mirror of
https://github.com/saitohirga/WSJT-X.git
synced 2024-11-18 01:52:05 -05:00
357f467ef0
git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/jtms3@2529 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
73 lines
3.3 KiB
Plaintext
73 lines
3.3 KiB
Plaintext
JTMSK: Possible New Mode for Meteor Scatter
|
|
-------------------------------------------
|
|
|
|
JTMSK is a digital mode designed for amateur meteor-scatter
|
|
communication. The "MSK" part of the name could mean "Meteor Scatter
|
|
King": the mode works at the same signal levels as the popular mode
|
|
FSK441, but at about twice the character transmission rate, so it
|
|
makes much better use of short pings. MSK also means "Minimum Shift
|
|
Keying", which is the modulation technique used in JTMSK. It is a
|
|
form of continuous-phase frequency-shift keying that achieves high
|
|
transmission rate with a narrow bandwidth, while maintaining a
|
|
constant-envelope waveform.
|
|
|
|
1. Transmitting
|
|
|
|
Messages are sent character-by character, 6 bits plus odd parity.
|
|
Message length in characters is automatically selected from among the
|
|
possibilities 5 7 9 11 13 17 19 23 29. (Note that these lengths share
|
|
no common factor.) Messages are padded with blanks when required, up
|
|
to the next available length.
|
|
|
|
|
|
Modulation is Minimum Shift Keying (MSK) at 2000 baud. A "0" bit is
|
|
sent as an audio sine wave at f0=1000 Hz, lasting 0.5 ms; a "1" bit is
|
|
a sine wave at f1=2000 Hz. The carrier phase increment over one bit
|
|
is thus 0.5 cycles (pi radians) at f0, or 1.0 cycles at f1. Odd
|
|
parity assures an odd number of 1's and even number of 0's in every
|
|
7-bit character, so the phase increment over a full character is
|
|
always an integer number of cycles.
|
|
|
|
2. Receiving
|
|
|
|
a. Detect pings and update display in real time. (rtping)
|
|
|
|
b. Compute real-to-complex FFT. Zap birdies, remove frequency
|
|
components outside the range 300 - 2700 Hz, convert to analytic
|
|
time-domain signal. (analytic)
|
|
|
|
c. Square the complex signal and compute FFT. Look for narrow
|
|
spectral lines at frequency 2000 + 2*(DF +/- Tol) Hz and
|
|
4000 + 2*(DF +/- Tol) Hz. (msdf)
|
|
|
|
d. If a signal is found, measure frequency offset DF and phase Dphi.
|
|
(DF and Dphi are measured relative to the original frequencies f0
|
|
and f1, not their doubled values.) Multiply cx by
|
|
exp(-twopi*i*DF*t - Dphi) to recover the standard-frequency
|
|
baseband signal. (tweak1)
|
|
|
|
e. Establish symbol and character sync by cross-correlating with
|
|
conjg(cwb), MSK waveform for the <space> character.
|
|
|
|
f. Find message length by computing ACF.
|
|
|
|
g. Decode the message by cross-correlating character-length segments
|
|
of cdat against complex waveforms for each possible character.
|
|
|
|
h. If ping is long enough, try folding the data and determining the
|
|
best-fit characters as above.
|
|
|
|
3. Parameters of fast modes:
|
|
|
|
+--------------------+-----------------------+-------------------+
|
|
| | Meteor Scatter | IonoScatter |
|
|
| | FSK441 JTMS JTMSK | JT6M ISCAT |
|
|
+--------------------+-----------------------+-------------------|
|
|
| T/R period (s) | 30 30 30 | 30 30 |
|
|
| Modulation type | 4-FSK MSK MSK | 44-FSK 41-FSK |
|
|
| Keying rate (baud) | 441 1378 2000 | 21.5 43.1 |
|
|
| Bit rate (bps) | 882 1378 2000 | - - |
|
|
| Characters/s | 147 197 286 | 14.3 32.3 |
|
|
| Bandwidth (Hz) | 1764 1378 2000 | 947 1809 |
|
|
+--------------------+-----------------------+-------------------+
|