File compression software condenses digital files or folders so they occupy less disk space. This is accomplished by reducing redundancy in the code of a file (lossless compression) or by eliminating some data from a file (lossy compression). An algorithm replaces redundancies or removes superfluous or irrelevant data to achieve this outcome. When a file is losslessly condensed it can be fully replicated when decompressed.

Operation

File compression software condenses, manages, and expands data as necessary. The software combines many files into a single unit smaller than the total size of the original content. Doing so makes storage and backup more efficient and facilitates transmission of electronic records. Standard forms of transmission engage .zip or .zipx formats.

Uncompressed information can be corrupted when transferred on the Internet. Therefore, compression helps avoid this and reduces transfer time. It permits the relocation of sizable files or folders that might be subject to data transmission limitations established by email providers or other services. It also saves download time.

To gain access to compressed data, the recipient must decompress it using decompression tools compatible with the format initially employed. Major operating systems, such as Mac OS X and Windows 7, 8, and Vista feature inbuilt file compression systems to archive data as zip content.

Formats

Many formats exist for compressing and decompressing files. Software programs are compatible with one or more of these, depending on design and intended functionality. The formats were developed in response to multiple operating systems with the need for different types of compression, as well as competition between software developers.

The most prevalent types of systems/formats include:

• LHA: Uses .lha or lzh files. Standard with Amiga systems.
• RAR: Uses .rar files. A proprietary format for Microsoft Windows.
• Tape archive: Uses .tar files. Popular with many *NIX operating systems.
• 7-Zip: Uses .7z files. Archive format.
• GNU Zip: Uses .gz files. Common with *NIX operating systems.
• StuffIt: Uses .sit files. Archive format for Mac and at times employed with other operating systems.
• Zip: Uses .zip files. The most extensively used archive format. Supported by a board selection of programs, including Microsoft Windows.
• WinAce: Uses .ace files. This format is common with CD/DVD images.

Compression Techniques

Lossless Compression

File compression software performing lossless compression reduces the total number of bits and bytes contained in a file, making it more convenient to store, download, or send electronically. After a file is condensed and combined with other units, it is placed into an archive to speed data transfer and keep it organized.

Once it is downloaded, decompressed, and restored to its original size, the expanded file is unchanged from its original state. The fundamental principle identifies redundancies or occasions when identical information is contained within the material.

When file compression software isolates recurring data, it's reduced by recording the occurrence only once and then refers to the initial instance in subsequent iterations instead of listing it again. For example, if a word appears multiple times in a text document, it is only necessary to record it once, and refer to the original instance in subsequent occurrences, thereby saving memory space. Numerous file compression software programs integrate variations of the LZ adaptive dictionary-based algorithm to condense files. Dictionary, in this context, is a method for cataloging portions of data.

Dictionary-based algorithms work well when compacting text files by finding repeating patterns to compress. Given the predominance of such patterns in written languages, achieving a significant reduction in the size of the file is possible. A group of factors, such as the size, type, and selected compression scheme, determines the actual file-reduction ratio. A decrease of 50 percent or greater is common when dealing with large text files. Such algorithms are less efficient when dealing with non-text-based material, including graphics and video, or music file formats such as .mp3 or .mov. The presence of substantial unique data prevents significant compression for these materials.

Lossy Compression

Lossy compression eliminates bits of information considered unnecessary o reduce the output size. It is deployed for media elements such as pictures, sound, and video. Lossy compression is irreversible; as a result, the compressed file cannot be reproduced as it was in its original form. This type of compression is not suitable for files where an exact reproduction is desired, as is the case with software databases, applications, and text files.

For example, in Bitmap pictures lossy compression is applied to reduce the file size. Compression software, in this case, shrinks the size by making alterations to the pixels underlying the picture. In an image of a field of grass, for instance, while the field appears uniformly green, in actuality the pixels depicting the grass contain minor differences. The program saves space by using one color value for all the grass in the picture, and rewrites the file to reflect this. If the compression quality is high enough, the change is not noticeable and the file size is substantially reduced.

MP3 content is another common application of lossy compression. Digital material is condensed by removing some audio information from the original recording. Sounds barely discernible to the human ear are removed. Higher levels of compression remove increasing volumes of information. Excessive compression leads to indistinguishable sounds in music files and blurriness when applied to images.

Selection

The majority of the programs possess the ability to archive and extract through basic right-click functionality. Drag-and-drop features and instruction wizards are also helpful for inexperienced users. Another valuable characteristic is the ability to perform archive conversions from one format to another. The capacity to batch convert files is helpful as well.

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