mirror of
https://github.com/saitohirga/WSJT-X.git
synced 2024-11-09 02:26:06 -05:00
43664a2e01
git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@3669 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
51 lines
2.8 KiB
Plaintext
51 lines
2.8 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 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 × N × m 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 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 1.486 s (16,384 samples) for pairs
|
||
of 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 ×
|
||
n × m Hz with n = 2, 3, 4 for the messages RO, RRR, and 73.
|