concept of frequency modulation (FM) 

Frequency Modulation (FM) is a modulation technique in which the frequency of the carrier wave is varied in accordance with the amplitude of the input signal (such as an audio signal). Unlike amplitude modulation, the amplitude of the carrier wave remains constant, making FM more resistant to noise and signal degradation. FM is widely used in radio broadcasting (FM radio) and other applications where high-fidelity sound transmission is important.