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138 lines
6.4 KiB
Plaintext
138 lines
6.4 KiB
Plaintext
[[JT65PRO]]
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=== JT65
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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 tone intervals or "`symbols`" of length 4096/11025 =
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0.372 s. Within each interval the waveform is a constant-amplitude
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sinusoid 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
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normally sent in each interval having a “1” in the following
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pseudo-random 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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[[JT4PRO]]
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=== JT4
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JT4 uses 72-bit structured messages nearly identical to those in
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JT65. Error control coding (ECC) uses a strong convolutional code with
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constraint length K=32, rate r=1/2, and a zero tail, leading to an
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encoded message length of (72+31) x 2 = 206 information-carrying
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bits. Modulation is 4-tone frequency-shift keying at 11025 / 2520 =
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4.375 baud. Each symbol carries one information bit (the most
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significant bit) and ony synchronizing bit (the least signicifant
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bit). The pseudo-random sync vector is the following sequence:
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000011000110110010100000001100000000000010110110101111101000
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100100111110001010001111011001000110101010101111101010110101
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011100101101111000011011000111011101110010001101100100011111
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10011000011000101101111010
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[[JT9PRO]]
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=== JT9
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JT9 is designed for making minimally valid QSOs at LF, MF, and HF. It
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uses 72-bit structured messages nearly identical (at the user level)
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to those in JT65. Error control coding (ECC) uses a strong
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convolutional code with constraint length K=32, rate r=1/2, and a zero
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tail, leading to an encoded message length of (72+31) × 2 = 206
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information-carrying bits. Modulation is nine-tone frequency-shift
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keying, 9-FSK. Eight tones are used for data, one for
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synchronization. Eight data tones means that three data bits are
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conveyed by each transmitted information symbol. Sixteen symbol
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intervals are devoted to synchronization, so a transmission requires a
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total of 206 / 3 + 16 = 85 (rounded up) channel symbols. The sync
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symbols are those numbered 1, 2, 5, 10, 16, 23, 33, 35, 51, 52, 55,
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60, 66, 73, 83, and 85 in the transmitted sequence. Each symbol lasts
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for 6912 sample intervals at 12000 samples per second, or about 0.576
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seconds. Tone spacing of the 9-FSK modulation is 12000/6912 = 1.736
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Hz, the inverse of the symbol duration. The total occupied bandwidth
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is 9 × 1.736 = 15.6 Hz.
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[[PROTOCOL_SUMMARY]]
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=== Summary
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Frequency spacing between tones, total occupied bandwidth, and
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approximate decoding thresholds are given for the various submodes of
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JT4, JT9, and JT65 in the following table:
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Submode Spacing BW S/N
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(Hz) (Hz) dB
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----------------------------
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JT4A 4.375 17.5 -23
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JT4B 8.75 35.0 -22
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JT4C 17.5 70.0 -21
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JT4D 39.375 157.5 -20
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JT4E 78.75 315.0 -19
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JT4F 157.5 630.0 -18
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JT4G 315.0 1260.0 -17
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JT9 1.7361 15.625 -27
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JT65A 2.6917 177.6 -25
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JT65B 5.3833 355.3 -24
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JT65C 10.767 710.6 -23
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Transmissions in all three modes are essentially the same length, and
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all use 72 bits to carry message information. At user level the modes
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support nearly identical message structures.
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JT4 and JT65 signal reports are constrained to the range –1 to –30
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dB. This range is more than adequate for EME purposes, but not enough
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for optimum use at HF. S/N values displayed by the JT4 and JT65
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decoders are clamped at an upper limit –1 dB, and the S/N scale is
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nonlinear above –10 dB.
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By comparison, JT9 allows for signal reports in the range –50 to +49
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dB. It manages this by taking over a small portion of "`message
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space`" that would otherwise be used for grid locators within 1 degree
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of the south pole. The S/N scale of the present JT9 decoder is
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reasonably linear (although it's not intended to be a precision
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measurement tool).
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JT9 is an order of magnitude better than JT65 in spectral
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efficiency. On a busy HF band, the conventional 2-kHz-wide JT65
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sub-band is often filled with overlapping signals. Ten times as many
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JT9 signals can fit into the same frequency range, without collisions.
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