WSJT-X/doc/source/jt65-protocol.adoc

// Status=review

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 with just
71 bits.  A 72nd bit serves as a flag to indicate that the 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 aim of source
encoding is to compress the common messages used for minimal 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
transmitter and receiver.  Each transmission is divided into 126
contiguous time intervals or symbols, each 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 s.  The
synchronizing tone is at 11025 × 472/4096 = 1270.5 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 1275.8 + 2.6917 Nm Hz, 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.
For EME (but conventionally not on the HF bands) the signal report
OOO is conveyed by reversing sync and data positions in the
transmitted sequence.  Shorthand messages dispense with the sync
vector and use intervals of 1.486 s (16,384 samples) for the
alternating tones.  The lower frequency is always 1270.5 Hz, the same
as that of the sync tone, and the frequency separation is 26.92 nm Hz
with n = 2, 3, 4 for the messages RO, RRR, and 73.