14 July 2020

demodulating sync F7B mode transmissions using F7B.exe software

F7B signal, apparently a "classic" MFSK-4
My friend cryptomaster coded a program (F7B.exe) for demodulating synchronous F7B, it can be freely used and distributed for the analysis of such signals. Unfortunately, SA program parses these transmissions as classic MFSK-4 but that is not what actually is on-air: indeed,  F7B transmission mode deals with signals consisting of two independent but synchronous channels CH1 CH2 each carrying teletype signals (FSK2 on 2 independent channels).

F7B.exe can be downloaded from here:
Warning: F7B.exe is coded to run under win/32-bit systems so you may encounter a comdlg32.ocx error (component not correctly registered) when try to run it under x64 systems, here is how to resolve comdlg32.ocx missing error:

F7B.exe needs as input file the  raw MFSK-4 stream produced by the demodulator of SA and output two ASCII-Bit files CH1.txt and CH2.txt in the same directory of the input file:

Moreover, the Octave script T207_detect.m  has been used for the check of T207/CIS-14 mode [1]: it can be freely downloaded from here (you will need GNU Octave package [2] to run the script):
the Octave script T207_detect_e.m also extracts the two world and bit interleaved channels:

I tested the demodulator on the two  F7B modes 100Bd/1000 and 96Bd/500, both actively used by Ukrainian Nets; by the way, two preliminary observations:

1) usually, the two F7B channels CH1 CH2 are T-207 secured according to the CIS-14 mode [1]. Since CIS-14 consists of two independent 5-bit MTK-2 channels (transmitted in 14-bit frames), it turns out that those F7B transmissions may carry up to four independent 5-bit channels (CH1a, CH1b, CH2a, CH2b);

2) both those F7B waveforms have their FSK2 counterparts.

I also tested F7B.exe on the Rockwell TE-204, a "special" time-frequency diversity FSK4 mode most commonly used by Allied Air Forces as an air-to-ground messaging system as well as in ground and naval applications.

100Bd/1000 F7B

Fig. 1 - 100Bd/1000 F7B
100Bd/1000 F7B transmissions occupies about 3000Hz bandwidth with the four tones at -1500, -500, +500, +1500 Hz respect to the center frequency. As said above, 100Bd/1000 F7B transmissions may carry up to four independent 5-bit channels. In this regard, it's worth noting in figure 2 that - at least in this sample - the two F7B channels adopt different CIS-14 checksums: mode 20 (0312) in channel CH1 and mode 3 (3120) in channel CH2: who knows, maybe the two F7B channels carry the same data but with different checksums just to increase the redundancy of the system. Note that CIS-14 100Bd/1000  (the FSK2 counterpart) uses the checksum mode 20.

Fig. 2 - two 100Bd/1000 F7B channels, each transporting two CIS-14 channels (CH1a, CH1b, CH2a, CH2b)

96Bd/500 F7B
Fig. 3 - 96Bd/500 F7B
96Bd/500 F7B transmissions occupies about 1500Hz bandwidth with the four tones at -750, -250, +250, + 750 Hz respect to the center frequency. The same earlier conclusion applies: that is since CIS-14 consists of two independent channels,  96Bd/500 F7B transmissions too may carry up to four independent 5-bit channels.
I don't know if it's a mere coincidence, anyway it's interesting to note in figures 4,5 that only one channel transports data, as well as the CIS-14 96Bd/500 (the FSK2 counterpart) does: i.e., either 96Bd/500 F7B either 96Bd/500 FSK2 use only one of their two "available" channels.  Both the waveforms use the CIS-14 checksum mode 20.

Fig. 4 - two 96Bd/500 F7B channels, only one of the two available CIS-14 channels is used

Fig. 5 - CIS-14 96Bd/500 FSK2 channels, only one of the two available 5-bit channels is used
In this transmission the two channels use different EOT signaling sequences (figure 6)

Fig. 5

Rockwell TE-204 FSK4
Although TE-204 does not definitely use the F7B mode, I decided to check it since regards two FSK2 channels. Indeed, TE-204 transmits the "mark" on 935 Hz for 6.67 msec period followed by a replicated 6.67 msec "mark" at 1815 Hz. Similarly, the "space" is transmitted at 1375 Hz for 6.67 msec followed by a replicated 6.67 msec "space" at 2255 Hz (figure 6). This "mode" provides an in-band frequency  and time diversity function  (thus the speed is the half of the measured one). As for above, from the perspective of the  data-transfer, the modem works as a 75Bd/880Hz FSK2 modem.

Fig. 6
Well, decoding separately the two FSK2 channels (935-1375, 1815-2255) we get obviously two time-shifted similar streams (figure 7)

Fig. 7 - the two TE-204 demodulated FSK2 streams
Demodulating it as if it were an F7B signal we get "01" sequences in channel CH1 and  (expected) duplicated data in channel CH2 (figure 8)

Fig. 8

https://yadi.sk/d/f0U80pRg3F58iA  (F7B signals)

[1] https://i56578-swl.blogspot.com/.../redefining-t-207-checksums.html
[2] https://www.gnu.org/software/octave/

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