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 >> > > >
