Part 5 of "Building a MIL-STD-188-110 HF Modem in Modern C++." Start with the overview , Part 1, the rules I build by , Part 2, the transmit chain , Part 3, the audio front end , or Part 4, acquisition . The code for this article is on GitHub: kg7hf/modem-core .

Part 4 found the burst on a line with one path. HF seldom has one. The signal reaches the receiver by more than one route through the ionosphere, and each route delivers its own copy of the burst, a few milliseconds after the one before. MIL-STD-188-110B specifies its serial-tone performance on exactly that. In 5.3.2.5, Table XX, the fading channels are two independent Rayleigh paths of equal average power: 2 ms apart for 600 to 2400 bps, and 5 ms apart for 75 to 300 bps and one of the 2400 bps rows. At 2400 symbols a second, that is 4.8 symbols and 12.

The transmitter sends the burst once; the second copy is the channel's doing.…