MFSK 21/13 tones 31.5/63 Bd (125 and 250 Hz spaced)

unid MFSK modem spotted on 12138.8 KHz/USB, according to the suppressed carrier during MFSK, and switching from 21-tones 31.5Bd (125Hz shift) to 13-tones 63Bd (250Hz shift).  Note also the espansion of the spectrum since the occupied bandwidth varies from 2600Hz (MFSK-21) to 3000 Hz (MFSK-13).

Fig. 1

Fig. 2

Apart from the two edge tones, the MFSK-13 uses the same ten odd tones of the MFSK-21 waveform:

Fig. 3

Transmissions may have different duration, maybe depending on the length of the messages to be sent: from few seconds up to 6 minutes (the longest I copied). Sometimes the transmissions use only the MFSK-21 waveform and two adiacent channels for the peers (maybe ISB?):

Fig. 4

The MFSK-21/13 signal was also copied on 9280.0 KHz (frequency of the carrier) and reported in radioscanner here. They ascribe the transmissions to not well identified "domestic" (Russian?) telecomm operators.

https://yadi.sk/d/m03PtZUS3MnwHY