mirror of
https://github.com/saitohirga/WSJT-X.git
synced 2024-11-23 20:58:55 -05:00
def93a8931
git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@5434 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
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, 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.
|