Compact discs (CDs), CD-Roms, Digital video discs (DVDs), Universal Multimedia Disc (UMD) and Minidiscs are all made of a thin layer metal, usually aluminum, covered in a protective coating of clear, hard plastic. In the case of CDs, sound is translated into electronic signals that are translated into a digital format. This is impressed into the metal of the disc as a series of millions of tiny pits. During playback, a laser reads the changes in the metal and translates these back into sound.
CD-Roms can hold data, and sound files and pictures; DVDs have rapidly overtaken video cassettes as a way of playing back movies; and the small size and large memory of minidiscs made them popular.
In the same way that the advent of the CD ousted vinyl singles and albums, improved technology that has led to increased storage capacity on handheld players, downloading music files from sites on the internet straight to these devices, as well as illegal file sharing, have led dramatic reductions in the number of CDs sold.
Frequency modulation, usually simply called FM, is a method of sending sound signals on radio waves. Amplitude modulation (AM) and FM are the two chief means of transmitting music and speech, although digital technology is increasing in popularity.