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- *JT65* is a mature mode described in the literature some years
ago. Details of the *JT9* protocol are presented in <<X16,Appendix A>>
of this Guide.

- To users already familiar with *JT65*, the most striking difference
between the two modes is the much smaller occupied bandwidth of JT9:
15.6 Hz, compared with 177.6 Hz for *JT65A*. Transmissions in the two
modes are essentially the same length, and both modes use exactly 72
bits to carry message information.  At the user level the two modes
support the same message structures.

- *JT65* signal reports are constrained to the range –1 to –30 dB —
more than adequate for EME purposes, but not enough dynamic range for
ideal use at HF and below.

- S/N values displayed by the *JT65* decoder are clamped at –1 dB,
because that’s all the original protocol can handle; the S/N scale in
present *JT65* decoders becomes increasingly nonlinear above –10 dB.

- By comparison, *JT9* allows for signal reports in the range –50 to
\+49 dB.  It manages this by co-opting a small amount of message space
otherwise used for grid locator's within 1 degree of the south
pole. The S/N scale of the present *JT9* decoder is reasonably linear,
although it’s not intended as a precision measurement tool. With clean
signals in a clean nose background, *JT65* achieves nearly 100%
probability of correct decoding down to S/N = –22 dB and 50% at –24
dB.  *JT9* is about 2 dB better, achieving 50% decoding at about –26
dB.  Both modes produce extremely low false-decode rates.

- Early experience suggests that under most HF propagation conditions
the two modes have comparable reliability, with perhaps a slight edge
to *JT9*. The tone spacing of *JT9* is about two-thirds that of
*JT65*, so in some disturbed ionospheric conditions in the higher
portion of the HF spectrum, *JT65* may do better. *JT9* is an order of
magnitude better in spectral efficiency. On a busy HF band, we often
find the 2-kHz-wide *JT65* sub-band filled wall-to-wall with signals.
Ten times as many JT9 signals could fit into the same space, without
overlap.