24 October 2023

CIS VFT 12-FSK async channels system (P-327-12)

Very interesting and rather rare capture sent me by my friend AngazU, of a CIS VFT (Voice-Frequency Telegraphy) waveform which uses twelve  independent FSK channels multiplexed onto a single 3 KHz wide HF channel. The transmission was recorded on 4567 Khz USB , just while monitoring the close well-known "buzzer". Regardless of the nickname we would like to give to these signals, the name of the equipmnet is P-327-12, or in Russian language П-327-12  (thanks to Cryptomaster for the info).
The P-327-12 equipment provides up to 12 channels of Voice-Frequency Telegraphy with a speed of up to 100 baud in one HF channel or 6 channels in wo standard HF channels. The occupied band of each VFT channel is 160Hz. The characteristic (upper and lower) frequencies of a single channel are determined by the formulas:
f1 = 180 + 240n Hz
f2 = 300 + 240n Hz

where f1 and f2 are the lower and upper characteristic frequencies (ie the FSK shift) of the nth channel. If there is no traffic onthe  nth input channel, only the higher frequency (f2) is transmitted. The "pilot tone", as usual, is transmitted on 3300 Hz.

Fig. 1 - CIS 12-FSK spectrum

It's worth noting that the P-327 system is asynchronous(!), i.e. the transmission speed of each channel can be any in the range of 0-100 or 200 Baud. For example, in this sample channels 1 and 3 have a speed of 50 Baud while channel 6 is keyed at the speed of 100 Baud (Figs. 2,3).

Fig. 2a

Fig. 2b
Be careful not to confuse the CIS-12 (AT-3400D equipment) and CIS VFT-12 (P-327-12 equipment) waveforms: although "at first sight" on the waterfall they look like the same signal, they sound different and have a slight difference in the occupied bandwidth; furthermore, while CIS-12 consists of 12 continuous PSK channels, the T-237-12 signal consists of twelve not simultaneous FSK channels (Figure 3).
Fig. 3 -comparison of the spectra of VFT-12 and CIS-12 signals

At least in this sample, the data do not appear to be formatted according to the Russian CIS-11 and/or CIS-14 standards (Figure 4).
Fig. 4 -  CIS-11 & CIS-14 test results
P-327 system was adopted as weapons to replace the P-318M voice-frequency telegraphy equipment. Thanks to the use of modern digital components and new technical solutions, it was possible to improve the quality of telegraph communications, simplify the process of operating equipment, and reduce weight and size indicators.

Fig. 5 - P327-12 system: note the 12 "slots" of the modem (https://infopedia.su/18xc490.html?ysclid=lo41wlbhdo570983862)



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