Much of radio broadcasting uses amplitude modulation (AM), although other types of modulation are possible. A convenient and efficient means of transmitting information is by the propagation of waves of electromagnetic radiation (see waves and wave motion). Sound waves in the audible range, such as speech and music, have a frequency that is too low for efficient transmission through the air for significant distances.
By the process of modulation, however, this low-frequency audio information the signal can be impressed on a carrier wave that has a much higher frequency and can propagate through space for great distances. The transmitter at a radio station generates a carrier wave having constant characteristics, such as amplitude and frequency. The signal containing the desired information is then used to modulate the carrier.
That is, the signal produces a variation in the form of a certain characteristic of the carrier wave. This new wave, called the modulated wave, will contain the information of the signal. In AM, it is the amplitude of the carrier wave that is made to vary so that it will contain the information of the signal. When the modulated wave reaches a radio receiver tuned to the proper frequency, it is demodulated, "revealing" the wave that was carried. (Demodulation is the process of reconstructing the original signal from the modulated wave. It is essentially the opposite of modulation.)
The signal can then reproduce the desired sound by using a loudspeaker. AM radio is still a popular form of radio broadcasting, but it does have a number of shortcomings.
The quality of reproduction is relatively poor because of inherent limitations in the technique and because of interference from other stations and other electrical signals, such as those produced by lightning or by electronic devices. When the frequency, rather than the amplitude, of the carrier wave is modulated, the process is called frequency modulation or FM.
