mirror of https://github.com/saitohirga/WSJT-X.git
65 lines
2.8 KiB
Plaintext
65 lines
2.8 KiB
Plaintext
JTMSK: Possible New Mode for Meteor Scatter
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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 can be one of {5 7 9 11 13 17 19 23 29}; messages are
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padded with blanks up to the next available length from this list.
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Modulation is Minimum Shift Keying (MSK) at 2000 baud. This is a form
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of constant-envelope, continuous-phase modulation, equivalent to FSK
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with offset equal to half the bit rate. JTMSK sends a "0" bit with
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0.5 ms of a sine wave at f0=1000 Hz, and a "1" bit at f1=2000 Hz.
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Carrier phase increment over one bit is therefore 0.5 cycles (pi
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radians) at f0 and 1.0 cycles (two*pi radians) at f1. With odd parity
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there is always an odd number of 1's and even number of 0's in a
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character, so the phase increment over a full character is always an
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integer number of cycles.
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2. Receiving
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a. Pings are detected (or mouse-picked data is selected) as in
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WSJT9, except that it's done in real time rather than at the end
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of an Rx interval.
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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, and convert to analytic
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time-domain signal. (analytic)
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c. Square the complex signal, cx2=cx*cx, and compute FFT. Look for
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narrow carriers at frequency 2000 + 2*DF +/- 2*Tol and 2000 Hz
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higher. (msdf)
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d. If carrier is found, measure frequency f and phase phi. Multiply
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cx by exp(-twopi*i*(f-1500)*t - phi) to recover the
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standard-frequency baseband signal. (tweak1)
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e. Establish symbol and character sync by cross-correlating with
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conjg(cwb), where cwb is the MSK waveform for the <space>
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character.
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f. Find message length by computing ACF (of what? cdat? soft
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symbol values?)
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g. Cross-correlate the complex standard-frequency signal against
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conjugated templates for each character in the JTMSK alphabet.
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Best match yields the decoded character.
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h. 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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i. If msglen is established and long enough, try folding the data and
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determining best-fit characters as above.
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3. Comparison of modes:
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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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Characters/s | 147 197 286 | 14.3 32.3
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Bandwidth (Hz) | 1764 1378 2000 | 947 1809
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