Data Compression

Data Compression

• Data compression means storing data in a format that requires less space than usual.
• Data compression is particularly useful in communications because it enables devices to transmit or store the same amount of data in fewer bits. 
• There are a variety of data compression techniques, but only a few have been standardized.
• The CCITT has defined a standard data compression technique for transmitting faxes and a compression standard for data communications through modems(CCITT V.42bis). 
• In addition, there are compression formats, such as ARC and ZIP.
• Data compression is also widely used in backup utilities, spreadsheet applications, and database management systems. 
• Certain types of data, such as bit-mapped graphics, can be compressed to a small fraction of their normal size.

Compression losses

• Lossless compression, every single bit of data that was originally in the file remains after the file is uncompressed. 
• All of the information is completely restored. 
• This is generally the technique of choice for text or spreadsheet files, where losing words or financial data could pose a problem. 
• The Graphics Interchange File (GIF) is an image format used on the Web that provides lossless compression.

• Lossy compression reduces a file by permanently eliminating certain information, especially redundant information. 
• When the file is uncompressed, only a part of the original information is still there (although the user may not notice it). 
• Lossy compression is generally used for video and sound, where a certain amount of information loss will not be detected by most users.

Types of Compressions	Advantages	Disadvantages
Lossless	Corresponds 100% to the original file	File size reduction only to about 25% (for piano music) and 70% (for rock music)
	Gapless
	Fully taggable with track infos and cover
	Conversion in any other format possible without loss of audio information
Lossy	Fully taggable with track infos and cover	Does not contain all of the original audio information.
	File size reduction to 4-22% of original size	MP3 not gapless, OGG Vorbis, however, is said to be gapless.
		Conversion into any other format only with loss of audio information