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These documentation source files are not the one true version, just a copy for testing purposes. DO NOT EDIT THESE FILES. To use this on Windows you will need a working asciidoc installation and the path to it must be included in your CMAKE_PREFIX_PATH (probably via a local CMake tool chain file). At the time of writing the official asciidoc package does not work on Windows. The latest development master does however work, it can be downloaded as a snapshot ZIP archive from here: https://github.com/asciidoc/asciidoc/archive/master.zip git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@5316 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
51 lines
2.9 KiB
Plaintext
51 lines
2.9 KiB
Plaintext
// Status=review
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JT65 was designed for making minimal QSOs via EME (``moon-bounce'') on
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the VHF and UHF bands. A detailed description of the protocol and its
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implementation in program _WSJT_ was published in {jt65protocol} for
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September-October, 2005. Briefly stated, JT65 uses 60 s T/R sequences
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and carefully structured messages. Standard messages are compressed so
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that two callsigns and a grid locator can be transmitted with just 71
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bits. A 72nd bit serves as a flag to indicate that the message
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consists of arbitrary text (up to 13 characters) instead of callsigns
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and a grid locator. Special formats allow other information such as
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add-on callsign prefixes (e.g., ZA/K1ABC) or numerical signal reports
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(in dB) to be substituted for the grid locator. The basic aim is to
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compress the common messages used for minimally valid QSOs into a
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minimum fixed number of bits. After compression, a Reed Solomon
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(63,12) error-control code converts 72-bit user messages into
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sequences of 63 six-bit channel symbols.
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JT65 requires tight synchronization of time and frequency between
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transmitting and receiving stations. Each transmission is divided into 126
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contiguous time intervals or symbols, each of length 4096/11025 =
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0.372 seconds. Within each interval the waveform is a constant-amplitude
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sinusoid at one of 65 pre-defined frequencies. Frequency steps
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between intervals are accomplished in a phase-continuous manner. Half
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of the channel symbols are devoted to a pseudo-random synchronizing
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vector interleaved with the encoded information symbols. The sync
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vector allows calibration of time and frequency offsets between
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transmitter and receiver. A transmission nominally begins at t = 1 s
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after the start of a UTC minute and finishes at t = 47.8 seconds. The
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synchronizing tone is at 11025 × 472/4096 = 1270.5 Hz, and is normally
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sent in each interval having a “1” in the following pseudo-random
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sequence:
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100110001111110101000101100100011100111101101111000110101011001
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101010100100000011000000011010010110101010011001001000011111111
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Encoded user information is transmitted during the 63 intervals not
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used for the sync tone. Each channel symbol generates a tone at
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frequency 1275.8 + 2.6917 × N × m Hz, where N is the value of the
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six-bit symbol, 0 ≤ N ≤ 63, and m is 1, 2, or 4 for JT65 sub-modes A,
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B, or C. JT65A is the submode always used at HF.
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For EME (but, conventionally, not on the HF bands) the signal report OOO
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is sometimes used instead of numerical signal reports. It is conveyed
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by reversing sync and data positions in the transmitted sequence.
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Shorthand messages for RO, RRR, and 73 dispense with the sync vector
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entirely and use time intervals of 1.486 s (16,384 samples) for pairs
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of alternating tones. The lower frequency is always 1270.5 Hz, the
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same as that of the sync tone, and the frequency separation is 26.92 ×
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n × m Hz with n = 2, 3, 4 for the messages RO, RRR, and 73.
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