There's more than one way to broadcast a radio signal. This brief essay from A Science Odyssey website describes the two methods most often used by radio stations to code sound information into radio waves.
At the most basic level, the radio broadcasts we hear take place in four stages. First, a microphone converts sound waves into electrical signals. Second, the electrical signal from the microphone changes, or modulates, a radio wave called a carrier radio wave, so that it will contain the sound information. This carrier wave is emitted by the broadcast tower. Third, a receiving antenna on a radio tuned to the proper frequency picks up the carrier wave, which then travels to the receiver or radio. Finally, the receiver detects the modulation in the carrier wave and converts it back into electrical signals. The electrical signals are sent to a loudspeaker, which changes them once again into sound waves.
Sound information -- including pitch and loudness -- is carried by radio waves in one of two ways. Depending on the type of radio station -- AM or FM -- the carrier wave is modified differently. Amplitude modulation, or AM, stations code information in the carrier wave by changing, or modulating, the amplitude, the height of the carrier wave's peaks and valleys. The speed at which the amplitude changes determines the pitch of the sound, and the amount that it changes determines its loudness. In contrast, frequency modulation, or FM, stations alter the frequency of the carrier signal. In this case, the speed at which the frequency changes determines the pitch, and the amount that it changes determines the loudness.
Both these methods of broadcasting radio signals have advantages and disadvantages. Generally, AM carrier waves have much longer wavelengths than FM carrier waves, and as a result, they can bend around obstacles like mountains and buildings better than FM waves and can travel greater distances before the signal fades. However, because information is coded in the amplitude of an AM carrier wave, things such as the hum from nearby appliances and sparks going off in passing cars can influence the amplitude and are more likely to interfere with the wave. Also, any unwanted change in the power of the transmission can cause an amplitude change that will distort the sound the radio produces. Since it is much less likely that the frequency of an FM carrier wave might be accidentally modulated, the quality of the sound is better.