Protocol
XPA2 utilizes a proprietary 16-tone MFSK waveform at the speed of 7.8125Bd, and the shift of 15.625Hz. Each tone represents one character:
| Hz | Character |
|---|---|
| 1000.000 | Space |
| 1015.625 | (Not used) |
| 1031.250 | Start of message |
| 1046.875 | End of transmission |
| 1062.500 | (Not used) |
| 1078.125 | Repeat previous character |
| 1093.750 | 0 |
| 1109.375 | 1 |
| 1125.000 | 2 |
| 1140.625 | 3 |
| 1156.250 | 4 |
| 1171.875 | 5 |
| 1187.500 | 6 |
| 1203.125 | 7 |
| 1218.750 | 8 |
| 1234.375 | 9 |
Broadcast procedure:
- Precisely two minutes of 400 ms of the 9 tone (1234.375Hz), and 1600 ms of the space tone (1000Hz) repeating.
- The tone string 6666622662626 (forming continuous tones). (Note the use of a Barker sequence - possibly relating to synchronization in multipath propagation conditions.)
- The repeat and start of message tones alternating (20 tones in total).
- 10 space tones (continuous 1000Hz tone string).
- 5-digit groups, each separated by a space.
- The end of message and repeat tones alternating (20 tones in total).
Messages in the form of "0____ 00001 00000 _____" are null messages.
One broadcast can contain multiple messages. The messages are separated by "00000 00000".