What characteristic is altered in frequency modulation compared to amplitude modulation?

In frequency modulation (FM), the primary characteristic that is altered compared to amplitude modulation (AM) is the frequency of the carrier wave. In FM, information is encoded by varying the frequency of the carrier wave in accordance with the amplitude of the input signal, while the amplitude of the carrier remains constant. This is fundamentally different from AM, where the amplitude of the carrier wave is varied to carry the information.

By changing the frequency rather than the amplitude, FM can provide a number of advantages, including better noise immunity and improved sound quality, particularly in audio broadcasting. The bandwidth of the signal does increase in FM relative to AM as it requires a wider frequency range to accommodate the frequency deviations that occur due to modulation. However, the essential distinguishing feature that defines frequency modulation is indeed the variation of the carrier frequency.