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A codec is a device or computer program capable of encoding and/or decoding a digital data stream or signal. The word codec is a portmanteau (a blending of two or more words) of 'compressor-decompressor' or, more accurately, 'coder-decoder'.
Historically a modem was a contraction of modulator/demodulator (modem was called dataset by telcos) and converted digital data from computers to analog for phone line transmission. On the receiving end the analog was converted back to digital. CODECs did the opposite (convert audio analog to digital and then computer digital sound back to audio).
There was no compression involved in CODECs, only coding and decoding.
An endec (encoder/decoder) 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.
A codec encodes a data stream or signal for transmission, storage or encryption, or decodes it for playback
or editing. Codecs are used in videoconferencing and streaming media applications. A video camera's analog-to-digital
converter (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 digital-to-analog converter (DAC) for analog display. The term codec is also used as 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. An example of this are the codecs used in the sound cards of personal computers.
* Lossy codecs: Many of the more popular codecs in the software world are lossy, meaning that
they reduce quality by some amount in order to achieve compression, but use some algorithm to create
the impression of the data being there. Smaller data sets ease the strain on relatively expensive storage
sub-systems such as non-volatile memory and
hard disk, as well as write-once-read-many formats such as CD-ROM, DVD and Blu-ray Disc
* Lossless codecs: There are also many lossless codecs which are typically used for archiving data in a compressed form while retaining all of the information present in the original stream. If preserving
the original quality of the stream is more important than eliminating the correspondingly larger data sizes,
lossless codecs are preferred. This is especially true if the data is to undergo further processing (for example editing) in which case the repeated application of processing (encoding and decoding) on lossy codecs will degrade the quality of the resulting data such that it is no longer identifiable (visually, audibly or both). Using more than one codec or encoding scheme successively can also degrade quality significantly. The decreasing cost of storage capacity and network bandwidth has a tendency to reduce the need for lossy codecs for some media.
Codecs are often designed to emphasize certain aspects of the media, or their use, 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.
Audio codecs for cell phones need to have very low latency between source encoding and playback; while audio codecs for recording or broadcast can use high-latency audio compression techniques to achieve higher fidelity at a lower bit-rate.
There are thousands of audio and video codecs ranging in cost from free to hundreds of dollars or more. This variety of codecs can create compatibility and obsolescence issues. By contrast, raw uncompressed PCM audio (44.1 kHz, 16 bit stereo, as represented on an audio CD or in a .wav or .aiff file) is a standard across multiple platforms.
Many multimedia data streams contain both audio and video, and often some metadata that permit synchronization of audio and video. Each of these three streams may be handled by different programs, processes, or hardware; but for the multimedia data streams to be useful in stored or transmitted form, they must be encapsulated together in a container format.
Lower bit rate codecs allow more users, but they also have more distortion. Beyond the initial increase in distortion, lower bit rate codecs also achieve their lower bit rates
by using more complex algorithms that make certain assumptions, such as those about the media and the packet loss rate. Other codecs may not make those same assumptions. When a user with a low bit-rate codec talks to a user with another codec, additional distortion is introduced by each transcoding.
The notion of AVI being a codec is incorrect as AVI is a container format, which many codecs might use (although not to ISO standard). There are also other well-known containers such as Ogg, ASF, QuickTime, RealMedia, Matroska, DivX and containers defined as ISO standards, such as MPEG Transport Stream, Program Stream, MP4 and ISO base media file format.
The article is based on materials from matroska.org, wikipedia.org.