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45 lines
1.8 KiB
Plaintext
45 lines
1.8 KiB
Plaintext
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// Status=review
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//Needs work!
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.The JT9 Protocol and its Implementation
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- *JT9* is a mode designed for making QSOs at HF, MF, and LF. The
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mode uses essentially the same 72-bit structured messages as *JT65*.
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- Error control coding (ECC) uses a strong convolutional code with
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constraint length K=32, rate r=1/2, and a zero tail. WIth 72-bit
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user messages, this leads to an encoded message length of
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(72+31) × 2 = 206 bits.
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- Modulation is 9-FSK: 8 tone frequencies for data, and one for
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synchronization. In a given transmission sixteen tone intervals
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(those numbered 1, 2, 5, 10, 16, 23, 33, 35, 51, 52, 55, 60, 66, 73,
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83, and 85 in the sequence) are devoted to synchronization. Thus, a
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transmission requires a total of (206 / 3) + 16 = 85 (rounded up) tone
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intervals.
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- Symbol lengths are chosen so that nsps, the number of samples
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per symbol (at 12000 samples per second) is a number with no prime
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factor greater than 7. This choice makes for efficient FFTs. Tone
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spacing of the 9-FSK modulation is:
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-----
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df = 1 / tsym = 12000 / nsps, equal to the keying rate
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-----
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- Symbol durations are approximately (TRperiod - 8) / 85, where
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TRperiod is the T/R sequence length in seconds.
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- The total occupied bandwidth is 9 × df. The generated signal has
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continuous phase and constant amplitude, so there are no key
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clicks. For experimental purposes, submodes of *JT9* were defined with
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transmission lengths greater than one minute.
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- Parameters of all submodes are summarized in the following table,
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along with approximate decoding thresholds measured by simulation on
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an additive white Gaussian noise (AWGN) channel. Numbers following
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*``JT9-''* in the submode names specify the T/R sequence length in
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minutes. When not otherwise specified in this Guide, *JT9* implies
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submode *JT9-1*, the only submode implemented in current versions of
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{wsjtx}.
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