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 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 | +--------------------+-----------------------+-------------------+