Thanks to my friend Mario, who late sent me any recordings, I had the chance to survey the Russian T-219 system, codenamed "Yachta" (Russian: ЯХТА). Yachta is simply a Russian analog voice scrambler featuring a Frequency Shift Keying (FSK) signal transmitted in the center of the spectrum, with the encrypted voice stream divided above and below the FSK signal (Figure 1). Although dating back to the Cold War era, the strategy is inactive utilized for tactical communication in the combat field as recordings are only a fewer days old and heard in the lower VHF scope (just above the advanced HF limit). The stream consists of unequal time segments, within which the 2 voice subchannels are swapped and inverted.
 |
| Fig. 1 - T-219 "Yachta" signal |
The FSK signal is utilized as a synchronization series and is transmitted at a rate of 100 bps with a 150 Hz shift (Figure 2).
 |
| 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.
 |
| 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 2 frequencies are preserved after each "shift" (Figure 4 shows 2 periods): that suggests that it's formed by switching (mechanically or electronically) 2 independent F1 F2 frequency generators which bear any inter-relationships or by utilizing a VCO system.
 |
| Fig. 4 - 2 periods of the FSK frequencies |
By the way, looking at the durations of 2 periods:
F1 = (2:0.001285470) = 1555,851167277338
F2 = (2:0,001422470) = 1406,005047558121
the shift is just about 150 Hz
