// Status=review

The most striking difference between JT65 and JT9 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 nearly identical message structures.

JT65 signal reports are constrained to the range –1 to –30 dB. This
range is more than adequate for EME purposes, but not really enough
for optimum use at HF and below. S/N values displayed by the JT65
decoder are clamped at an upper limit –1 dB, and in present JT65
decoders the S/N scale is nonlinear above –10 dB.

By comparison, JT9 allows for signal reports in the range –50 to +49
dB. It manages this by taking over a small portion of ``message
space'' that would otherwise be used for grid locators within 1 degree
of the south pole. The S/N scale of the present JT9 decoder is
reasonably linear (although it's not intended to be a precision
measurement tool).  

With clean signals and a clean noise background, JT65 achieves nearly
100% decoding down to S/N = –22 dB and about 50% success at –24
dB. JT9 is about 2 dB better, achieving 50% decoding at –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. 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 perform
better.

JT9 is an order of magnitude better in spectral efficiency. On a busy
HF band, the conventional 2-kHz-wide JT65 sub-band is often filled
with overlapping signals. Ten times as many JT9 signals can fit into
the same frequency range, without collisions.

JT65 signals often decode correctly even when they overlap. Such
behavior is much less likely with JT9 signals, which fill their occupied
bandwidth more densely. JT65 may also be more forgiving of small 
frequency drifts.