A codec is a software devised for encoding and decoding a digital data stream signal. The word codec itself is a combination of compressor-decompressor, or coder-decoder.

Codecs encode a stream or signal for transmission, storage or encryption and decode it for viewing or editing. Codecs are often used in streaming solutions and personal media storage, due to the fact of the compression factor they provide, which often maintains a very reasonable overall audio and video quality.

Most codecs are lossy, in order to get a reasonably small file size (for example, DivX, WMV, etc). There are lossless codecs as well, but for most purposes the almost imperceptible increase in quality is not worth the considerable increase in data size. The main exception is if the data will undergo more processing, especially editing, in the future, in which case the repeated lossy encoding could degrade the quality of the eventual file too much. Using more than one codec or encoding scheme throughout processing can also degrade quality but there are many situations where this cannot be avoided.

  Common Video Codecs and Standards

Below are listed the most widely-used video codecs with a brief description.

• H.261: Used primarily in older videoconferencing / videotelephony products. Developed by the ITU-T group, this was the first practical digital video compression standard. Essentially all subsequent standard video codec designs are based on it. It included such well-established concepts as YCbCr color representation, the 4:2:0 sampling format, 8-bit sample precision, 16x16 macroblocks, block-wise motion compensation, 8x8 block-wise discrete cosine transformation, zig-zag coefficient scanning, scalar quantization, run+value symbol mapping, and variable-length coding.

• MPEG-1 Part 2: Used for Video CDs, and also sometimes for online video. The quality is roughly comparable to that of VHS. If the source video quality is good and the bitrate is high enough, VCD can look better than VHS, and all in all very good, but VCD requires high bitrates for this. However, to get a fully compliant VCD file, bitrates higher than 1150 kbit/s and resolutions higher than 352 x 288 should not be used. When it comes to compatibility, VCD has the highest compatibility of any digital video/audio system. Almost every computer in the world can play this codec, most DVD players support it.

• MPEG-2 Part 2: Used on DVD, SVCD, and in most digital video broadcasting and satellite TV distribution systems. When used on a standard DVD, it offers good picture quality and supports widescreen. When used on SVCD, it is not as good as DVD but is certainly better than VCD. In terms of technical design, the most significant enhancement in MPEG-2 relative to MPEG-1 was the addition of support for interlaced video. MPEG-2 is now considered an aged codec, but has tremendous market acceptance and a very large installed base.

• H.263: Used primarily for videoconferencing, videotelephony, and internet video. H.263 represented a significant step forward in standardized compression capability for progressive scan video. Especially at low bit rates, it could provide a substantial improvement in the bit rate needed to reach a given level of fidelity.

• MPEG-4 Part 2: An MPEG standard that can be used for internet, broadcast, and on storage media. It offers improved quality relative to MPEG-2 and the first version of H.263. Its major technical features beyond prior codec standards consisted of object-oriented coding features and a variety of other such features not necessarily intended for improvement of ordinary video coding compression capability. It also included some enhancements of compression capability, both by embracing capabilities developed in H.263 and by adding new ones such as quarter-pel motion compensation. Like MPEG-2, it supports both progressive scan and interlaced video.

• MPEG-4 Part 10: A technically aligned standard with the H.264, also referred to as AVC. This emerging new standard is the current state of the art of ITU-T and MPEG standardized compression technology, and is rapidly gaining adoption into a wide variety of applications. It contains a number of significant advances in compression capability, and it has recently been adopted into a number of company products, including the PlayStation Portable, iPod, the Nero Digital product suite, Mac OS X v10.4, as well as HD DVD/Blu-ray Disc.

• DivX, Xvid, and 3ivx: Different implementations of the MPEG-4 Part 2 standard.

• VP6: TrueMotion VP6, is a video codec developed by On2 Technologies as a successor to earlier efforts such as VP3 and VP5. The VP6 codec has been used in products for broadcasting in the field, such as with BBC reporters and QuickLink software. Currently, On2 VP6 is the video compression technology that Macromedia/Adobe licensed for its Flash 8 family of products. 

• Sorenson 3: A codec that is popularly used by Apple's QuickTime, basically the ancestor of H.264. Many of the QuickTime movie trailers found on the web use this codec.

• Theora: Developed by the Xiph.org Foundation as part of their Ogg project, based upon On2 Technologies' VP3 codec, and christened by On2 as the successor in VP3's lineage, Theora is targeted at competing with MPEG-4 video and similar lower-bitrate video compression schemes, but without significant success.

• WMV (Windows Media Video): Microsoft's family of video codec designs including WMV 7, WMV 8, and WMV 9. It can do anything from low resolution video for dial up internet users to HDTV. It is widely used on Media Centre PCs. WMV can be viewed as a version of the MPEG-4 codec design.

• RealVideo: Developed by RealNetworks and aimed for streaming media over Internet. A popular codec technology a few years ago, now fading in importance and popularity for a variety of reasons.

• Cinepak: A very early codec used by Apple's QuickTime.

All of the codecs above have their qualities and drawbacks. The tradeoff between bit rate and fidelity (including artifacts) is usually considered the most important figure of technical merit.

  The DivX Codec

This codec allows to compress a complete DVD to a fraction of its original size. DivX video compression is so efficient you can fit the entire contents of a DVD on a regular data CD with virtually no loss in quality. Now you can take any digital video content, from home movies to your personal DVD collection1 and save it on CDs to share with friends or store it on your computer's hard drive without spending huge amounts on extra storage.

Traditionally, video on the Internet was grainy, low resolution and postage stamp-sized. Today all that is changing. Since the introduction of DivX® video technology five years ago, video content providers finally have a real solution to deliver their creations to the world without sacrificing visual quality or shelling out major cash for extra bandwidth. Millions of people worldwide are already using their standard high speed connections to download feature-length movies from the Internet in less than the time it takes to watch them.

Not only has DivX® video technology enabled the fast and secure delivery of high-quality, highly compressed digital video, it also frees digital video content from the confines of the computer monitor by allowing viewers to easily transfer it to a certified device and enjoy it right on the living room TV. With the full range of DivX® Certified devices flooding the market since early 2006, you can now easily take that DivX movie created on your computer, pop it into your DivX Certified DVD player and enjoy it on your big screen TV. The DivX codec even lets you make movies specifically for your favorite device, be it Portable, High Definition, Handheld or Home Theater.

• Licence: Propietary

• Website: divx.com

  The XviD Codec

Xvid enables very high compression rates that allow to effectively work with digital video on home computers, just like the DivX codec does. To give an example: uncompressed digital video is huge and requires about 100 GB per hour at PAL resolution. The same video would require just 500 MB per hour at very high quality when compressed with Xvid. That is a compression ratio of 200:1.

Xvid is Free Software and released under the GNU GPL license. This means that the source code of the software is publically available and programmers are allowed to make modifications to the code. Also, redistribution of Xvid is permitted under the terms of the GPL license.

So the GPL grants recipients more freedoms than it’s common with other software. Therefore, and due to its very high image quality that has been approved in independent tests conducted by third parties the Xvid codec has gained great popularity in the recent past and has become the codec of choice when it comes to exchanging digital video.

Advantages

The most clear XviD advantages over its competitor codec, DivX, is the fact of being distributed under the GNU GPL licence, this means the codec does not have time or feature limitations, and any programmer can get its source code to review it freely. Additionally, the XviD codec might provide faster video encoding and decoding speeds, while maintaining a superior and award-winning image quality.

• Licence: GNU GPL

• Website: xvid.org

  Questions? Comments?

If you have any questions or need support regarding this guide or article, don't hesitate to ask at the DivXLand.org forum personally.

  Related Topics

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Introduction to container formats

MPEG format specifications for DVD and VCD