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Improved JT4 decoder in WSJT

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  • Joe Taylor
    To: Microwave users of WSJT Based on results of some fairly extensive tests of the JT4x modes on terrestrial 23 cm paths, carried out by members of the UK
    Message 1 of 1 , Jul 6, 2009
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      To: Microwave users of WSJT

      Based on results of some fairly extensive tests of the JT4x
      modes on terrestrial 23 cm paths, carried out by members of
      the UK microwave group, I have implemented some improvements
      to the JT4 decoder.

      If you have read the description of JT2 and JT4 modes at
      http://physics.princeton.edu/pulsar/K1JT/JT2_JT4.TXT , you
      will have seen the following table that shows tone spacings,
      total bandwidths, and threshold S/N limits for each of the
      JT4 submodes:

      Mode Spacing BW S/N limit Possible Applications
      (Hz) (Hz) (dB)
      ----------------------------------------------------
      JT4A 4.375 17.5 -23 EME at VHF/UHF
      JT4B 8.75 35.0 -22
      JT4C 17.5 70.0 -21
      JT4D 39.375 157.5 -20 2.3, 3.4 GHz EME
      JT4E 78.75 315.0 -19 5.7 GHz EME
      JT4F 157.5 630.0 -18 10 GHz EME
      JT4G 315.0 1260.0 -17

      The seven JT4x submodes are intended to provide a range of
      tone-spacing options so as to accommodate a variety of
      propagation conditions -- in particular, different amounts
      of Doppler spreading on the path.

      Experiments by the UK microwave gang have established that
      on 23 cm paths of a few hundred km, submodes JT4A-C will
      seldom decode, even with what appears to be adequate signal
      strength. JT4D decodes sometimes, and JT4E-G usually
      decode. I interpret this to mean that the frequency
      spreading on the tested paths is typically several tens of
      Hz, perhaps sometimes approaching 100 Hz. If the spread
      exceeds the tone spacing, decoding ability will be severely
      degraded.

      Of course, to be safe one might always use JT4G, with its
      315 Hz tone spacing; but as the table shows, with the old
      decoder this can cost up to 7 dB in lost sensitivity when
      the path spread is actually much smaller.

      The new JT4 decoder obviates this problem by trying each of
      the detection bandwidths 4.375, 8.75, 17.5, 39.375, 78.75,
      157.5, and 315 Hz, in turn, until a good decode is obtained.
      The effect is that for an unspread signal, JT4G is now
      just as sensitive as JT4A. Decoding now takes somewhat
      longer, but the impact is small enough that with modern
      computers you probably won't notice it.

      I have not yet seen many examples of *.WAV files for 1296
      MHz troposcatter signals, so I am in a sense "flying blind"
      in attempts to improve JT4 for this application. I suspect
      that the next important area for improvement will be to
      implement AFC in software. Some user input (and a
      collection of example files) would be most helpful here!

      To download a trial version of WSJT with these improvements,
      use one of the following links:

      ################################################################
      For Windows:

      http://physics.princeton.edu/pulsar/K1JT/WSJT704r1268.EXE

      To install, simply execute the downloaded file.

      ################################################################
      For Ubuntu Linux 9.04 (and probably other recent
      Debian-based Linux distributions):

      http://physics.princeton.edu/pulsar/K1JT/wsjt7_r1268.tgz

      To install and run, put the downloaded file in your login
      directory and type the following commands in a
      command-prompt window:

      $ tar xzf wsjt7_r1268.tgz
      $ cd wsjt
      $ python wsjt.py

      ################################################################

      I look forward to receiving some reports from users!

      -- 73, Joe, K1JT
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