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38 lines
2.0 KiB
Plaintext
38 lines
2.0 KiB
Plaintext
// Status=review
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The JT65 protocol was described in a {jt65protocol} in 2005; details
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of the JT9 protocol are presented in the next section 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 nearly identical message structures.
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JT65 signal reports are constrained to the range –1 to –30 dB — more
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than adequate for EME purposes, but not enough dynamic range for ideal
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use at HF and below. S/N values displayed by the JT65 decoder are
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clamped at an upper limit –1 dB, because that’s all the original
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protocol can handle. Moreover, the S/N scale in present JT65 decoders
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becomes increasingly nonlinear above –10 dB. By comparison, JT9
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allows for signal reports in the range –50 to +49 dB. It manages this
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by co-opting a small portion of ``message space'' that would otherwise
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be used for grid locators within 1 degree of the south pole. The S/N
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scale of the present JT9 decoder is reasonably linear (although it’s
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not intended as a precision measurement tool). With clean signals and
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a clean nose background, JT65 achieves nearly 100% probability of
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correct decoding down to S/N = –22 dB and 50% at –24 dB. JT9 is about
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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 do
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better. JT9 is an order of magnitude better in spectral efficiency.
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On a busy HF band, we often find the 2-kHz-wide JT65 sub-band filled
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wall-to-wall with signals. Ten times as many JT9 signals can fit
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into the same frequency range, without overlap.
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