Windows Media Player

A codec is a device or software program that encodes and decodes digital data streams or signals. Like the word modem, which is a shortened form of modulator-demodulator, the word codec is a portmanteau of any of the following: Compressor-Decompressor, Coder-Decoder, or Compression/Decompression algorithm.

Codecs encode a stream or signal for transmission, storage, or encryption and decode it for viewing or editing. They are often used in videoconferencing and streaming media. A video camera's analog-to-digital converter (ADC), for example, converts analog signals into digital signals, which are then passed through a video compressor for digital transmission or storage. A receiving device then runs the signal through a video decompressor and a digital-to-analog converter (DAC) for analog display. An audio compressor converts analog audio signals into digital signals for transmission or storage. A receiving device then converts the digital signals back to analog using an audio decompressor for playback.

The raw encoded form of audio and video data is often called essence, to distinguish it from the metadata information that together make up the information content of the stream and any "wrapper" data that is then added to aid access to or improve the robustness of the stream.

Most codecs are "lossy," in order to create reasonably small file sizes. Although lossless codecs exist, 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 future processing, especially editing, in which case the repeated lossy encoding degrades the quality of the eventual file to unacceptable levels. Using more than one codec or encoding scheme throughout processing can also degrade quality, but there are many situations where this cannot be avoided. Many codecs are designed to emphasize certain aspects of the media to be encoded. For example, a digital video (using a DV codec) of a sports events like baseball or soccer need to encode motion well but not necessarily exact colors, whereas a video of an art exhibit needs to encode color and surface texture very well. There are hundreds or even thousands of codecs ranging from free ones to ones costing hundreds of dollars or more.

Many multimedia data streams contain both audio and video data and often some form of metadata that permits synchronization of the audio and video. Each of these three streams may be handled by different programs, processes, or hardware, but for the multimedia data stream to be useful in stored or transmitted form, it must be encapsulated in a "container" format.

While many people claim AVI is a codec, they are incorrect. AVI (nowadays) is a container format, which many codecs might use (although not to ISO). There are other well known alternative containers such as Ogg, ASF, QuickTime, RealMedia and MP4.