[[JT65PRO]] === JT65 JT65 was designed for making minimal QSOs via EME ("`moon-bounce`") on the VHF and UHF bands. A detailed description of the protocol and its implementation in program _WSJT_ was published in {jt65protocol} for September-October, 2005. Briefly stated, JT65 uses 60 s T/R sequences and carefully structured messages. Standard messages are compressed so that two callsigns and a grid locator can be transmitted in just 71 information bits. A 72^nd^ bit serves as a flag to indicate that a message consists of arbitrary text (up to 13 characters) instead of callsigns and a grid locator. Special formats allow other information such as add-on callsign prefixes (e.g., ZA/K1ABC) or numerical signal reports (in dB) to be substituted for the grid locator. The basic aim is to compress the most common messages used for minimally valid QSOs into a minimum fixed number of bits. After compression, a Reed Solomon (63,12) error-control code converts 72-bit user messages into sequences of 63 six-bit channel symbols. JT65 requires tight synchronization of time and frequency between transmitting and receiving stations. Each transmission is divided into 126 contiguous tone intervals or "`symbols`" of length 4096/11025 = 0.372 s. Within each interval the waveform is a constant-amplitude sinusoid at one of 65 pre-defined frequencies. Frequency steps between intervals are accomplished in a phase-continuous manner. Half of the channel symbols are devoted to a pseudo-random synchronizing vector interleaved with the encoded information symbols. The sync vector allows calibration of time and frequency offsets between transmitter and receiver. A transmission nominally begins at t = 1 s after the start of a UTC minute and finishes at t = 47.8 seconds. The synchronizing tone is at 11025 × 472/4096 = 1270.46 Hz, and is normally sent in each interval having a “1” in the following pseudo-random sequence: 100110001111110101000101100100011100111101101111000110101011001 101010100100000011000000011010010110101010011001001000011111111 Encoded user information is transmitted during the 63 intervals not used for the sync tone. Each channel symbol generates a tone at frequency 11025 × 472/4096 + 11025/4096 × (N+2) × m, where N is the value of the six-bit symbol, 0 ≤ N ≤ 63, and m is 1, 2, or 4 for JT65 sub-modes A, B, or C. Sub-mode JT65A is always used at HF. For EME (but, conventionally, not on the HF bands) the signal report OOO is sometimes used instead of numerical signal reports. It is conveyed by reversing sync and data positions in the transmitted sequence. Shorthand messages for RO, RRR, and 73 dispense with the sync vector entirely and use time intervals of 16384/11025 = 1.486 s for pairs of alternating tones. The lower frequency is always 1270.46 Hz, the same as that of the sync tone, and the frequency separation is 110250/4096 = 26.92 Hz multiplied by n × m, with n = 2, 3, 4 for the messages RO, RRR, and 73. [[JT4PRO]] === JT4 JT4 uses 72-bit structured messages nearly identical to those in JT65. Error control coding (ECC) uses a strong convolutional code with constraint length K=32, rate r=1/2, and a zero tail, leading to an encoded message length of (72+31) x 2 = 206 information-carrying bits. Modulation is 4-tone frequency-shift keying at 11025 / 2520 = 4.375 baud. Each symbol carries one information bit (the most significant bit) and ony synchronizing bit (the least signicifant bit). The pseudo-random sync vector is the following sequence: 000011000110110010100000001100000000000010110110101111101000 100100111110001010001111011001000110101010101111101010110101 011100101101111000011011000111011101110010001101100100011111 10011000011000101101111010 [[JT9PRO]] === JT9 JT9 is designed for making minimally valid QSOs at LF, MF, and HF. It uses 72-bit structured messages nearly identical (at the user level) to those in JT65. Error control coding (ECC) uses a strong convolutional code with constraint length K=32, rate r=1/2, and a zero tail, leading to an encoded message length of (72+31) × 2 = 206 information-carrying bits. Modulation is nine-tone frequency-shift keying, 9-FSK. Eight tones are used for data, one for synchronization. Eight data tones means that three data bits are conveyed by each transmitted information symbol. Sixteen symbol intervals are devoted to synchronization, so a transmission requires a total of 206 / 3 + 16 = 85 (rounded up) channel symbols. The sync symbols are those numbered 1, 2, 5, 10, 16, 23, 33, 35, 51, 52, 55, 60, 66, 73, 83, and 85 in the transmitted sequence. Each symbol lasts for 6912 sample intervals at 12000 samples per second, or about 0.576 seconds. Tone spacing of the 9-FSK modulation is 12000/6912 = 1.736 Hz, the inverse of the symbol duration. The total occupied bandwidth is 9 × 1.736 = 15.6 Hz. [[PROTOCOL_SUMMARY]] === Summary Frequency spacing between tones, total occupied bandwidth, and approximate decoding thresholds are given for the various submodes of JT4, JT9, and JT65 in the following table: Submode Spacing BW S/N (Hz) (Hz) dB ---------------------------- JT4A 4.375 17.5 -23 JT4B 8.75 35.0 -22 JT4C 17.5 70.0 -21 JT4D 39.375 157.5 -20 JT4E 78.75 315.0 -19 JT4F 157.5 630.0 -18 JT4G 315.0 1260.0 -17 JT9 1.7361 15.625 -27 JT65A 2.6917 177.6 -25 JT65B 5.3833 355.3 -24 JT65C 10.767 710.6 -23 Transmissions in all three modes are essentially the same length, and all use 72 bits to carry message information. At user level the modes support nearly identical message structures. JT4 and JT65 signal reports are constrained to the range –1 to –30 dB. This range is more than adequate for EME purposes, but not enough for optimum use at HF. S/N values displayed by the JT4 and JT65 decoders are clamped at an upper limit –1 dB, and the S/N scale is nonlinear above –10 dB. By comparison, JT9 allows for signal reports in the range –50 to +49 dB. It manages this by taking over a small portion of "`message space`" that would otherwise be used for grid locators within 1 degree of the south pole. The S/N scale of the present JT9 decoder is reasonably linear (although it's not intended to be a precision measurement tool). JT9 is an order of magnitude better than JT65 in spectral efficiency. On a busy HF band, the conventional 2-kHz-wide JT65 sub-band is often filled with overlapping signals. Ten times as many JT9 signals can fit into the same frequency range, without collisions.