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WSJT-X is an experimental version of WSJT implementing JT9, a new mode designed
especially for use on the MF and LF bands.  JT9 shares many characteristics with the
popular modes JT65 and JT4.  All three are designed for making QSOs under extreme
weak-signal conditions.  They use nearly identical message structure and source
encoding.  JT65 is used for EME on the VHF/UHF bands, and for worldwide QRP
communication at HF.  JT4 is used mainly on the microwave bands. In contrast, JT9 is
optimized for the 1.8 MHz, 472 kHz, and 137 kHz bands.  It has been found to be
useful also at HF, and even for EME at VHF, while using less than  ten percent of the
bandwidth of JT65.
JT9 offers five choices for the duration of timed T/R sequences: submodes JT9-1,
JT9-2, JT9-5, JT9-10, and JT9-30 use 1, 2, 5, 10, and 30 minutes, respectively.  Submodes with longer transmissions trade reduced throughput for smaller bandwidth and
increased sensitivity.  The slowest sub-mode, JT9-30, has total bandwidth 0.4 Hz and
operates at signal-to-noise ratios as low as -40 dB measured in the standard 2.5 kHz
reference bandwidth.

The JT9 Protocol
JT9 is a mode designed for making QSOs at MF and LF.  The mode uses essentially
the same 72-bit structured messages as JT65.  Error control coding (ECC) uses a
strong convolutional code with constraint length K=32, rate r=1/2, and a zero tail,
leading to an encoded message length of (72+31) × 2 = 206 information-carrying bits. 
Modulation is 9-FSK: 8 tones are used for data, one for synchronization.  Sixteen
symbol intervals are used for synchronization, so a transmission requires a total of 
206 / 3 + 16 = 85 (rounded up) channel symbols.  Symbol durations are approximately
(TRperiod - 8) / 85, where TRperiod is the T/R sequence length in seconds.  Exact
symbol lengths are chosen so that nsps, the number of samples per symbol (at 12000
samples per second) is a number with no prime factor greater than 7.  This choice
makes for efficient FFTs.  Tone spacing of the 9-FSK modulation is df = 1 / tsym =
12000 / nsps, equal to the keying rate.  The total occupied bandwidth is 9 × df.  The
generated signal has continuous phase and constant amplitude, and there are no key
clicks.
Parameters of five JT9 sub-modes are summarized in the following table, along with
approximate S/N thresholds measured by simulation on an AWGN channel.  Numbers
following “JT9-” in the sub-mode names specify TRperiod in minutes.
Submode nsps Symbol Duration(s) Tone Spacing (Hz)Signal Bandwidth(Hz)S/N Threshold*(dB) QSO Time (minutes)
JT9-1       6912              0.58                           1.736                         15.6                                  -27                            6
JT9-2      15360             1.28                            0.781                          7.0                                  -30                           12
JT9-5      40960             3.41                            0.293                          2.6                                  -34                           30
JT9-10    82944             6.91                            0.145                          1.3                                  -37                           60
JT9-30   252000            21.00                          0.048                          0.4                                  -42                         180

+Very Sensitve
-slow to VERY slow
-No QSO really to speak of
-Learning curve on reporting sequence
-Can be confused with a birdie signal


Mode Menu
JT9
Mode Audio Sample
FFT Window Screenshot