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51 lines
2.9 KiB
Plaintext
51 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 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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