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5-1-5 Compression and Decompression of Information

Because of their nature, the data files of multimedia have a larger size (amount of data) than the data files of only text information. If such a large data file is used as it is, a large recording area is required for storing the data file, and the load also increases when the file is exchanged across the network. Thus, in order to improve the efficiency of data storage and reduce the network load, the size (i.e., amount of data) of information is reduced in accordance with fixed rules (i.e., procedures). The process of reducing the size of information is called compression and the process of restoring the compressed information is called decompression.

The concept (i.e., format) of compression is broadly classified into the following two types of compression:

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• • Lossless compression
  It is a compression technique that enables a complete return to the original data from the compressed data.
• • Lossy compression
  It is a compression technique that does not enable a complete return to the original data from the compressed data. In the case of images, since humans hardly ever notice the differences even when the restoration process is more or less incomplete, it does not result in a big problem. In comparison with lossless compression, the compression rate, which indicates the compression effect of the amount of data, can be increased; that is, the amount of data can be reduced.

The typical information compression and decompression formats (i.e., standards) are as follows:

• • JPEG (Joint Photographic Experts Group)
  It is an international standard of a still image compression and decompression format that supports full color.
• • MPEG (Moving Picture Experts Group)
  It is an international standard of a moving image compression and decompression format.
• • ZIP
  It is a file compression format supported by several types of free software in addition to software products such as WinZip. It is used not only for compressing a single file but also for archiving, where several files are collectively compressed into one file. At an international level, it is a de facto standard (i.e., industry standard) file compression format.
• • GZIP (GNU ZIP)
  It is a file compression format that is mainly used in UNIX. Although its name includes ZIP, it is not compatible with ZIP, and there is no archive function either.
• • LZH
  It is a file compression format supported by the free software LHA. It is mainly used as a file compression format within Japan.
• • MR (Modified Read) / MMR (Modified Modified Read)
  It is a compression and decompression format (i.e., encoding format) that is mainly used for facsimile.