Data compression (also referred to as digital compression or just compression) is
|“||[s]queezing information so that it requires less space to store or transmit. When speech is compressed, for example, pauses are eliminated. Compression is generally expressed as a ratio. For example, an 8-to-1 ratio means that the information requires one-eighth of its original space. . . . The greater the compression ratio, the higher the chance for loss of quality in image, sound, or motion.||”|
In video transmission, for example, individual elements of the picture that do not change from frame to frame (when the background of a scene remains the same, for example) are not resent for each frame — just a code that tells the receiver/decoder that no change has taken place. This allows less information to be sent, requires less bandwidth, and allows more channels to be transmitted.
Data compression is useful because it helps reduce the consumption of expensive resources, such as hard disk space or transmission bandwidth. On the downside, compressed data must be decompressed to be used, and this extra processing may be detrimental to some applications.
For instance, a compression scheme for video may require expensive hardware for the video to be decompressed fast enough to be viewed as it is being decompressed (the option of completely decompressing the video before watching it may be inconvenient, and requires storage space for the decompressed video). The design of data compression schemes therefore involves trade-offs among various factors, including the degree of compression, the amount of distortion introduced, and the computational resources required to compress and decompress the data.
The two general categories of data compression are:
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