Hi all, I've been asked a number of questions about fundamentals concerning the new "Fast JT9" submodes in WSJT-X. This seems like a good time to review the basics of *all* WSJT modes, including their particular strengths, limitations, optimum propagation types, etc.
A useful comparison among all the WSJT modes is presented in a graph posted here: http://physics.princeton.edu/pulsar/K1JT/wsjt_modes.pdf The graph shows the bandwidths of signals in each WSJT mode, plotted as a function of message duration -- the time required to transmit the message once at the keying rate used for that mode. The slow modes fall on the right side of the figure. The transmissions last for about 48 s (JT4, JT9, and JT65) or 110 s (WSPR), and message information (including encoded redundancy for error correction) is sent just once per transmission. These modes are optimized for propagation types that yield more-or-less steady signals over a full transmission. The fast modes (toward upper left of the figure) use faster keying rates. Message durations are much less than the T/R sequence length, so information is repeated many times in a single transmission. These modes are designed to take advantage of short signal enhancements such as meteor pings or scatter maxima. JT4, JT9, and JT65 use structured messages compressed into exactly 72 bits of user information. In contrast, JTMS, FSK441, ISCAT, and JT6M permit arbitrary message lengths up to 28 characters. For comparison purposes in the plotted figure, message lengths were taken as 19 characters: two 6-character callsigns, a four-character grid or report, and the required spaces separating them. Fast keying requires relatively wide bandwidth. Slow modes can get by with smaller bandwidths or can use many more tones, as in JT65, which brings other advantages. Slow keying with wide tone spacing is a good choice for propagation types such as 10 GHz EME, where inevitable Doppler spreading renders signals much wider than their natural modulation-induced widths. The WSJT modes with structured messages and strong forward error correction have become very popular for a variety of applications including EME at VHF and above and weak-signal DXing on the MF and HF bands. The fast JT9 modes seem likely to become popular for scatter propagation; JT9H works fairly well also for meteor scatter on 6 meters. It would be nice if the JT9H message duration (0.425 s) could be made smaller, for meteor-scatter use. Unfortunately, this would require more bandwidth, which is impossible with traditional ham SSB transceivers. For this reason JTMS and FSK441 remain the best choices for meteor scatter at 144 MHz and above, and probably also at 50 MHz at power levels 100 W and less. It would be nice to have a mode with SSB-compatible bandwidth, message duration less than 0.2 s, and strong forward error correction. A (presently hypothetical) mode labeled JTMS-FEC in the figure represents a possible way to address this need. Experiments with such a mode are planned for the near future. -- 73, Joe, K1JT ------------------------------------------------------------------------------ _______________________________________________ wsjt-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/wsjt-devel
