what are codecs?

Answer 1

Short for compressor/decompressor, a codec is any technology for compressing and decompressing data. Codecs can be implemented in software, hardware, or a combination of both. Some popular codecs for computer video include MPEG, Indeo and Cinepak. In telecommunications, (short for coder/decoder) a device that encodes or decodes a signal. ...

There is another definition of Codec i.e.
It is an abbreviation for Coder-Decoder. It's an analog-to-digital (A/D) and digital-to-analog (D/A) converter for translating the signals from the outside world to digital, and back again.

Answer 2

A codec is a device or program capable of performing encoding and decoding on a digital data stream or signal. The word codec may be a combination of any of the following: 'Compressor-Decompressor', 'Coder-Decoder', or 'Compression/Decompression algorithm'.

An endec is a similar (but not identical) concept for hardware. In the middle 20th Century a "codec" was hardware that coded analog signals into Pulse-code modulation and decoded it 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 ADC converts its 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, then a DAC for analog display. A "codec" is a generic name for a video conferencing unit.

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. Originally this was in order to achieve a relatively small (compressed) file size such that the resultant files could be readily exchanged (across non-broadband networks) and stored on relatively expensive media (non-volatile memory, hard-disk storage, etc., as opposed to write-once read-many formats such as CD-ROM and DVD).

There are lossless codecs but for most purposes the slight increase in quality (some would say almost imperceptible increase) may not be worth the often considerable increase in data size. The main exception to this is if the data is to undergo any future 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).

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 event, such as baseball or soccer, needs to encode motion well but not necessarily exact colors, while a video of an art exhibit needs to perform well encoding color 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.

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 in a container format.

While many people explain that AVI is a codec, they are incorrect. 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 and MP4.