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When data is stored it occupies space on a storage device such as a hard disk or floppy disk. Buying storage space costs money. To reduce the cost of storing data users often attempt to reduce the size of the files they want to store without losing any of the data.
Data compression is the act of reducing the size of a file without losing any of the data it contains. Decompression (also known as uncompression or expansion) is the act of taking a compressed file and converting it back to its original size and format. Because compression alters the format of the data in a file a compressed file must be decompressed before it can be used.
How much a file can be compressed by will depend upon the method being used to compress the file and the data that is stored in the file.
The three most common applications for which data compression is used are :
Software is often distributed on CD-ROMs. The software must be installed onto a hard disk before it can be used. Software manufacturers usually compress the files that make up a package on the CD-ROM. When the software is installed onto a hard disk the files are decompressed by the installation software.
| Advantages | Because the software is compressed it is possible to fit extra data onto the CD such as help and tutorial information or demos of other programs. |
| Disadvantages | Because the software is compressed it can not be run directly off the CD-ROM. It must be installed first. |
Files stored on hard disks can also be compressed. Some utilites will compress and decompress individual files whilst others can compress an entire hard disk automatically as files are saved and loaded.
| Advantages | Files take up less space so you can store more files on a hard disk. |
| If a file takes up less space to save then the cost of storing the file is reduced. | |
| Disadvantages | You must buy a compression program to compress or decompress files. | The compression program will use up some of the computer's memory, leaving less available for other programs. |
Compressed files may load more quickly or more slowly than files that are not compressed. This will depend upon how well compressed a file is and the speed of different components of your computer hardware.
Compression is also used when data is transmitted over a telephone line. Modems will only allow the transmission of data at very slow speeds (e.g. 3Kb per second). To transmit a picture the size of an A4 page can take over ten minutes. If data that is to be transmitted can be compressed before it is sent then there can be considerable advantages :
| Advantages | Faster transmission time. |
| Reduced transmission costs. | |
| Disadvantages | Compression can only be used if both the transmitting and receiving modems support the same compression procedure. |
There are a variety of agreed standards for compressing data that is sent over telephone lines. Some modem manufacturers claim compression ratios of 1:4 (i.e. data can be reduced to a quarter of its original size before being transmitted). Such high compression rates are rarely achieved.
GCSE ICT Companion 04 - (C) P Meakin 2004