Why do some modems use more "rectangular" waveforms instead of what appears to be the optimum waveform for HF modems? Or are there downsides to raised cosine waveforms?
In terms of bandwidth, it seems to me that for most uses, a 500 Hz bandwidth is a wise choice. This seems to be a good tradeoff in width vs potential throughput for keyboard modes and even some higher speed data modes like Pactor 2 can do under better conditions. Also, 500 Hz filters have been commonly available for CW. With more rigs using DSP filters, I admit that it is less of an issue to tailor make it to one particular width. How about a two tone, DPSK scheme with 50 (maybe even 25?), and also 100 and 200 baud rates? Even if you would initially require manual adjustment. Then we also should have a mode that can run in a voice channel, probably 2.4 to at most 2.7 KHz width. How about an 8 tone DBPSK and maybe switchable (manually at first) to higher PSK rates? All of these modes could have similar timing for ARQ. How about 0.5 seconds pause between transmissions and awaiting an ACK or NAK or control signal? With right control signals you could change the length of time for the packet burst. But for starters, maybe just a simple packet size. 73, Rick, KV9U Walt DuBose wrote: >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 >> >>
