(i) Various electrical machines and noises cause amplitude disturbance in the transmission of amplitude-modulated wave. This makes the reception noisy. So, there is a need for different type of modulation, which can reduce the noise factor. Frequency modulation (FM) was proposed as a means of improving the signals-to-noise ratio of a radio system. The first practical system was put forward in 1936 as an alternative to AM in an effort to make radio transmissions more resistance to noise.
(ii) Fidelity or audio quality of amplitude-modulated transmission is poor. This types of transmission is also not good for musical programmes. There is a need to eliminate amplitude-sensitive noise. This is possible if we eliminate amplitude variations. In other words, there is a need to keep the amplitude of the carrier constant. This is precisely what we do in frequency modulation.
