mirror of
https://github.com/saitohirga/WSJT-X.git
synced 2024-11-23 20:58:55 -05:00
2171b79a94
git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@2594 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
32 lines
1.7 KiB
Plaintext
32 lines
1.7 KiB
Plaintext
JT8 is a mode designed for amateur QSOs and beacon-like transmissions
|
|
at MF and LF. The mode uses the same 72-bit user messages as JT65,
|
|
augmented by a 12-bit cyclic redundancy check (CRC). Error-control
|
|
coding uses a convolutional code with constraint length K=16, rate
|
|
r=1/2, and a zero tail, leading to an encoded message length of
|
|
(72+12+15)*2 = 198 bits. Modulation is 8-FSK, so a transmission
|
|
requires 198/3 = 66 information-carrying channel symbols. Two 8x8
|
|
Costas arrays are added to each transmission for purposes of time and
|
|
frequency synchronization. A full transmission thus contains 66 + 16
|
|
= 82 symbols. Tone spacing df of the 8-FSK modulation is equal to the
|
|
keying rate; symbol duration tsym = 1/df, and the total occupied
|
|
bandwidth is 8*df. The actual transmission length TxT is slightly
|
|
less than the T/R sequence time, to allow for possible message
|
|
decoding before the next transmission starts.
|
|
|
|
Parameters of the five JT8 sub-modes are summarized in the following
|
|
table, along with S/N thresholds measured by simulation on an AWGN
|
|
(additive white Gaussian noise) channel. Parameter nsps is the number
|
|
of samples per symbol at sample rate 1500 Hz.
|
|
|
|
-----------------------------------------------------
|
|
Mode T/R TxT nsps df tsym BW S/N*
|
|
(m) (s) (Hz) (s) (Hz) (dB)
|
|
-----------------------------------------------------
|
|
JT8-1 1 52.5 960 1.562 0.64 12.6 -26.9
|
|
JT8-2 2 112.0 2048 0.732 1.37 5.9 -30.2
|
|
JT8-5 5 293.9 5376 0.279 3.58 2.2 -34.4
|
|
JT8-10 10 587.8 10752 0.140 7.17 1.1 -37.5
|
|
JT8-30 30 1791.3 32768 0.046 21.85 0.4 -42.3
|
|
-----------------------------------------------------
|
|
* Noise power measured in a 2500 Hz bandwidth.
|