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275 lines
13 KiB
Plaintext
275 lines
13 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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[[QRA64_PROTOCOL]]
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=== QRA64
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TBD
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[[PROTOCOL_SUMMARY]]
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=== Comparison of Slow Modes
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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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QRA64A 1.736 111.1 -28?
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QRA64B 3.472 222.2
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QRA64C 6.944 444.4
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QRA64D 13.889 888.9
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QRA64E 27.228 1777.8
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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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=== ISCAT
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ISCAT messages are free-form, up to 28 characters in length.
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Modulation is 42-tone frequency-shift keying at 11025 / 512 = 21.533
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baud (ISCAT-A), or 11025 / 256 = 43.066 baud (ISCAT-B). Tone
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frequencies are spaced by an amount in Hz equal to the baud rate. The
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available character set is
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----
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0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ /.?@-
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----
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Transmissions consist of sequences of 24 symbols: a synchronizing
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pattern of four symbols at tone numbers 0, 1, 3, and 2, followed by
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two symbols with tone number corresponding to (message length) and
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(message length + 5), and finally 18 symbols conveying the user's
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message, sent repeatedly character by character. The message always
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starts with +@+, the beginning-of-message symbol, which is not
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displayed to the user. The sync pattern and message-length indicator
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have a fixed repetition period, recurring every 24 symbols. Message
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information occurs periodically within the 18 symbol positions set
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aside for its use, repeating at its own natural length.
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For example, consider the user message +CQ WA9XYZ+. Including the
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beginning-of-message symbol +@+, the message is 10 characters long.
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Using the character sequence displayed above to indicate tone numbers,
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the transmitted message will therefore start out as shown in the first
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line below:
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----
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0132AF@CQ WA9XYZ@CQ WA9X0132AFYZ@CQ WA9XYZ@CQ W0132AFA9X ...
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sync## sync## sync##
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----
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Note that the first six symbols (four for sync, two for message
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length) repeat every 24 symbols. Within the 18 information-carrying
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symbols in each 24, the user message +@CQ WA9XYZ+ repeats at its own
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natural length, 10 characters. The resulting sequence is extended as
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many times as will fit into a Tx sequence.
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=== MSK144
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(this section needs work ...)
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MSK144 is intended for meteor-scatter QSOs on the VHF bands. Standard
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messages are structured in the same way as those in the slow modes,
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with a 72 bits of user information. Forward error correction is
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implemented by first augmenting the 72 message bits with an 8-bit CRC
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calculated from the message bits. The CRC is used to detect and
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eliminate most false decodes at the receiver. The resulting 80-bit
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augmented message is then mapped to a 128-bit codeword using a
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(128,80) binary low-density-parity-check (LDPC) code designed
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specifically for this purpose. Two 8-bit synchronizing sequences are
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added to make a message frame 144 bits long. Modulation is Offset
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Quadrature Phase-Shift Keying (OQPSK) at 2000 baud. Even-numbered bits
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are conveted over the in-phase channel, odd-numbered bits on the
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quadrature channel. Individual symbols are shaped with half-sine
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profiles, thereby ensuring a generated waveform with constant
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envelope, equivelent to a Minimum Shift Keying (MSK) waveform. Frame
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duration is 72 ms so the effective character transmission rate for
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standard messages is as high as 250 cps.
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MSK144 also supports short-form messages that can be used after QSO
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partners have exchanged callsigns. These consist of 4 bits that
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encode a signal report, R+report, RRR, or 73, together with a 12-bit
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hash code based on the ordered pair of callsigns is use. A specially
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designed LDPC (32,16) code provides error-correction, and an 8-bit
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synchronizing vector is appended to make up a 40-bit frame.
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Short-message duration is thus 20 ms, and short messages can be
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conveyed by very short meteor "pings".
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As in the other fast modes in WSJT-X, the 72 ms or 20 ms frames of
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MSK144 messages are repeated without gaps for the full duration of a
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transmission cycle. For most purposes, a cycle duration of 15s is
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recommended for MSK144.
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The modulated MSK144 signal occupies the full bandwidth of a SSB
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transmitter, so transmissions are always centered on audio frequency
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1500 Hz. For best results, transmitter and receiver filters should be
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adjusted to provide the flattest possible response over the range
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300Hz to 2700Hz. Further, the maximum permissible frequency offset
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between you and your QSO partner should be ± 100 Hz.
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Details:
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Standard 72ms MSK frames contain 144 bits and consist of a standard
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JT-mode 72-bit message augmented with 56 bits for error detection and
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correction. The 72+56=128-bit codeword is combined with two 8-bit sync
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words to form a 144-bit frame. The frame is constructed as
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S8,D48,S8,D80, where S8 represents an 8-bit sync word and D48,D80
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represent the first 48 bits and last 80 bits of the 128-bit codeword,
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respectively. (At present, the 128-bit codeword is re-ordered to put
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even/odd bits at the beginning/end of the codeword — this is a
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holdover from JTMSK and is probably not necessary.) The 144-bit frame
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is repeated for the duration of a transmission cycle.
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=== Summary
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[[SLOW_TAB]]
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.Parameters of Slow Modes
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[width="90%",cols="3h,^3,^2,^1,^2,^2,^2,^2,^2,^2",frame=topbot,options="header"]
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|===============================================================================
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|Mode |FEC Type |(k,n) | Q| Mod | Baud |BW (Hz)|fSync|TxT (s)|S/N (dB)
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|JT4A |K=32, r=1/2|(206,72)| 2| 4-FSK| 4.375| 17.5 | 0.50| 47.1 | -23
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|JT9A |K=13, r=1/2|(206,72)| 8| 9-FSK| 1.736| 15.6 | 0.19| 49.0 | -27
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|JT65A |RS |(63,12) |64|65-FSK| 2.692| 177.6 | 0.50| 46.8 | -25
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|QRA64A|QRA |(63,12) |64|64-FSK| 1.736| 111.1 | 0.25| 48.4 | -28
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| WSPR |K=32, r=1/2|(162,50)| 2| 4-FSK| 1.465| 5.9 | 0.50|110.6 | -29
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|===============================================================================
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.Parameters of Fast Modes
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[width="90%",cols="3h,^3,^2,^1,^2,^2,^2,^2,^2,^2",frame="topbot",options="header"]
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|=============================================================================
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|Mode |FEC Type |(k,n) | Q| Mod | Baud |BW (Hz)|fSync|TxT (s)|S/N (dB)
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|ISCAT-A | - | - |42|42-FSK| 21.5 | 905 | 0.17| 1.176 |
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|ISCAT-B | - | - |42|42-FSK| 43.1 | 1809 | 0.17| 0.588 |
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|JT9E |K=32, r=1/2|(206,72)| 8| 9-FSK| 25.0 | 225 | 0.19| 3.400 |
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|JT9F |K=32, r=1/2|(206,72)| 8| 9-FSK| 50.0 | 450 | 0.19| 1.700 |
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|JT9G |K=32, r=1/2|(206,72)| 8| 9-FSK|100.0 | 900 | 0.19| 0.850 |
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|JT9H |K=32, r=1/2|(206,72)| 8| 9-FSK|200.0 | 1800 | 0.19| 0.425 |
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|MSK144 |LDPC |(128,72)| 2| OQPSK| 2000 | 2000 | 0.11| 0.072 | -5
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|MSK144 Sh|LDPC |(32,16) | 2| OQPSK| 2000 | 2000 | 0.20| 0.020 | -5
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|=============================================================================
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