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What is Compression?

Compression is a reversible conversion of data to a format that requires fewer bits, usually performed so that the data can be stored or transmitted more efficiently. The size of the data in compressed form (C) relative to the original size (O) is known as the compression ratio (R=O/C). If the inverse of the process, decompression, produces an exact replica of the original data then the compression is lossless. Lossy compression, usually applied to image data, does not allow reproduction of an exact replica of the original image, but has a higher compression ratio. Thus lossy compression allows only an approximation of the original to be generated. For image compression, the fidelity of the approximation usually decreases as the compression ratio increases.

Compression is analagous to folding a letter before placing it in a small envelope so that it can be transported more easily and cheaply. Compressed data, like the folded letter, is not easily read and must first be decompressed, or unfolded, to restore it to its original form.

The success of data compression depends largely on the data itself and some data types are inherently more compressible than others. Generally some elements within the data are more common than others and most compression algorithms exploit this property, known as redundancy. The greater the redundancy within the data, the more successful the compression of the data is likely to be. Fortunately, digital video contains a great deal of redundancy and thus is very suitable for compression.

A device (software or hardware) that compresses data is often know as an encoder or coder, whereas a device that decompresses data is known as a decoder. A device that acts as both a coder and decoder is known as a codec.

A great number of compression techniques have been developed and some lossless techniques can be applied to any type of data. Development, in recent years, of lossy techniques specifically for image data has contributed a great deal to the realisation of digital video applications.

Compression techniques used for digital video can be categorized into three main groups:

[General purpose compression techniques]

© Colin E. Manning 1996