Protocol
XPA utilizes a proprietary 17-tone MFSK waveform. The speed is 10 bd. Each tone represents one character:
| Hz | Character |
|---|---|
| 520 | Start of transmission #1 |
| 600 | Start of header |
| 680 | Space |
| 720 | End of transmission |
| 760 | 0 |
| 800 | 1 |
| 840 | 2 |
| 880 | 3 |
| 920 | 4 |
| 960 | 5 |
| 1000 | 6 |
| 1040 | 7 |
| 1080 | 8 |
| 1120 | 9 |
| 1160 | Start of message |
| 1200 | Repeat previous character |
| 1280 | Start of transmission #2 |
Broadcast beginning procedure:
- Precisely two minutes of 400 ms of the start of transmission #1 tone (520 Hz) and 1600 ms of the start of transmission #2 tone (1280 Hz) repeating.
- The start of header tone (600 Hz) and the 9 tone (1120 Hz) repeating (40 tones in total).
- Tone string 666662266262644444444446666622662626 (forming continuous tones).
- The 3-digit schedule ID and 1-digit number of messages (in case of traffic) or 000 (in case of null messages), separated by spaces and repeated thrice. (the repeat previous character tone is not used here)
- The start of header tone (600 Hz) and the 9 tone (1120 Hz) repeating (40 tones in total).
- Tone string 666662266262644444444446666622662626 (forming continuous tones).
- The repeat tone (1200 Hz) and the start of message tone (1160 Hz) repeating (20 times in total).
- 10 space tones (continuous 680 Hz tone string).
The transfer of message groups is interrupted every 64 groups by the tone string 6666622662626 (forming continuous tones). This tone string, the same that is also used various times at the beginning at the transmission, is a Barker sequence, and possibly relates to synchronization dealing with multipath propagation conditions.
Messages in the form of "0____ 00001 00000 _____" are null messages.
The groups of multiple messages in one broadcast are separated by "00000 00000".