mirror of
https://github.com/saitohirga/WSJT-X.git
synced 2025-10-31 13:10:19 -04:00
52 lines
2.9 KiB
Plaintext
52 lines
2.9 KiB
Plaintext
|
// 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 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 time 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.
|