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