mirror of
https://github.com/saitohirga/WSJT-X.git
synced 2024-11-30 16:18:54 -05:00
42 lines
2.0 KiB
Plaintext
42 lines
2.0 KiB
Plaintext
|
// 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. Moreover, the S/N scale
|
|||
|
|
in present JT65 decoders 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 nose background, JT65 achieves nearly
|
|||
|
|
100% decoding down to S/N = –22 dB and about 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. 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.
|