Data compression is any of several methods used to reduce the size of files of digital (numeric) data. Such data may represent text, sounds, or still or moving pictures. Data compression enables a storage device, such as a computer’s hard drive, to hold more information. It also enables information to be transmitted faster over a communication system, such as a cellular telephone network or the Internet.
A file compressed using lossless compression can be expanded to exactly re-create the original data. Lossless compression is typically used for text files. The various methods can reduce the size of a text file by about 40 to 75 percent. A common method, called variable length coding, uses short codes to represent commonly used characters or words and longer codes for those used less often. Lossy compression is typically used for sound and still or moving picture files. When expanded, the files provide only an approximation of the original data. But unless a file has been compressed greatly, the losses are barely perceptible to the human ear and eye. Lossy compression can reduce the size of a file by 90 percent or more.
International standards organizations have developed standard methods for compression of different types of files. Such methods include JPEG (Joint Photographic Experts Group) for color still images; MPEG-4 (Motion Picture Experts Group) and H.263 for moving pictures; and MP3 (MPEG audio layer 3) for music.
