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42 lines
2.0 KiB
Plaintext
42 lines
2.0 KiB
Plaintext
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// 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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