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81 lines
3.2 KiB
Plaintext
81 lines
3.2 KiB
Plaintext
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// 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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audio waveform is computed on-the-fly, with 16-bit integer samples
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sent to the audio output device at a 48000 Hz rate. Generated JT65
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and JT9 signals have continuous phase and constant amplitude, and
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there are no key clicks. The transmitter's power amplifier need not
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be highly linear.
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.Receiving
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_WSJT-X_ acquires 16-bit integer samples from the audio input device
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at a 48000 Hz rate and immediately downsamples the stream to 12000 Hz.
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Spectra from overlapping segments are computed for the waterfall
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display and saved for the decoder at intervals of 3456/12000 = 0.288
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s, half the JT9 symbol length.
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.Decoding
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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. The functional blocks are different for
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the JT65 and JT9 modes. In dual-mode JT9+JT65 operation on computers
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with more than one CPU, decoding computations for the two modes are
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done in parallel.
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The basic decoding algorithm for JT65 mode is described in the 2005
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{jt65protocol} paper. The following list summarizes the corresponding
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algorithm for JT9 mode. Blocks are labeled here 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 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 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
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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 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 many of
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them turn out to be undecodable, the decoding loop can take an
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inconveniently long time. For this reason the step labeled +decode9+
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is programmed to ``time out'' and report failure for a given signal if
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it takes too long. The choices *Fast | Normal | Deepest* on the
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*Decode* menu provide the user with a three-step adjustment of the
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timeout limit.
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