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Video compression formats explained

Divx and Xvid

divxDivx is currently the most popular video compression format. It can reduce the size of video files without having a detrimental effect on quality, and hence is useful for transferring video files across the internet. It is also a useful format for backing up DVD-videos - a standard 4.7GB DVD can contain four or more full DVD movies if they are compressed using the Divx codec. Some of today's DVD players will support files burnt to disc using DivX compression - you will need to check to see if yours does. If you need to, you can buy a DivX compatible DVD player from Amazon for under £30.

The DivX codec is not included with Windows, so your version of Windows Media Player may not play DivX encoded files. This is because, allegedly, Microsoft did not want to people to be able to use DivX to compress movies. The Microsoft video compression standard uses the Advanced Systems Format (ASF) as a container for compressed files of the form MPEG-4, Windows Media Audio (WMA) and Windows Media Video (WMV). However, a French hacker, Jerome Rota, cracked a version of the MPEG-4 format and modified it to create an AVI file. (A file encoded in DivX has the file extension .avi). Rota was subequently employed by Jordan Greenhall to form the DivX company. DivX is thus a direct competitor to Windows ASF.

The Divx software itself is an application that uses the MPEG-4 standard to compress digital video.

If you need to download the DivX codec, it is freely available from Divx.com although you will have to pay if you want the application that actually encodes your MPEG video to the avi format. There are also many other applications available from sources other than DivX that will convert video data from one format to another. Try a seach on "video conversion" to see some of the software currently available.

xvidXvid (DivX spelt backwards) has the same roots as Divx but is now an open source video compression format, which works in a similar way to DivX. The main differences are that Xvid is supported by Linux and other operating systems, although it is not as efficient as DivX 5.x in terms of quality and compression.

All of the above compression formats are lossy. Lossy compression is a compression format that loses some of the original data when it is compressed, so that when the data is decompressed it is slightly different, although very close to the original. MP3 is a lossy compression format for wav files, and jpeg is a lossy compression format for bmp (bitmap) files.

Codecs

A video codec is a program that is used to compress or decompress (ie. play) video files.

Video Compression Standards

MPEG stands for "Moving Pictures Expert Group", which was a group established in 1988 to define compression standards for digital video and audio formats. There are three standards currently in use , and two in the pipeline. They are:

  • MPEG-1 - The standard compression format for audio and video, characterised by the .mpg file extension. Level 3 of MPEG-1 is the popular audio compression format known as MP3.
  • MPEG-2 - Used for DVD compression and digital set top boxes. Can also be used for HDTV.
  • MPEG-4 - As mentioned above, the standard compression format for files transferred across the internet.

The two new formats under development are MPEG-7 and MPEG-21. Both are concerned with Multimedia Content.

VOB Files

VOB stands for DVD Video Object. It the file type that is found on a DVD movie disc and contains the video data. Basically, it is a multiplexed MPEG-2 video, audio and subtitle stream.