in the FRH1135 recording) is like this for the first case: The complex amplitude of the dashes surrounding the region is (5.146,-5.941) and (7.164,1.704) before respectively after. It means that the phase has drifted by about 45 degrees over the time interval and I can assume that the phase of a dot would be the average of the of the surroundings which means that I would expect an amplitude of something like (6.155,-2.118) When computing the amplitude as the average of I and Q over the three time intervals I get: reg 0 (1.043,-0.339) pwr 2.90 1.20 reg 1 (7.109,-1.924) pwr 59.94 54.24 reg 2 (0.551, 0.503) pwr 4.05 0.55 This is obviously a dot. The complex amplitude for the center position fits closely to what one would expect. The numbers after pwr is the RMS power followed by the sum of squares of the I and Q amplitudes. Under the assumption it is really a dot, I can compute the noise power level as (1.2+0.55+0.947)/3=0.899 while the signal power would be 54.24 for a S/N of 60.33 Under the assumption there is no dot, the noise power would be 18.66 on the average and 62 times higher in the center region compared to the average of the two surrounding regions that have to be key-up. I am writing this to MOON-NET and the Linrad list because I hope someone could help in translating what I can compute into the probability of having a dot in situations where it is not so obvious. I assume it is reasonably easy for someone who has the appropriate knowledge of statistics (that I do not have.) I would also be interested to know how one computes a probability for a dot based on the RMS powers for the three regions: 1.2, 54.24 and 0.55. I guess the ratio 54.24/( 0.5*(1.2+0.55)) translates directly to a probability (much greater than one) for a dot and another (close to zero) for a space. ----------------------------------------------------------------- In the second case, with five possible waveforms a typical case from the same recording is: Surrounding dashes: (8.202,-0.480),(7.078,-6.384) Evaluation for a dot on each of the 5 positions: reg 0 ( 0.169,-0.420) pwr 2.406 0.205 reg 1 ( 7.134, 1.780) pwr 60.375 54.075 reg 2 ( 0.429,-0.005) pwr 5.370 0.184 reg 3 ( 0.025,-0.094) pwr 2.603 0.009 reg 4 (-0.664, 0.193) pwr 1.888 0.478 Evaluation for a dash: reg 123 (2.667,-0.473) pwr 23.443 7.339 This is obviously case 3, but how do I convert the result from amplitudes/powers to five probabilities (that sum up to 1.00) ??? Anyone knows? I expect two sets of probabilities. One based on the integrated power over 1 respectively 3 morse code time units and another based on RMS powers. (Sometimes an EME signal changes its phase rapidly during a QSB minimum and then RMS powers could be more adequate than averaged I and Q signals.) I have a feeling it should be an easy problem - but not so to me:-( A realistic case, from the famous unkn422.wav file by AF9Y is like this: Surrounding dashes (-3.078,0.987),(-2.545,-1.678) reg 0 ( 0.476,-0.060) pwr 6.127 0.230 reg 1 (-2.457,-0.325) pwr 17.513 6.146 reg 2 (-2.192,-0.368) pwr 15.392 4.941 I know this is a dot because it is a part of the Y in AF9Y, but I do not know what criterion to apply for the software to take a decicion that a dot is much more probable than a space or vice versa. Leaving the choice to a later state when some more parts are decoded is not very attractive since too many uncertainties will make it difficult to take decisions for longer regions. Any help would be much appreciated. 73 Leif / SM5BSZ --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Linrad" group. To post to this group, send email to linrad@xxxxxxxxxxxxxxxx To unsubscribe from this group, send email to linrad-unsubscribe@xxxxxxxxxxxxxxxx For more options, visit this group at http://groups.google.com/group/linrad -~----------~----~----~----~------~----~------~--~---LINRADDARNIL