Nokia Adaptive MSG-Terminal, FSK 401Bd/301Bd 780Hz shift

This is an interesting copy of transmissions sourced by the Nokia Adaptive MSG-Terminal (Nokia Sanomalaite M/90) in "dual" 401 (401.5) and 301 Baud mode, both running with constant 780 Hz shift and heard on 10225.0 KHz/USB (offset = 1700 Hz) at 0605 UTC.

The three transmissions resemble a PtP ARQ system where the caller station use the 401Bd mode to send data and the called station use the 301 Bd mode for ACKs, also note that in the second transmission the 401Bd frame is sent without the preamble: maybe it is a segment which belongs to the first transmission. However, during that same recording the 401Bd mode has been used also as stand-alone (Fig. 2).

Fig. 2

Just the preamble is probably the "business card" of this waveform: talking with my friend KarapuZ about this device, he pointed the fact that the preamble it's always modulated at a speed of 301 Baud.
Indeed, at a first glance the speed seem to be constant in the two modes: 401 and 301 Baud (Fig. 3)

Fig. 3

but trying to demodulate the 401Bd signal it's possible notice some distorsions in the preamble which are not present when the stream is forced to 301 Baud (Fig. 4)

Fig. 4

These distorsions are not present in the case of the 301Bd signal (Fig. 5)

Fig. 5

In order to prove this characteristic, KarapuZ sent me a recording of the Nokia M/90 running at 151 (150.5) Bd and also in this case it's possibile notice that same behavior in the preamble (Figs. 6,7)

Fig. 6

Fig. 7

KarapuZ also suggested to demodulate the 151Bd signal using the differential decoding: after the removal of the synchronization bits (vertical solid columns in the 4-bit period stream) and inverting the polarity, a period of 8 bits with 1 stop bit is obtained. Next, it's difficult to state how to process the information, as a rule, such terminals use off-line encryption. Since the removal of 2 bits of four, the data signaling rate is 150/2 = 75 bps.