external stress applied to an object or substance, tending to cause a decrease in volume (see pressure
). Gases can be compressed easily, solids and liquids to a very small degree if at all. Water, for example, is practically incompressible, thus making it especially useful for hydraulic machines
. According to the kinetic-molecular theory of gases
, when the molecules of a gas are brought close enough together by compression, the gas (under certain conditions of temperature) undergoes liquefaction
. This principle is applied commercially to several gases, including liquid oxygen and the so-called bottled gas (a mixture of hydrocarbons) used as a fuel. Boyle's law deals with the decrease in the volume of a gas in relation to the increase of pressure upon it (see gas laws
). The ability or the degree to which an internal-combustion engine reduces the volume of its fuel mixture preparatory to firing is called its compression. Also, a region of high pressure in a fluid is called a compression; thus sound waves are said to propagate at compressions and rarefactions (regions of low pressure) of their medium, such as air.
The Columbia Electronic Encyclopedia™ Copyright © 2022, Columbia University Press. Licensed from Columbia University Press. All rights reserved.
Direct pushing force, in line with the axis of the member: the opposite of tension.
Illustrated Dictionary of Architecture Copyright © 2012, 2002, 1998 by The McGraw-Hill Companies, Inc. All rights reserved
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.
(in Russian, kompressiia), a force effect on a gaseous body that reduces the volume occupied by the gas and increases its pressure and temperature. Compression is achieved by means of compressors and during the operation of internal combustion engines and other equipment.
Russian technical literature usually uses the word szhatie instead of kompressiia, although the meaning of the former is more general, since it also covers solids and volume reduction upon cooling.
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
Reduction of the effective gain of a device at one level of signal with respect to the gain at a lower level of signal, so that weak signal components will not be lost in background and strong signals will not overload the system.
A system of forces which tend to decrease the volume or shorten rocks.
Reduction in the volume of a substance due to pressure; for example in building, the type of stress which causes shortening of the fibers of a wooden member.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
1. The state of being compressed, or being shortened by a force.
2. The change in length produced in a test specimen by a compressive load.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.
The ratio of the difference between the equatorial radius (major semi-axis) and the polar radius (minor semi-axis) of the earth to the equatorial radius. Also called compression. The flattening of the earth is the ellipticity of the spheroid, and it equals the ellipticity of the ellipse forming a meridianal section of the spheroid. If a and b represent the major and minor semi-axes of the spheroid, respectively, and f is the flattening of the earth, f= (a – b)/a. The magnitude of the flattening is sometimes expressed by stating the numerical value of the reciprocal of the flattening, a/(a – b).
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved
Engineering an increase in pressure of the charge in an engine or compressor obtained by reducing its volume
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
(Or "compaction") The coding of data to save
storage space or transmission time. Although data is already
coded in digital form for computer processing, it can often be
coded more efficiently (using fewer bits). For example,
replaces strings of repeated characters
(or other units of data) with a single character and a count.
There are many compression algorithms and utilities.
Compressed data must be decompressed before it can be used.
The standard Unix
compression utilty is called compress
's superior gzip
has largely replaced it. Other
compression utilties include pack
When compressing several similar files, it is usually better
to join the files together into an archive
of some kind
for example) and then compress them, rather than
to join together individually compressed files. This is
because some common compression algorithms build up tables
based on the data from their current input which they have
already compressed. They then use this table to compress
subsequent data more efficiently.
See also TIFF
, Lempel-Ziv Welch,
.Web Content Compression FAQ
Reducing the dynamic range of an audio signal,
making quiet sounds louder and loud sounds quieter. Thus,
when discussing digital audio, the preferred term for reducing
the total amount of data is "compaction". Some advocate this
term in all contexts.
This article is provided by FOLDOC - Free Online Dictionary of Computing (foldoc.org)
archive programSoftware that compresses multiple files and folders into a single file (the "archive"). If a folder or a hierarchy of folders is compressed, the file/folder relationship is maintained within the archived file, and it is restored when the archive is decompressed back to its original state. Although there are numerous compression formats, many archive programs (archivers) support the very popular ZIP format. See archive, archive formats, ZIP file and data compression.
data compressionThere are two categories of data compression. The first reduces the size of a single file to save storage space and transmit faster. The second is for storage and transmission convenience.
#1 - Compressing a Single File
The JPEG image, MPEG video, MP3 audio and G.7xx voice formats are widely used "lossy" methods that analyze which pixels, video frames or sound waves can be removed forever without the average person noticing (see lossy compression). GIF images have no loss of pixels but may have a loss of colors (see GIF).
JPEG files can be reduced as much as 80%; MPEG enables a two-hour HD movie to fit on a single disc, and MP3 sparked a revolution by reducing CD music 90%. For a list of compression methods, see codec examples. See JPEG, GIF, MPEG, MP3, G.7xx and interframe coding.
#2 - Compressing a Group of Files (Archiving)
The second "lossless" category compresses and restores data without the loss of a single bit. Although this is widely used for documents, this method is not aware of the content's purpose. It merely looks for repeatable patterns of 0s and 1s, and the more patterns, the higher the compression ratio. Text documents compress the most, while binary and already-compressed files (JPEG, MPEG, etc.) compress the least.
Although lossless methods such as the ZIP format are used to reduce the size of a single, huge file, they are widely used to compress several files into one "archive." It is convenient to store and considerably more convenient to transmit a single file than to keep track of multiple files. See lossless compression, archive, archive formats and capacity optimization.
Lossless Methods (Dictionary and Statistical)
The widely used dictionary method creates a list of repeatable phrases. For example, GIF images and ZIP and JAR archives are compressed with this method (see LZW). The statistical method converts characters into variable length strings of bits based on frequency of use (see Huffman coding).
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