mirror of
https://github.com/saitohirga/WSJT-X.git
synced 2024-11-19 10:32:02 -05:00
ace8f6418e
bad data. 2. Fix a bug that could allow a Koetter-Vardy false decode instead of a valid Berlekamp-Massey decode, sometimes leading to program crash. 3. Many more edits in the User's Guide, *.adoc files. git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@3664 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
45 lines
2.2 KiB
Plaintext
45 lines
2.2 KiB
Plaintext
// Status=review
|
||
|
||
The JT65 protocol was described in a {jt65protocol} in 2005; details
|
||
of the JT9 protocol are presented in the next section 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 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 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 can fit into
|
||
the same frequency range, without overlap.
|
||
|
||
JT65 signals often decode correctly even when they overlap. Such
|
||
behavior is much less likely with JT9 signals, which fill their occupied
|
||
bandwisth more densely. JT65 may also be more forgiving of small
|
||
frequency drifts.
|