11 January 2018

MFSK-11 125Bd/250Hz waveform


Unid MFSK-11 waveform spotted this morning on 7973.0 KHz on USB,  already heard on March 13, 2017 (same frequency) and March 23, 2016 (9300.0 KHz). The waveform is an 11-ary frequency shift keying (FSK) modulation with eleven orthogonal tones spaced by 250 Hz (first tone at 650 Hz), one tone (or symbol) at a time; the tones are transmitted at a rate of 125 tones (symbols) per second (Fig. 1).

Fig. 1
The 792 msec ACF value means a frame length of 99 symbols/sec, given the speed of 125 Baud. Once demodulated (Fig. 2) I converted the 0-A symbols into their quadribit rapresentation using the Hex table 0=0000, ..., A=1010 supposing that each tone represents 4 bits of data.

Fig. 2
The resulting bitstream confirms the framing of 99 quadribit symbols, ie a 396-bit length period as shown in Figure 3.

Fig. 3
It's worth noting that each 99-symbols frame consists of nine blocks, each consisting of just 11 symbols: this is an interesting relation between the number of symbols-per-block and the number of the levels (both 11)!
Presumably it's a CIS waveform, although I did not find any confirmation in other logs. Comments are welcome.

➤ 𝒖𝒑𝒅𝒂𝒕𝒆
I want to thanks my friends KarapuZ and cryptomaster who sent me some other useful informations and samples about this waveform. Particularly, a record from KarapuZ must be mentioned: in that case it's possible to observe a 6864-bit (1716 symbols) period 

Fig. 4