WSJT-X/doc/user_guide/protocols.adoc

[[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.