50Bd/75 FSK is the T600 waveform variant utilized by Russian Navy on 18.1 kHz, commonly for submarine communications.
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| Fig. 1 - 50Bd/75 waveform |
As seen in CIS 36-50, frames are constructed from data blocks consisting of 7-bit words with a fixed ratio of '1's vs. '0's of 4 to 3 or vice versa, depending on polarity of reception (Figure 2).
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| Fig. 2 - 4/3 ratio test on a five-message transmission (s1 and s2 data blocks are the same) |
Multiple Russian military naval communication stations share this frequency (18.10 KHz) and the call sign "RDL". The 24h transmission schedule has frequent flash-override messages in A1A Morse, FSK-Morse and T600 75 Hz, as shown in the lower image of Figure 1.
https://yadi.sk/d/0M7f_H_WOD9LTA (T600-75 bitstream)
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