Glossary definition of Compression

Compression is the reduction in size of data, using a finite algorithm. Compression is very useful where there are limited resources for storing or transmitting data, which is often the case with mobile phones, as they have both limited memory for storing information, and limited bandwidth for communicating it.

Compression algorithms can be divided into two classes-those that retain all the original information through the compression process-which are said to be lossless-and those that ditch the least significant information in order to save more space, which are known as lossy. JPEG is an example of a lossy format; heavily compressed JPEG images often have large areas of colour with blocky boundaries and can distort clean edges and straight lines.

The most common use of compression in mobile phones is the conversion audio data (such as speech in phone calls) for transmission; both ADPCM and ACELP involve a compression step. Image formats often include some kind of compression option, with both JPEG and PNG incorporating sophisticated compression techniques. Compression is also used in video as there is a lot of information that can be represented using fewer bytes than the uncompressed raw version, or even omitted entirely.