This signal (reported here) is logged as FSK 4-tone with baudrate 150Bd and shift 4000Hz, or else CIS FTM-4 (Pic. 1). Looking in depth, they use only 4 of 24 combinations of the four tones (Pic. 2), so the encoded characters which are transmitted are reduced to 4. This mode is called Frequency-Time Matrix (FTM) or, in this case, FTM-4. The coding method is five-digit group of characters plus spaces. The four symbols are:
A) 1-2-3-4
B) 2-4-1-3
C) 4-3-2-1
D) 3-1-4-2
B) 2-4-1-3
C) 4-3-2-1
D) 3-1-4-2
Looking at the segment in Pic. 2, we could find the occurences of each symbol, for example "A" occurs 4 times, "B" occurs 3 times, "C" occurs 3 times and "D" occurs 3 times.
Since the baudrate of the signal, i.e. 150Bd, the real data-rate is 150/4 = 37.5 Baud.
Since the baudrate of the signal, i.e. 150Bd, the real data-rate is 150/4 = 37.5 Baud.
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| Pic. 1 |
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| Pic. 2 |
The repetitions of 4 symbols is clearly visible analyzing the demodulated bistream, unless some few errors due to the demodulation:
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| Pic. 3 |
The structure of the message seen has 48 bits period: some parts are well distinct
although it is unclear what they mean (sinc?, address?, commands?, ...)
although it is unclear what they mean (sinc?, address?, commands?, ...)
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| Pic. 4 |
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