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42 lines
2.1 KiB
Plaintext
42 lines
2.1 KiB
Plaintext
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// Status=review
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//This section needs work!
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- *JT65* is a mature mode described in the literature some years
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ago. Details of the *JT9* protocol are presented in <<X16,Appendix A>>
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of this Guide.
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- To users already familiar with *JT65*, the most striking difference
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between the two modes is the much smaller occupied bandwidth of JT9:
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15.6 Hz, compared with 177.6 Hz for *JT65A*. Transmissions in the two
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modes are essentially the same length, and both modes use exactly 72
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bits to carry message information. At the user level the two modes
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support the same message structures.
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- *JT65* signal reports are constrained to the range –1 to –30 dB —
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more than adequate for EME purposes, but not enough dynamic range for
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ideal use at HF and below.
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- S/N values displayed by the *JT65* decoder are clamped at –1 dB,
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because that’s all the original protocol can handle; the S/N scale in
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present *JT65* decoders becomes increasingly nonlinear above –10 dB.
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- By comparison, *JT9* allows for signal reports in the range –50 to
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\+49 dB. It manages this by co-opting a small amount of message space
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otherwise used for grid locator's within 1 degree of the south
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pole. The S/N scale of the present *JT9* decoder is reasonably linear,
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although it’s not intended as a precision measurement tool. With clean
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signals in a clean nose background, *JT65* achieves nearly 100%
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probability of correct decoding down to S/N = –22 dB and 50% at –24
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dB. *JT9* is about 2 dB better, achieving 50% decoding at about –26
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dB. Both modes 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, with perhaps a slight edge
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to *JT9*. The tone spacing of *JT9* is about two-thirds that of
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*JT65*, so in some disturbed ionospheric conditions in the higher
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portion of the HF spectrum, *JT65* may do better. *JT9* is an order of
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magnitude better in spectral efficiency. On a busy HF band, we often
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find the 2-kHz-wide *JT65* sub-band filled wall-to-wall with signals.
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Ten times as many JT9 signals could fit into the same space, without
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overlap.
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