28 August 2020

CIS Navy VLF 50Bd/75 FSK (T600 75Hz)

50Bd/75 FSK is the T600 waveform variant utilized by Russian Navy on 18.1 kHz, commonly for submarine communications.

Fig. 1 - 50Bd/75 waveform
As seen in CIS 36-50, frames are constructed from data blocks consisting of 7-bit words: a packet of payload data of basically arbitrary length is surrounded by a start and an end sequence (EOM). Sometimes blocks of data already transmitted are observed to be repeated, verifying the contents by the recipient can be performed easily this way. Idle sequences of reversals, i.e. strictly alternating sequences of "0"s and "1"s, of 36 Bd and 50 Bd are used to introduce and to terminate a transmission or also (50 Bd only) to separate data blocks.
A data block itself consists of three sections: start sequence, (encrypted) payload data, end of message (EOM) sequence; all payload data has a fixed ratio of '1's vs. '0's of 4 to 3 or vice versa, depending on polarity of reception (Figure 2). 

Fig. 2 - 4/3 ratio test on a five-message transmission (s1 and s2 data blocks are the same)
After a single "1" bit (following the last "01" reversal pair), the 28-bit (four-words) start sequence 1010111 1101011 0100000 1010000 is sent: the 4/3 ratio is not followed here to make the start sequence distinguishable from the actual data. Payload comes next; its first part are two equal words 0101101 0101101 followed by a repeated ten-word (7 bits each) group: likely the Message Indicator or the session key. All subsequent data (arbitrary length) do not obey a special regularity anymore. The end sequence shows five equal 7-bit words (5 x 0001000), again disregarding the 4/3 ratio of the data section (Figure 3).
Fig. 3 - structure of a data block

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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