| Codec is a portmanteau of "Compressor-Decompressor",
which describes a device or program capable of performing transformations on a data stream or
signal. Codecs can both put the stream or
signal into an encoded form (often for transmission, storage or encryption) and retrieve, or decode that form for viewing or
manipulation in a format more appropriate for these operations. Codecs are often used in videoconferencing and streaming
media solutions.
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 get a reasonably small file size. 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 in
the future, in which case the repeated lossy encoding would damage the eventual quality too much.
Many multimedia data streams need to 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, they must be encapsulated
together. This function is carried out by a video file format (or
container), such as *.mpg, *.avi, *.mov, *.mp4, *.rm, *.ogg or *.tta. Some of these formats are limited to containing streams conforming to a small fixed set of codecs, while others
are intended to be more general purpose.
An endec is a similar (but not identical) concept for hardware.
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