Re: [digitalradio] Re: NBEMS QST article/digital weak signal FM
Thank you for that explanation. I didn't know the modulation mode would make a difference. It would have been interesting to test the theory with Skip. Unfortunately, we live too far apart for VHF/FM. Thanks again... Tony - K2MO Tony, You do not need to test only with me! You can test with anyone else the proper distance away who has both 2 meter FM and SSB capability and an interface. In fact, such a test will be more informative with one other than just myself. Andy's sked page is one way to arrange for tests, and an email to this reflector might also uncover someone who would like to work with you and is the right distance away. In fact, you can sometimes just rotate a beam to reduce a signal to become however weak you need it to be. You could also use contacts on HF to arrange for a sked with someone at the right distance and with the necessary equipment. The IC-706MKII, FT-857, and FT-897 are all popular rigs with multimode capability, as are the IC-746Pro and TS-2000. This kind of thing is what ham radio is all about - go for it! :-) 73, Skip KH6TY
[digitalradio] Re: NBEMS QST article/digital weak signal FM
As more people try using digital modes on 2 meter FM, the overall best performing mode will automatically surface, but for the longest range on digital modes (not counting CW), it is really necessary to use SSB, and in that case, we have found that MFSK16 is just too critical for tuning to be used with transceivers without a TCXO. Skip, how about to try MFSK16 with RSID? The RSID solves the intial tuning on key down. Once the signal is tuned, AFC shall track it. 73, Vojtech
Re: [digitalradio] Re: NBEMS QST article/digital weak signal FM
Vojtech, Another good suggestion! :-) I see the wheels have been turning in Vojtech's mind! RSID is already in fldigi, so will try that. I hope others reading this will also try that, and all the modes, and let us know their experiences. Testing is slowed down by the necessity to find someone else with the same setup, but that should become easier to do as time goes on. 73, Skip KH6TY - Original Message - From: Vojtech Bubnik bubn...@seznam.cz To: digitalradio@yahoogroups.com Sent: Monday, February 23, 2009 1:02 PM Subject: [digitalradio] Re: NBEMS QST article/digital weak signal FM As more people try using digital modes on 2 meter FM, the overall best performing mode will automatically surface, but for the longest range on digital modes (not counting CW), it is really necessary to use SSB, and in that case, we have found that MFSK16 is just too critical for tuning to be used with transceivers without a TCXO. Skip, how about to try MFSK16 with RSID? The RSID solves the intial tuning on key down. Once the signal is tuned, AFC shall track it. 73, Vojtech No virus found in this incoming message. Checked by AVG - http://www.avg.com Version: 8.0.138 / Virus Database: 270.11.2/1965 - Release Date: 2/21/2009 3:36 PM
[digitalradio] Re: NBEMS QST article/digital weak signal FM
Hi Tony, The original reason we went to DominoEx instead of MFSK16 or PSK63 was because at VHF (we were originally using SSB), transceiver drift, in addition to multipath, is a major problem and MFSK16 is much more critical for mistuning or drift (on SSB) than DominoEx. But, when using FM, frequency drift should not be as great a problem, and it may well turn out that MFSK16 will work better overall, but we have not yet made enough comparisons to find out. FM is not usually used way under limiting, so this is rather new ground for us. On SSB, between two stations that both have TCXO's, MFSK16 works really well, but unfortunately, it is not possible to control whether or not a TCXO is being used. Multipath is generally not a problem until the reflected signal is seen crossing the main signal on the waterfall, and then a slow beat note can be heard, regardless of signal strengths. At that time, the main signal is often completely cancelled by the out of phase reflected signal, and there is simply no resulting signal to decode until the reflected signal is seen to move off to the side of the main signal. Once it has, the mode with the lowest minimum S/N will work the best. Over the long path, propagation often appears to be very steady, with no atmospheric distortion, and the limiting factor is then the minimum S/N of the mode. However, at other times, there is a persistent, fast flutter, and at those times, MFSK16 might prove to be the best mode to use. We have yet to find out. As more people try using digital modes on 2 meter FM, the overall best performing mode will automatically surface, but for the longest range on digital modes (not counting CW), it is really necessary to use SSB, and in that case, we have found that MFSK16 is just too critical for tuning to be used with transceivers without a TCXO. 73, Skip KH6TY 4c. Re: NBEMS QST article/digital weak signal FM Posted by: Tony d...@optonline.net kt2q Date: Sat Feb 21, 2009 4:44 pm ((PST)) Skip, White noise tests show DominioEX-4 to be a bit more sensitive than MFSK16, but it doesn't seem to handle HF distortion nearly as well. I was surprised that it did better than MFSK16 with multipath and was wondering if you thought the better throughput was due to MFSK16 tuning issues rather than actual robustness? Tony - KHMU Announce your digital presence via our Interactive Sked Page at http://www.obriensweb.com/sked Recommended software: Winwarbler, FLDIGI, DM780, or Multipsk Yahoo! Groups Links * To visit your group on the web, go to: http://groups.yahoo.com/group/digitalradio/ * Your email settings: Individual Email | Traditional * To change settings online go to: http://groups.yahoo.com/group/digitalradio/join (Yahoo! ID required) * To change settings via email: mailto:digitalradio-dig...@yahoogroups.com mailto:digitalradio-fullfeatu...@yahoogroups.com * To unsubscribe from this group, send an email to: digitalradio-unsubscr...@yahoogroups.com * Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
[digitalradio] Re: NBEMS QST article/digital weak signal FM
Hi Tony. I suppose the reason is that we are comparing MFSK16/DominoEX over FM versus MFSK16/DominoEX over SSB. I believe they are just different animals. SSB only shifts signals in frequency. FM does much more complex (in mathematical sense) transformation. Skip is doing interesting pioneering work. Digital modulation over common voice FM transceiver will have different noise and distortion properties than SSB. It will be influenced by noise properties of FM, preemphasis/deemphasis, how FM is modulated (pulling VFO inside the phase loop?) etc. Pulling varactor inside the phase loop will distort low frequencies (phase loop acts against the modulation), therefore baseband modulation is difficult. It brings back the memories of my teenage packet radio obsession. At that time the modulation modes were limited mainly by circuit complexity and there was no DSP. Common handhelds were modulated with BELL202 1200Bd two tone synchronous modulation. 9k6 enabled transceivers allowed direct modulation of the VFO varactor by bypassing all the microphone circuit, preemphasis and clipping. I suppose Skip's target is the first group of transceivers, where the modulation/demodulation amplitude and phase response is unknown. Skip, it would be interesting, if you could investigate, which modulation bandwidths and at which center audio frequency the common FM transceivers work best with common HF weak signal digital modes. Keep the good work. Someone able to do the math? 73, Vojtech OK1IAK White noise tests show DominioEX-4 to be a bit more sensitive than MFSK16, but it doesn't seem to handle HF distortion nearly as well. I was surprised that it did better than MFSK16 with multipath and was wondering if you thought the better throughput was due to MFSK16 tuning issues rather than actual robustness? Tony - KHMU
[digitalradio] Re: NBEMS QST article/digital weak signal FM
Hi Vojtech, Thanks for the tip. I totally forgot about the possible effect of deemphasis and what effect the center audio frequency might have. Our goal with NBEMS has always been able to reach at least 100 miles reliably, in order to span the largest expected disaster area to reach Internet or phone connectivity. We were recently surprised to find over a 117 mile path on 2 meters, that, on the average, FM with DominoEx actually worked better than SSB with DominoEx, even with the poorer S/N of FM compared to SSB. The surprise was an unexpected, consistent, fast flutter which did not seem to affect FM nearly as badly as SSB. Thanks to Tony's wondering, we will continue to evaluate different modes (and different audio center frequencies!) and post the results here. 73, Skip KH6TY Skip, it would be interesting, if you could investigate, which modulation bandwidths and at which center audio frequency the common FM transceivers work best with common HF weak signal digital modes. Keep the good work. Someone able to do the math? 73, Vojtech OK1IAK White noise tests show DominioEX-4 to be a bit more sensitive than MFSK16, but it doesn't seem to handle HF distortion nearly as well. I was surprised that it did better than MFSK16 with multipath and was wondering if you thought the better throughput was due to MFSK16 tuning issues rather than actual robustness? Tony - KHMU
Re: [digitalradio] Re: NBEMS QST article/digital weak signal FM
Vojtech, Thank you for that explanation. I didn't know the modulation mode would make a difference. It would have been interesting to test the theory with Skip. Unfortunately, we live too far apart for VHF/FM. Thanks again... Tony - K2MO - Original Message - From: Vojtech Bubnik bubn...@seznam.cz To: digitalradio@yahoogroups.com Sent: Sunday, February 22, 2009 7:53 AM Subject: [digitalradio] Re: NBEMS QST article/digital weak signal FM Hi Tony. I suppose the reason is that we are comparing MFSK16/DominoEX over FM versus MFSK16/DominoEX over SSB. I believe they are just different animals. SSB only shifts signals in frequency. FM does much more complex (in mathematical sense) transformation. Skip is doing interesting pioneering work. Digital modulation over common voice FM transceiver will have different noise and distortion properties than SSB. It will be influenced by noise properties of FM, preemphasis/deemphasis, how FM is modulated (pulling VFO inside the phase loop?) etc. Pulling varactor inside the phase loop will distort low frequencies (phase loop acts against the modulation), therefore baseband modulation is difficult. It brings back the memories of my teenage packet radio obsession. At that time the modulation modes were limited mainly by circuit complexity and there was no DSP. Common handhelds were modulated with BELL202 1200Bd two tone synchronous modulation. 9k6 enabled transceivers allowed direct modulation of the VFO varactor by bypassing all the microphone circuit, preemphasis and clipping. I suppose Skip's target is the first group of transceivers, where the modulation/demodulation amplitude and phase response is unknown. Skip, it would be interesting, if you could investigate, which modulation bandwidths and at which center audio frequency the common FM transceivers work best with common HF weak signal digital modes. Keep the good work. Someone able to do the math? 73, Vojtech OK1IAK White noise tests show DominioEX-4 to be a bit more sensitive than MFSK16, but it doesn't seem to handle HF distortion nearly as well. I was surprised that it did better than MFSK16 with multipath and was wondering if you thought the better throughput was due to MFSK16 tuning issues rather than actual robustness? Tony - KHMU
Re: [digitalradio] Re: NBEMS QST article/digital weak signal FM
Skip, The surprise was an unexpected, consistent, fast flutter which did not seem to affect FM nearly as badly as SSB. I recently had a 100+ mile QSO on 2 meter CW. The contact involved a lot of aircraft scatter with frequency shifts in excess of 50Hz. There were other signal components mixed in as well. Sine your digital mode tests involved FM and SSB, I would imagine that the lack of Doppler on FM would add stability to the signal and could be part of the reason for the improvement. We see this often with FM satellites where the Doppler shift is not detected in the audio as it is on on SSB/CW satellites. Just a thought... Tony - K2MO - Original Message - From: kh6ty kh...@comcast.net To: digitalradio@yahoogroups.com Sent: Sunday, February 22, 2009 8:58 AM Subject: [digitalradio] Re: NBEMS QST article/digital weak signal FM Hi Vojtech, Thanks for the tip. I totally forgot about the possible effect of deemphasis and what effect the center audio frequency might have. Our goal with NBEMS has always been able to reach at least 100 miles reliably, in order to span the largest expected disaster area to reach Internet or phone connectivity. We were recently surprised to find over a 117 mile path on 2 meters, that, on the average, FM with DominoEx actually worked better than SSB with DominoEx, even with the poorer S/N of FM compared to SSB. The surprise was an unexpected, consistent, fast flutter which did not seem to affect FM nearly as badly as SSB. Thanks to Tony's wondering, we will continue to evaluate different modes (and different audio center frequencies!) and post the results here. 73, Skip KH6TY Skip, it would be interesting, if you could investigate, which modulation bandwidths and at which center audio frequency the common FM transceivers work best with common HF weak signal digital modes. Keep the good work. Someone able to do the math? 73, Vojtech OK1IAK White noise tests show DominioEX-4 to be a bit more sensitive than MFSK16, but it doesn't seem to handle HF distortion nearly as well. I was surprised that it did better than MFSK16 with multipath and was wondering if you thought the better throughput was due to MFSK16 tuning issues rather than actual robustness? Tony - KHMU
