As electronic equipment became more refined (with improved frequency stability) and the demands for error free communication increased, frequency shift keying (FSK) was introduced. In this mode the transmitter is continuously on, but transmits alternately on two different frequencies, one representing Mark level and the other one Space level. As amplitude noise is additive, it can be eliminated by clipping all spikes above the carrier level. The difference between the two frequencies (the frequency deviation) is called the shift, and may be for example 170, 425 or 850 Hz.
For many years FSK was the mainstay of digital radio communication.