mirror of
https://github.com/saitohirga/WSJT-X.git
synced 2024-11-18 18:12:12 -05:00
118ad4e1b7
git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@5434 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
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.
|