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95 lines
4.3 KiB
Plaintext
95 lines
4.3 KiB
Plaintext
// Status=review
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//Needs work!
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JT9 is a mode designed for making minimal QSOs at LF, MF, and HF. It
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uses 72-bit structured messages nearly identical (at the user level)
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to those in JT65. Error control coding (ECC) uses a strong
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convolutional code with constraint length K=32, rate r=1/2, and a zero
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tail, leading to an encoded message length of (72+31) × 2 = 206
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information-carrying bits. Modulation is 9-FSK: eight tones are used
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for data, one for synchronization. Thus, three data bits or one
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synchronization bit are conveyed by each transmitted symbol. Sixteen
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symbol intervals are devoted to synchronization, so a transmission
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requires a total of 206 / 3 + 16 = 85 (rounded up) channel
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symbols. The sync symbols are those numbered 1, 2, 5, 10, 16, 23, 33,
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35, 51, 52, 55, 60, 66, 73, 83, and 85 in the transmitted sequence.
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Each symbol lasts for 6912 sample intervals at 12000 samples per
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second, or about 0.576 s. Tone spacing of the 9-FSK modulation is
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12000/6912 = 1.736 Hz, the inverse of the symbol duration. The total
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occupied bandwidth is 9 × 1.736 = 15.6 Hz. A generated JT9 signal has
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continuous phase and constant amplitude. There are no key clicks, and
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the transmitter's power amplifier need not be highly linear.
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.Transmitting
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Immediately before the start of a transmission WSJT-X encodes a
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user’s message and computes the sequence of tones to be sent. The
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transmitted audio waveform is computed on-the-fly, using 16-bit
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integer samples at a 48000 Hz rate. The digital samples are converted
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to an analog waveform in the sound card or equivalent D/A interface.
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.Receiving and Decoding
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WSJT-X acquires 16-bit integer samples from the sound card at a 48000
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Hz rate, and immediately downsamples the stream to 12000 Hz. Spectra
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from overlapping segments are computed for the waterfall display and
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saved at intervals of 0.188 s, half the JT9 symbol length. As shown
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in screen shots earlier in this guide, a JT9 signal appears in the
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*Cumulative* spectrum as a nearly rectangular shape about 16 Hz wide.
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Although there is no clearly visible “sync tone” like the one at the
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low-frequency edge of a JT65 signal, by convention the nominal
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frequency of a JT9 signal is taken to be that of the lowest tone, at
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the left edge of the spectrum.
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At the end of a reception sequence, about 50 seconds into the UTC
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minute, received data samples are forwarded to the decoder. For
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operator convenience the decoder goes through its full procedure
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twice: first at the selected Rx frequency, and then in the full
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displayed frequency range (or in JT9+JT65 mode, the displayed range
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above the blue *JT65 nnnn JT9* marker). Decoding of clean JT9 signals
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in a white-noise background starts to fail below signal-to-noise
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ratio –25 dB and reaches 50% copy at -26 dB.
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Each decoding pass can be described as a sequence of discrete blocks.
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The blocks are labeled here with the names of functional procedures in
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the code.
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sync9: Use sync symbols to find candidate JT9 signals
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in the specified frequency range
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Then, at the frequency of each plausible candidate:
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downsam9: Mix, filter and downsample to 16 complex
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samples/symbol
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peakdt9: Using sync symbols, time-align to start of JT9 symbol
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sequence
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afc9: Measure frequency offset and any possible drift
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twkfreq: Remove frequency offset and drift
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symspec2: Compute 8-bin spectra for 69 information-carrying
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symbols, using the time- and frequency-aligned data;
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transform to yield 206 single-bit soft symbols
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interleave9: Remove single-bit symbol interleaving imposed at the
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transmitter
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decode9: Retrieve a 72-bit user message using the sequential
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Fano algorithm for convolutional codes
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unpackmsg: Unpack a human-readable message from the 72-bit
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compressed format
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With marginal or unrecognizable signals the sequential Fano algorithm
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can take exponentially long times. If the +sync9+ step in the above
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sequence finds many seemingly worthy candidate signals, and if many of
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them turn out to be undecodable, the decoding loop can take a very
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long time. For this reason the step labeled +decode9+ is programmed
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to “time out” and report failure if it is taking too long. The
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choices *Fast | Normal | Deepest* on the Decode menu provide a
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three-step adjustment of this timeout limit.
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