CIS FSK scanning SelCall (ACS/C)

This is an example of a CIS ACS/C (Automatic Channel Selection/Control) scanning SelCall [1], in this case a scan set of seven channels is used. The signal was recorded by my friend AngazU in two occasions and presents some interesting aspects. As shown in Figure 1, the call consists of five scanning cycles: the 3 centrals ones, each consisting of all the seven channels of the scan list (1-7), plus the first and the latter ones consisting respectively of the four upper channels (4-7) and the three lower ones (1-3): this way each channel of the scan list is "worked" four times. Since the scanning mode, the call (and probably the system) is asynchronous.

Fig. 1 - the five CIS ACS/C cycles

The main parameters are (Figure 2):

- used modulation in channels is FSK at 150 bps with a shift of 200 Hz;
- separation between channels is 4 Khz, for a total bandwidth of ~24 KHz *
- each FSK segment has a duration of 4000ms (see below) or 600 bits, which makes a 28s duration for a complete scanning cycle;
- the scanning call lasts 1m 50s;
- transition time between two consecutive channels is pratically zero. 

* the occupied bandwidth in Hz may be computed as ~[(N-1) × 4000 + 2×Br], in this sample N=7

Fig. 2 - main parameters

The FSK segments exhibit a 300 ms ACF and consist of a 45-bit repeated sequence (Figure 3), obviously all the segments transmit the same data.

Fig. 3 - a demodulated bitstream

It's worth noting that a 15-bit encoding would make sense.

As from Figure 1, the last FSK segment is shorter (~ 2240ms instead of 4000ms): the sequence before the trailing "1s" seems to be 1 bit off from the previous pattern, maybe it's a streaming failure (Figure 4).

Fig. 4 - last FSK segment

The second recording (Figure 5) shows the same scan set arrangement (4-7, 1-7, 1-7, 1-7, 1-3) except for a short FSK segment in the third scan cycle (Figure 6): it's not possible to know if it is intentional or is a malfunction or maybe - as assumed for the last segment - a streaming failure.

Fig. 5

Fig. 6

About the length of the FSK segments it's interesting to notice in Figure 7 that the very first segment, and only this, lasts 4040ms and consists of an initial preamble consisting of a 460ms "01"s sequence followed by a ~ 3580ms length data of the selcall: probably the preamble signals the start of the scanning cycles.

Fig. 7 - "01"s preamble in the first FSK segment

Even more interesting is that the preamble seems to be keyed at the speed of 160 bps and thus consisting of 73-bit length reversals: Figures 8, 9 clearly indicate the different speeds.

Fig. 8 - different modulation speeds detected with the "zero-crossing" method

Fig. 9 - 160 and 150 bps FSK demodulations

The scanning system makes probably use of the CIS Selcall waveform [2] "Vishnya" (from the name of the R-016V "Вишня" radio  equipment): indeed modulation, bps, shift and ACF match. It must be said, however, that although I referred to the transmission as a "call" it could also be an LQA or other type of message/signaling. Apparently - at least in this portion of band - there is nor reply from the called station.

By the way, also this (quite rare and old) signal was heard using a remote SDR located in Ukraine... unfortunately, many interesting signals are on-air in that unfortunate area.

https://disk.yandex.com/d/MuMLNeTCkb4npg

[1] http://signals.radioscanner.ru/base/signal251/
[2] http://signals.radioscanner.ru/base/signal106/