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What is Data Compression?

R. Kayne
R. Kayne

Data compression is a general term for a group of technologies that encode large files in order to shrink them down in size. Smaller files take up less room, leaving more storage space. They are also faster to transfer over a network, whether that network is the Internet, an intranet, or a local area network (LAN).

In the 1970s, various techniques were available to archive files, or place them together in a single package to avoid sending multiple files between computers. The idea was soon augmented with data compression techniques, so the term “archive” is now often used to describe a compressed file.

Music files often are compressed so that listeners can fit more songs onto devices.
Music files often are compressed so that listeners can fit more songs onto devices.

Compressing data involves applying an algorithm that makes some of the repetitive bits unnecessary. It's a bit like a kind of shorthand map that gets stored with the compressed file. When decompressed, the map restores all of the missing bits, reconstituting the complete file. This technique can be used with text, graphics, executable programs, and multimedia files, though some types of files compress better than others.

Today’s most commonly recognized data compression technique was originally used in the DOS operating system prior to Microsoft Windows™ becoming ubiquitous in the mid-1990s. Author Phil Katz eventually termed these compressed files zipped files — the idea being that when the files were unzipped (decompressed), the full contents “popped” out. Files used with this method have the extension, .zip.

Files that are extremely large even when compressed can be split into pieces before being sent over a network. The pieces are collected and reassembled on the receiving end. The leading compression technique for large files, also from the days of DOS, is called RAR, after author Eugene Roshal. Programs that support Roshal ARchive files can create a set of RARs from a large multimedia file, for example, or decompress an existing RAR to reassemble a movie or program. These files have the extension .rar, or for multi-part files, part01.rar, part02.rar or .r01 r02.

Various music formats also use other data compression techniques to shink the files while maintaining as much of the original quality as possible. The most obvious example is the .mp3 format. In this case, however, the compressed file is not an archive and cannot be decompressed. The bits that are removed to achieve the smaller file size are gone for good. Other techniques used for music files retain more quality but also result bigger file sizes.

Data compression programs are widely available online. A zipped or RAR'd file requires a program that supports that method to unzip or unRAR it. Most programs support multiple types of compressed files, and many of these programs are freeware.

Discussion Comments


@Terrificli -- while pirates do love compressing movies and songs to smaller files so they can be shared on the Internet, those technologies don't benefit just people wanting to download entertainment and not pay for it. For example, most old time radio (OTR) programs are available on the Internet in MP3 format. Often, we're talking about shows such as "The Jack Benny Program" or "Burns and Allen" that are effectively public domain and might have been lost had they not been converted to digital formats that don't deteriorate. The MP3 format, then, has been essential in preserving those old shows. No illegality there.

Also, there are a good number of independent bands and filmmakers that have taken advantage of file compression to share their creations easily with others.

Indeed, there is a problem when compression is used to share files illegally, but none when it is used to make sharing files that are public domain or compressed and made available by the people who own the material or have the right to use it. It all goes to the intention of the user as to whether an MP3, MP3, AVI or anything else is legal or not.


@Soulfox -- yes, you will lose some video quality with those formats. That is because MP4, AVI and several others save space the same way the MP3 format does. They are called "lossy" formats because some information is stripped out in an attempt to make files small enough to download. There are some digital formats that are not lossy, but they result in large files and are rarely used.

Here's the good news, thought. The loss in quality is hard to detect by most people. You get a picture and sound that is, in most cases, more than good enough so people don't mind the loss in quality.

That is exactly why piracy is so rampant these days -- big old movies and songs can be shrunk down to sizes that are manageable and downloadable.


The same is true of video. When you see file extensions such as MP4 or AVI, you are looking at a file that carries video and is considerably smaller than the original.

Here's a question. This article points out how MP3 files are audio files that are smaller than the originals but sacrifice quality for size. Is the same true of MP4, AVI and other file formats that carry video?

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    • Music files often are compressed so that listeners can fit more songs onto devices.
      By: Jürgen Fälchle
      Music files often are compressed so that listeners can fit more songs onto devices.