Please allow me to make some comments based on research done here in San Antonio.
As some may know, SouthWest Research Institute is located here in San Antonio and has done work on high baud rate modes with very poor SNRs. Also they are part of the current project that is "flying" a space craft passt Saturn or Jupitor going to Pluto. Note that the project and space craft were launched after Pluto was demoted. There research has shown that a "tone/carrier" with more than 8 phases is going to be very hard to detect and that in fact to get it to be error free, the decode time may be as long as using a simpler mode with ARQ. They showed one case where it took 4 seconds to decode 1 second of transmission. There is really great potential in OFDM types of modems if bandwidth is not a concern. However, if we want to keep the bandwidth under 500 Hz, then you are going to be limited to the number of tones you use, the baud rate, gaurd band, etc, etc. There are notes about brick wall filters and of course tone shaper filters, etc. One solution suggested to me was that each tone be individually shaped/filtered before transmitting and then each tone have an individual brick wall filter before it is decoded. I believe that there is not going to be one mode or mode configuration that works well on 3-30 MHz...we will probably have several sets of configurations or perhaps even one optimum configuration for each band or set of conditions. The more options you have that you can adjust of the fly or that can be used adaptively the better off the mode will be. Someone (perhaps all) needs to keep technical notes on what modes work best on what band. Also, we need to come to an agreement on what mazimum bandwidth and user throughput we want as well as how robust and how sensitive we want the mode to be. My personal belief is that we go for 500 Hz wandwidth, 400-800 WPM user throughput 99.9% error free and work down below a 0 dB SNR (0 to -5 dB) on a poor CCIR channel. Put your thinking caps on and make your wish list. 73 & CLU, Walt/K5YFW KV9U wrote: > > > Some of us did try Chip modes when Nino first came out with them, but > they did not seem to perform as well as existing modes. > > I really implore to our treasured programmers to see if they can come up > with some modes that can compete with Pactor modes. Especially some ARQ > modes that can work on MS OS. > > We know from Pactor 2, that a raised cosine shaped pulse is likely a > very good basic waveform. Then for the most robust mode, a two tone > DBPSK modulation is used and as the conditions improve, the modulation > changes to DQPSK and then with further improvements to 8-DPSK and even > 16-DPSK for maximum throughput when conditions are very good. This is > what enables Pactor 2 to send about 700 bits per second at the peak > speed and do it in only a 500 Hz wide span. > > We know this can be done at the higher speeds under good conditions with > sound card modes since SCAMP was even faster than P2, although a much > wider signal. The problem with SCAMP was that it had no fallback position. > > Pactor 3 is runs an occupied bandwidth of about 2.4 kHz, but raw speed > is over 2700 bps. Instead of 2 tones, P3 uses up to 18, separated by 120 > Hz and modulated at 100 baud DBPSK or DQPSK. > > SCS has some fairly detailed data on Pactor 3 at: > > http://www.scs- ptc.com/download /PACTOR-III- Protocol. pdf > <http://www.scs-ptc.com/download/PACTOR-III-Protocol.pdf> > > I wish someone could explain why we can not have a sound card mode that > is roughly the same as Pactor 2 at least. Even if there was no ARQ at first. > > And how different is Pactor 3, than what the SSTV hams are using > everyday? Aren't they using OFDM with QAM? If you recall what Tom Rink > said back in 1995 on the TAPR HF SIG: > > "As mentioned in the introduction, PACTOR-II uses a two-tone DPSK modulation > system. Due to the raised cosine pulse shaping, the maximum required > bandwidth > is only around 450 Hz at minus 50 dB. ASK, which was also tested in the > early > stage, provided poorer results in weak conditions compared with a higher > DPSK > modulation, as different amplitude levels are more difficult to > distinguish in > noisy channels than more phase levels. Additionally, ASK increases the Crest > Factor of the signal. For these reasons, it is not used in the final > PACTOR-II > protocol. Basic information on these items can also be found in the > first part > of this series." > > Although not ASK, doesn't QAM employ amplitude changes as part of the > modulation scheme? > > What happens if you use a multitone DPSK? It seems to a non-engineering > person like myself, that a lot of what P2 and P3 are made up of are > really a series of PSK100 or PSK200 tones (carriers). > Isn't Q15X25 a similar modulation scheme? It even runs at 83.33 baud > rather than a minimum of 100 baud such as P2. > > Why did it not work as well as P modes? > > Or is it because it has no coding such as Reed-Solomon block coding or > Viterbi convolutional coding? > > 73, > > Rick, KV9U > > Jose_Angel Amador Fundora wrote: > > >Nino: > > > >I have not had luck with Chip...not a single QSO so far. > > > >On 40 meters local NVIS test it did not work. > > > >Maybe the 300 baud chip rate was too fast for it to work. > > > >Would it be prefarable to use it on a "close to the MUF, single ray > link"?? I would like to try it on the air. > > > >How has been the actual experience with Chip modes? > > > >73 de Jose, CO2JA > > > > > > > >---------- Original Message ------------ --------- --------- ---- > >From: "Nino Porcino \(IZ8BLY\)" <[EMAIL PROTECTED] com > <mailto:iz8bly%40katamail.com>> > >Reply-To: digitalradio@ yahoogroups. com > <mailto:digitalradio%40yahoogroups.com> > >Date: Mon, 22 Jan 2007 18:18:07 +0100 > > > > > > > >>Walt/K5YFW wrote: > >> > >> > >> > >>>if you may be receiving 1, 2 and 3 hop signals. How does this affect > BPSK > >>>and QPSK signals from for example PSK31/63/125? > >>> > >>> > >>the 3 different signals will sum at the receiver, but, having each one a > >>different phase, the sum is destructive with the result that they tend to > >>cancel. If the paths are stable you notice a drop in the signal > strength but > >>if paths are unstable (as it is often the case) one signal may win > over the > >>others and the phase of the PSK decoder will wander back and forth. The > >>clock recovery is also problematic because of the unstability of the > >>reference. > >> > >>Among the possible solutions to multipath there is the spread spectrum > >>modulation (as in Chip64) where the symbols at the receiver aren't > expected > >>at a precise timing, but are decoded in a "clockless" manner. In Chip64 > >>signal scope you can actually see the signal trace wandering left and > rigth > >>due to path hopping or see the ghosted trace of the secondary path. > >> > >>Nino/IZ8BLY > >> > >> > >> > >> > >> > > > > > > > >___________ ___ ____________ __ ____________ __ ____________ __ > >Correo enviado por ElectroMAIL. Facultad Eléctrica. CUJAE Dominio: > electrica.cujae. edu.cu > > > > > > > > > > > > > > > > > > > >
