19 December 2016

PSTN V-series modem over a radio link ?

This transmission has been copied on 6421.5 KHz/USB at 0808 UTC and consits of five parts:
1) initial FSK 300Bd call and link negotiation, between the calling and the answering modem;
2) PSK 1200Bd data transfer from the calling modem;
3) FSK 300Bd from the answering modem (a mode switch request ?);
4) PSK 600Bd data transfer from the calling modem;
5) FSK 300bd likely as ACK and link disconnect;
Each FSK/PSK frame is preceeded by a 1500Hz short tone (Fig. 1) and looking at the strength of the PSK frames they seem to be sent by the calling modem.

Fig. 1
The FSK signal initialing the link has a speed of 300Bd and shift of ~170Hz (Fig. 2). Once demodulated,  it exhibits a 10-bit length period and matches the 8N1 format: one start bit, one stop bit, no parity and 8 data bits (Fig. 3).
Fig. 2

Fig. 3

Speeds, carriers and PSK constellations related to the PSK frames are shown below in Figs 4,5 (note the amplitude failure in the initial 1200Bd PSK frame that impairs its study):

Fig. 4
Fig. 5

It's interesting to note that the waveforms of the PSK frames match some of the ITU-T V series recommendations for data communication modems over the Public Switched Telephone Network (PSTN):
1200 Baud, 1800 Hz carrier, PSK-4 (2400 bps gross) --> ITU-T V.26
 600 Baud,  1200 Hz carrier, PSK-4 (1200 bps gross) --> ITU-T V.22 (Bell 212A)
(ITU-T recommendations can be read here)

My friend KarapuZ, who pointed me to ITU modems, heard some other V-serie modem on HF: maybe some tests or arrangements as proof-of-practice about the feasibility of such unusual solution (PSTN modem over HF). It's worth noting what seems to be a sort of adaptive solution, the 1200-600 Baud switch, and that the military often use such modems but on VHF repeaters.
Comments are welcome.

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