Thanks to my friend Mario, who recently sent me some recordings, I had the opportunity to study the Russian T-219 system, codenamed "Yachta" (Russian: ЯХТА). Yachta is a Russian analog voice scrambler featuring a Frequency Shift Keying (FSK) signal transmitted in the center of the spectrum, with the encrypted voice stream split above and below the FSK signal (Figure 1). Although dating back to the Cold War era, the system is still used for tactical communication in the combat field as recordings are only a few days old and heard in the lower VHF range (just above the upper HF limit). The stream consists of unequal time segments, within which the two voice subchannels are swapped and inverted.
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| Fig. 1 - T-219 "Yachta" signal |
The FSK signal is used as a synchronization sequence and is transmitted at a rate of 100 bps with a 150 Hz shift (Figure 2).
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| Fig. 2 - T-219 FSK parameters |
As shown in Figure 3, after FSK demodulation the resulting synchronization bitstream is an M-sequence based on the irreducible polynomial x^52+x^49+1.
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| Fig. 3 - the M-sequence based on the irreducible polynomial x^52+x^49+1 |
It turned out that during the formation of the FSK signal the pahses of the two frequencies are preserved after each "shift" (Figure 4 shows two periods): that suggests that it's formed by switching (mechanically or electronically) two independent F1 F2 frequency generators which bear some inter-relationships or by using a VCO system.
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| Fig. 4 - two periods of the FSK frequencies |
By the way, looking at the durations of two periods:
F1 = (2:0.001285470) = 1555,851167277338
F2 = (2:0,001422470) = 1406,005047558121
the shift is just about 150 Hz
