mirror of https://github.com/saitohirga/WSJT-X.git
76 lines
3.0 KiB
Plaintext
76 lines
3.0 KiB
Plaintext
// Status=review
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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 then computed on-the-fly, using 16-bit
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integer samples at a 48000 Hz rate. Digital samples are converted to
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an analog waveform in the sound card or equivalent D/A interface.
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.Receiving
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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.
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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 over the full
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displayed frequency range. Each decoding pass can be described as a
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sequence of discrete blocks. These functional blocks are different
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for the JT65 and JT9 modes.
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.Decoding
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The basic decoding algorithm for JT65 mode was described in the
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{jt65protocol} paper. The following list summarizes the corresponding
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algorithm for JT9 mode, with blocks labeled with the names of
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functional procedures in the code.
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[horizontal]
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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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[horizontal]
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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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Decoding of clean JT9 signals in a white-noise background starts to
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fail below signal-to-noise ratio -25 dB and reaches 50% copy at -26
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dB.
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With marginal or unrecognizable signals the sequential decoding
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algorithm can take exponentially long times. If the +sync9+ step in
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the above sequence finds many seemingly worthy candidate signals, and
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if many of them turn out to be undecodable the decoding loop can take
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an inconveniently long time. For this reason the step labeled
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+decode9+ is programmed to ``time out'' and report failure if it is
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taking too long. The choices *Fast | Normal | Deepest* on the Decode
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menu provide the user with a three-step adjustment of this timeout
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limit.
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