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52 lines
2.9 KiB
Plaintext
52 lines
2.9 KiB
Plaintext
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// 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 in just 71
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information bits. A 72^nd^ bit serves as a flag to indicate that a
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message consists of arbitrary text (up to 13 characters) instead of
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callsigns and a grid locator. Special formats allow other information
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such as add-on callsign prefixes (e.g., ZA/K1ABC) or numerical signal
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reports (in dB) to be substituted for the grid locator. The basic aim
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is to compress the most common messages used for minimally valid QSOs
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into a 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
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126 contiguous time intervals or symbols of length 4096/11025 = 0.372
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s. Within each interval the waveform is a constant-amplitude sinusoid
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at one of 65 pre-defined frequencies. Frequency steps between
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intervals are accomplished in a phase-continuous manner. Half of the
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channel symbols are devoted to a pseudo-random synchronizing vector
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interleaved with the encoded information symbols. The sync vector
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allows calibration of time and frequency offsets between transmitter
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and receiver. A transmission nominally begins at t = 1 s after the
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start of a UTC minute and finishes at t = 47.8 seconds. The
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synchronizing tone is at 11025 × 472/4096 = 1270.46 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 11025 × 472/4096 + 11025/4096 × (N+2) × m, where N is the
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value of the six-bit symbol, 0 ≤ N ≤ 63, and m is 1, 2, or 4 for JT65
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sub-modes A, B, or C. Sub-mode JT65A is always used at HF.
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For EME (but, conventionally, not on the HF bands) the signal report
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OOO is sometimes used instead of numerical signal reports. It is
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conveyed by reversing sync and data positions in the transmitted
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sequence. Shorthand messages for RO, RRR, and 73 dispense with the
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sync vector entirely and use time intervals of 16384/11025 = 1.486 s
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for pairs of alternating tones. The lower frequency is always 1270.46
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Hz, the same as that of the sync tone, and the frequency separation is
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110250/4096 = 26.92 Hz multiplied by n × m, with n = 2, 3, 4 for the
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messages RO, RRR, and 73.
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