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These documentation source files are not the one true version, just a copy for testing purposes. DO NOT EDIT THESE FILES. To use this on Windows you will need a working asciidoc installation and the path to it must be included in your CMAKE_PREFIX_PATH (probably via a local CMake tool chain file). At the time of writing the official asciidoc package does not work on Windows. The latest development master does however work, it can be downloaded as a snapshot ZIP archive from here: https://github.com/asciidoc/asciidoc/archive/master.zip git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@5316 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
42 lines
2.0 KiB
Plaintext
42 lines
2.0 KiB
Plaintext
// Status=review
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The most striking difference between JT65 and JT9 is the much smaller
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occupied bandwidth of JT9: 15.6 Hz, compared with 177.6 Hz for JT65A.
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Transmissions in the two modes are essentially the same length, and
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both modes use exactly 72 bits to carry message information. At the
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user level the two modes support nearly identical message structures.
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JT65 signal reports are constrained to the range –1 to –30 dB. This
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range is more than adequate for EME purposes, but not really enough
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for optimum use at HF and below. S/N values displayed by the JT65
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decoder are clamped at an upper limit –1 dB. Moreover, the S/N scale
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in present JT65 decoders is nonlinear above –10 dB.
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By comparison, JT9 allows for signal reports in the range –50 to +49
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dB. It manages this by taking over a small portion of ``message
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space'' that would otherwise be used for grid locators within 1 degree
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of the south pole. The S/N scale of the present JT9 decoder is
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reasonably linear (although it’s not intended to be a precision
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measurement tool).
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With clean signals and a clean nose background, JT65 achieves nearly
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100% decoding down to S/N = –22 dB and about 50% at –24 dB. JT9 is
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about 2 dB better, achieving 50% decoding at about –26 dB. Both modes
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produce extremely low false-decode rates.
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Early experience suggests that under most HF propagation conditions
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the two modes have comparable reliability. The tone spacing of JT9 is
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about two-thirds that of JT65, so in some disturbed ionospheric
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conditions in the higher portion of the HF spectrum, JT65 may perform
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better.
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JT9 is an order of magnitude better in spectral efficiency. On a busy
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HF band, the conventional 2-kHz-wide JT65 sub-band is often filled
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with overlapping signals. Ten times as many JT9 signals can fit into
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the same frequency range, without collisions.
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JT65 signals often decode correctly even when they overlap. Such
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behavior is much less likely with JT9 signals, which fill their occupied
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bandwidth more densely. JT65 may also be more forgiving of small
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frequency drifts.
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