> Well, over here, the biggest challenges are technical, such as "Will > the signal survive ionospheric paths?". Only one way to find that one out. ;) >
Digital voice on HF will NEVER catch on because it is a strong signal mode, and HF is the home of weak signals, well except for 80 meters. By strong signal I mean a lot of power is needed per path length. Sure you can hit your local repeater with 100 milliwatts, but try going more than a few miles with that power - especially over the horizon. DStar is AMBE vocoded voice fed to GMSK modulation while G4GUO/AOR's HF digital protocol is AMBE vocoded voice fed to OFDM (specificially 36 carrier DQPSK) modulation. Depending on how you measure performance, GMSK generally requires about twice the bandwidth as OFDM for the same data and error rate, so there will be additional regulatory issues in the USA for HF GMSK, at least below 28 mhz. I've not thought about the s/n required for CW to be intelligible, but I bet it's just a hair above zero. 10 db s/n generally produces amateur quality ssb, and 15 db s/n is the minimum for commercial quality ssb. G4GUO needed 25 db over a 70 km path on 40 meters for reliable communications (see first link below). When the ARRL reviewed the AR9800, they needed 18 db s/n to avoid syllable dropout (see second link below). Although focusing on satellite communications, Phil Karn (KA9Q) give an excellent explanation for poor digital performance at low s/n - "Decrease the power of a SSB signal by 1 dB and the result is simply a 1 dB reduction in audio S/N. Most might not even notice. (Even the differences between the simulated SSB signals given here are a little subtle to me.) But a 1dB change in S/N to an RF demodulator and FEC decoder can make the difference between perfect operation and no operation. Being inherently nonlinear, digital modulation has a threshold effect much like that of wideband FM, only more pronounced. The stronger (i.e., more efficient) the modulation and coding, the sharper the threshold." (see third link below) Imaging this effect with QSB right around the threshold... And as Tony says, HF multipath is a killer for these demodulators. http://www.arrl.org/tis/info/HTML/digital_voice/ http://www.arrl.org/members-only/prodrev/pdf/pr0402.pdf http://www-mobile.ecs.soton.ac.uk/speech_codecs/voicedemo/ Okay, my Elecraft K1 battery is charged so I think I'll go to the park with my portable G5RV and see if the 5W band to JA land is open this afternoon. 73 -- John
