FSKAn FSK-2 signal as spectrum and sonagram

FSK, or: frequency-shift keying, was the start of machine-to-machine communications on HF. The information is coded and transmitted by a signal which changes between at least two frequencies. The upper frequency is called mark, the lower space, and the difference is the shift. Mark represents “1”, space “0”. With these digits you may compose texts in different alphabets.

The speed is measured in bit/second, or baud. Bit/second simply is bit/second, whereas baud stands for “symbol/second”. Both might be the same, but often that’s not true.
If one bit (0 or 1) is 20 ms long, that’s 50 bit/second, or 50 bps.

“Asynchronous code” consists of special start and/or stop bit. In the widely use(d) Baudot code, the stop bit is of 1.5 times longer than the length of an ordinary bit.

Frequency notation: The professional way is to give the virtual carrier frequency as HF frequency, i.e.: “4.583 kHz, 500 Hz shift” means mark and space on 4.582,750 kHz and 4.583,250 kHz, respectively.

On this page, you will find FSK signals with just two states – mark and space. And just one channel. See the MFSK page for signals of “multi-frequency-shift keying”, and with FSK transmissions showing more than one channel.