WSJT-X/doc/source/jt9-protocol.adoc

// Status=review
//Needs work!

JT9 is designed for making minimal QSOs at LF, MF, and HF.  It uses
72-bit structured messages nearly identical (at the user level) to
those in 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 nine-tone frequency-shift keying, 9-FSK.
Eight tones are used for data, one for synchronization.  Eight data
tones means that three data bits are conveyed by each transmitted
information symbol.  Sixteen symbol intervals are devoted to
synchronization, so a transmission requires a total of 206 / 3
+ 16 = 85 (rounded up) channel symbols. The sync symbols are those
numbered 1, 2, 5, 10, 16, 23, 33, 35, 51, 52, 55, 60, 66, 73, 83, and
85 in the transmitted sequence.

Each symbol lasts for 6912 sample intervals at 12000 samples per
second, or about 0.576 s.  Tone spacing of the 9-FSK modulation is
12000/6912 = 1.736 Hz, the inverse of the symbol duration.  The total
occupied bandwidth is 9 × 1.736 = 15.6 Hz.  A generated JT9 signal has
continuous phase and constant amplitude, and there are no key clicks.
The transmitter's power amplifier need not be highly linear.