by Peggy Barnett

Created on: July 31, 2008

Zip files are compression files. Since a file is data saved for future use, a file requires a storage technique, either zip or non-zip. Compression is a technique used for storing a file in a smaller amount of space than would usually be required for the file, an advantage when saving larger-sized files.

Two types of compression have been developed to store data in a smaller package: lossless compression and lossy compression. With lossless compression, there is no loss of data in the file during the storage process. When a lossless compression file is opened, all of the data reconstructed is identical to the data saved.

Familiar zip files, such as WinZip and pkzip, use lossless compression techniques, such as the Lempel-Ziv technique. Based on an algorithm, the Lempel-Ziv technique compiles an alternate list of character strings and codes, which are created from the original data, and temporarily replaces the original data with this alternate text, which requires fewer characters, and therefore, less space. Another lossless compression technique, called run-length encoding, replaces strings of identical characters with the total number of identical characters in the string, reducing the amount of data to be saved. The processes involved in the Lempel-Ziv technique and run-length encoding are reversed when the related compressed files are opened.

Lossy compression techniques require loss of a small amount of data. Lossy compression is commonly used in MP3 (MPEG, Motion Picture Experts Group, Audio Layer 3) files and JPEG (Joint Photographic Experts Group) files. Through the use of perceptual noise shaping, or perceptual encoding, in MP3 files, sounds beyond the range of the human ear are removed from audio files prior to storage, which reduces the size of the files. Since less audio data has been saved, the newly created file can be played at a slower speed than the original audio, to create the same sound, another advantage of this compression technique.

Video file compression techniques typically remove pixels from areas in which a single color seems to occupy a lot of space. This occurs in three steps. In the first step, the video data is broken into blocks. Once in the blocks, in step two, some data in adjacent areas is changed to identical value data (a slightly different shade of a color in one pixel may be changed to reflect the same shade as the pixel next to it). In the third step, the run-length encoding technique is used to replace the strings of identical characters with the total number of identical characters involved, reducing the total amount of data saved. Since data is actually changed in a video lossy compression technique, some original data is lost.

Whether lossless or lossy compression is used, zip files, including MP3 and JPEG files, have increased the ease of storing large files. Low-cost and free zip file applications have helped to increase usage of this popular technique.

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