Spanish Universities are still conducting intensive tests of the HFDVL modem (hfdvl-modem-ofdm-73-tone) on 14350.0 KHz on USB, I not tried to listen on the other 14825.5 KHz test-frequency. While the previous tests consisted of transmission of about 30 and 10 seconds duration, in these samples (at least) a different "test format" is used: it seems to me that one side of the link just sends data (long signals) while the other peer seems just send a sort of ACK (shorter signals). The durations are fixed, ie 1'06'' and 0'11 secs.
The tests involve two transmitting modems, more likely located at Las Palmas - Canary Islands (14350.0 KHz just belongs to the University of Las Palmas) and Madrid, since the HFDVL waveform is developed by the groups "CeTIC" from the Universidad de Las Palmas de Gran Canaria and "GAPS" from Universidad Politécnia de Madrid, other than Aeropuertos Españoles y Navegación Aérea. As far as I know, the modem manufacturer is MM CICOM Telecomunicaciones from Spain.
Prior to the transmission of data, a three part preamble is transmitted.
- 633.1/3 ms (19 signal element periods) in ACK signals
- 433.1/3 ms (13 signal element periods) in DATA signals
while in the 24-May recording the tone has a duration of 433.1/3 ms (13 signal element periods). Part two consists of 2 signal element periods of (KNOWN) data and part three consists of 1 signal element period of only even carriers followed by 3 signal element periods of only odd carriers: part two and three sum to 6 signals and make a 200ms duration (Pic. 3).
My friend KarapuZ pointed my attention on the part two: according to his analysis, this is BPSK 2400Bd modulated and carrier is 60 Hz down-shifted in respect of the unmodulated tone frequency. The 60 Hz shift is probably added for the initial synchronization. Following his tips I replicated his results (Pic. 4).
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Pic. 1 |
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| Pic. 2 |
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| Pic.3 |
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| Pic. 4 |
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