// Status=review
//Needs work!
.The JT9 Protocol and its Implementation

- *JT9* is a mode designed for making QSOs at HF, 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.  WIth 72-bit
user messages, this leads to an encoded message length of 
(72+31) × 2 = 206 bits.

- Modulation is 9-FSK: 8 tone frequencies for data, and one for
synchronization.  In a given transmission sixteen tone intervals
(those numbered 1, 2, 5, 10, 16, 23, 33, 35, 51, 52, 55, 60, 66, 73,
83, and 85 in the sequence) are devoted to synchronization.  Thus, a
transmission requires a total of (206 / 3) + 16 = 85 (rounded up) tone
intervals.

- 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
-----

- Symbol durations are approximately (TRperiod - 8) / 85, where
TRperiod is the T/R sequence length in seconds.


- The total occupied bandwidth is 9 × df. The generated signal has
continuous phase and constant amplitude, so there are no key
clicks. For experimental purposes, submodes of *JT9* were defined with
transmission lengths greater than one minute.

- Parameters of all submodes are summarized in the following table,
along with approximate decoding thresholds measured by simulation on
an additive white Gaussian noise (AWGN) channel. Numbers following
*``JT9-''* in the submode names specify the T/R sequence length in
minutes.  When not otherwise specified in this Guide, *JT9* implies
submode *JT9-1*, the only submode implemented in current versions of
{wsjtx}.