66 lines
3.4 KiB
Plaintext
66 lines
3.4 KiB
Plaintext
JT9 is a mode designed for amateur QSOs at MF and LF. The mode uses
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the same 72-bit structured messages as JT65. Error control coding
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(ECC) uses a strong convolutional code with constraint length K=32,
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rate r=1/2, and a zero tail, leading to an encoded message length of
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(72+31)*2 = 206 information-carrying bits. Modulation is 9-FSK: 8
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tones for data, one for synchronization. Sixteen symbol intervals are
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used for synchronization, so a transmission requires a total of 207/3
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+ 16 = 85 channel symbols. Symbol durations tsym are approximately
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(TRperiod-8)/85, where TRperiod is the T/R sequence length in seconds.
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Exact 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 df=1/tsym=12000/nsps, equal to the
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keying rate. The total occupied bandwidth is 9*df. The generated
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signal has continuous phase, and there are no key clicks.
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Parameters of five JT9 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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channel. Numbers following "JT9-" in the sub-mode names specify the
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T/R sequence length in minutes.
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--------------------------------------------------------------------------
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Mode nsps nsps2 df tsym BW S/N* Tdec Tfree Factors
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12000 1500 (Hz) (s) (Hz) (dB) (s) (s) of nsps nfft3
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--------------------------------------------------------------------------
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JT9-1 6912 864 1.736 0.58 15.6 -26.9 52.5 7.5 2^8 3^3 2048
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JT9-2 15360 1920 0.781 1.28 7.0 -30.2 112.3 7.7 2^10 3 5 2048
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JT9-5 40960 5120 0.293 3.41 2.6 -34.4 293.6 6.4 2^13 5 6144
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JT9-10 82944 10368 0.145 6.91 1.3 -37.5 591.0 9.0 2^10 3^4 12288
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JT9-30 252000 31500 0.048 21.00 0.4 -42.3 1788.5 11.5 2^5 3^2 5^3 7 32768
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--------------------------------------------------------------------------
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* Noise power measured in a 2500 Hz bandwidth.
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NB: nfft3 might be doubled and used with a sin^2 window.
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Transmitting
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------------
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1. Source encode the structured message to 72 bits
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2. Apply convolutional ECC (K=32, r=1/2) to yield (72+31)*2 = 206 bits
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3. Interleave to scramble the bit order
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4. Assemble 3-bit groups to make (206+1)/3 = 69 symbols
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5. Gray-code the symbol values
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6. Insert 16 sync symbols ==> 69+16=85 channel symbols, values 0-8
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Receiving
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---------
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1. Apply noise blanking with the timf2 method
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2. Filter to 1000 Hz bandwidth and downsample (1/8) to 1500 Hz, saving
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complex data to array c0(2,700,000).
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3. Compute spectra at half-symbol steps. Use for waterfall display
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s(22000) and save in ss(184,22000) and savg(22000) for detecting
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sync vectors.
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4. At time Tdec, find sync vectors in ss(); get approx DF or list of DFs
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5. Do full-length FFT, NFFT1=96*nsps2, zero-padded as required.
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6. For each candidate signal, do inverse FFT of length 1536 (or 3072?).
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This yields 16 complex samples per symbol; sync tone should be
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close to zero frequency.
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7. Use afc65b method to get improved values of DF, DT.
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8. Tweak freq and time offset to 0.
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9. Compute 8-bin spectra of 69 data symbols: ssym(0:7,69). Re-order the
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bins to remove Gray code.
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10. Compute soft symbols for 206 bits (bit 207 is always 0).
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11. Remove interleaving
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12. Pack bits into bytes, send to Fano decoder
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13. If Fano succeeds, remove source encoding and display user message.
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