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32 lines
1.7 KiB
Plaintext
32 lines
1.7 KiB
Plaintext
JT8 is a mode designed for amateur QSOs and beacon-like transmissions
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at MF and LF. The mode uses the same 72-bit user messages as JT65,
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augmented by a 12-bit cyclic redundancy check (CRC). Error-control
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coding uses a convolutional code with constraint length K=16, rate
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r=1/2, and a zero tail, leading to an encoded message length of
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(72+12+15)*2 = 198 bits. Modulation is 8-FSK, so a transmission
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requires 198/3 = 66 information-carrying channel symbols. Two 8x8
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Costas arrays are added to each transmission for purposes of time and
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frequency synchronization. A full transmission thus contains 66 + 16
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= 82 symbols. Tone spacing df of the 8-FSK modulation is equal to the
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keying rate; symbol duration tsym = 1/df, and the total occupied
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bandwidth is 8*df. The actual transmission length TxT is slightly
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less than the T/R sequence time, to allow for possible message
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decoding before the next transmission starts.
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Parameters of the five JT8 sub-modes are summarized in the following
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table, along with S/N thresholds measured by simulation on an AWGN
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(additive white Gaussian noise) channel. Parameter nsps is the number
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of samples per symbol at sample rate 1500 Hz.
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-----------------------------------------------------
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Mode T/R TxT nsps df tsym BW S/N*
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(m) (s) (Hz) (s) (Hz) (dB)
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-----------------------------------------------------
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JT8-1 1 52.5 960 1.562 0.64 12.6 -26.9
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JT8-2 2 112.0 2048 0.732 1.37 5.9 -30.2
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JT8-5 5 293.9 5376 0.279 3.58 2.2 -34.4
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JT8-10 10 587.8 10752 0.140 7.17 1.1 -37.5
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JT8-30 30 1791.3 32768 0.046 21.85 0.4 -42.3
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-----------------------------------------------------
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* Noise power measured in a 2500 Hz bandwidth.
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