Codec

Related concepts
An endec is a similar yet different concept mainly used for hardware. In the mid 20th century, a "codec" was hardware that coded analog signals into Pulse-code modulation (PCM) and decoded them back. Late in the century the name came to be applied to a class of software for converting among digital signal formats, and including compander functions.

Codecs (in the modern, software sense) encode a stream or signal for transmission, storage or encryption and decode it for viewing or editing. Codecs are often used in videoconferencing and streaming media applications. A video camera's analogue-to-digital converter (ADC) converts its analogue 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, then a digital-to-analogue converter (DAC) for analogue display. A "codec" is a generic name for a video conferencing unit.

An audio compressor converts analogue audio signals into digital signals for transmission or storage. A receiving device then converts the digital signals back to analogue using an audio decompressor, for playback. An example of this are the codecs used in the sound cards of personal computers.

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.


Compression quality
Most codecs are lossy, allowing the compressed data to be made smaller than otherwise. This aids transmission across networks and storage on relatively expensive media, such as non-volatile memory and hard disk, as well as write-once read-many formats such as CD-ROM and DVD.

There are also lossless codecs, but for most purposes the slight increase in quality might not be worth the increase in data size, which is often considerable. The main exception to this is if the data is to undergo further processing (for example editing) in which case the repeated application of lossy codecs (repeated encoding and subsequent decoding) will almost certainly degrade the quality of the edited file such that it is readily identifiable (visually or audibly or both). Using more than one codec or encoding scheme whilst creating a finished product can also degrade quality significantly (however there are many situations where this is all but unavoidable). The decreasing cost of storage capacity and network bandwidth may obviate the need for lossy codecs for some media over time.

Codecs are often designed to emphasise certain aspects of the media to be encoded. For example, a digital video (using a DV codec) of a sports event, such as baseball or soccer, needs to encode motion well but not necessarily exact colours, while a video of an art exhibit needs to perform well encoding colour and surface texture. There are hundreds or even thousands of codecs ranging from those downloadable for free to ones costing hundreds of dollars or more. This can create compatibility and obsolescence issues. By contrast, lossless PCM audio (44.1 kHz, 16 bit stereo, as represented on an audio CD or in a .wav or .aiff file) offers more of a persistent standard across multiple platforms and over time.

Many multimedia data streams need to contain both audio and video data, and often some form of metadata that permits synchronisation of 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 in a container format.

The widely spread notion of AVI being a codec is incorrect as AVI (nowadays) is a container format, which many codecs might use (although not to ISO standard). There are other well known alternative containers such as Ogg, ASF, QuickTime, RealMedia, Matroska, DivX, and MP4.