The Mot-valise codec comes from “ Co deur- DEC odor” and indicates a process able to compress or decompress a Signal, Analogique or Numérique, in a Format of data. This process can exist in material form or Logiciel it. For example, in the middle of the 20th century, word codec was used to indicate a type of material making it possible to code an analogical signal in PCM and to decode it in return.
The codecs encodent flows or signals for the transmission, storage or the encoding of Donnée S. Of another dimensioned, they decode these flows or signals for edition or visionnage.
The original intention of the codecs is to be able to treat a maximum of data with a minimum of resources. They are used for applications like telephony, the videoconferences or the diffusion of media on Internet.
Types of codecs
There exists a multitude of codecs, but they can be divided into two big families according to their manner of compressing the data. Indeed, one distinguishes two types of compression.
the nondestructive compression (one also says without losses or lossless ) makes it possible to find the initial signal such as it was before coding.
the destructive compression (with losses or lossy ) takes into account the characteristics of the data to be compressed and can withdraw information being able to be considered “nonrelevant”; for example, information nonperceptible by the human being is withdrawn from the formats JPEG and Vorbis.
Compression Vorbis for example, compress the following sound of the “psycho-acoustic” criteria taking of account the not or not very audible Frequency S of the sound Spectrum such as the very acute Harmonic S and frequencies. It is a “destructive” compression, because it loses sound information. The codecs video MPEG, DivX or XviD use algorithms taking of account the retinal Persistance, the differentiation of the Couleur S by the eye and other “imperfections” of the sight to remove details which the human being almost does not perceive in normal weather. They are also compressions known as destructive.
Codecs, standards and containers
The codec is often a word pass key and errors are often made. First of all, one should not confuse codec and standard or Format of data. The standard defines the manner of treating the data, the codec implements this manner. It is said that the codec implements the standard. For example, MPEG-4 AVC/H.264 is a video standard, whose implementation is the codec X264 but there is the different one.
A second current error is to confuse the codecs and the containers. As its name indicates it, a container contains flows Audio and/or Vidéo which is encodés with a codec. Each container accepts some codecs audios and vidéos, and has characteristics which are clean for him (Sous-titrage, chapitrage…). The container is often determined by the extension of the files.
Codecs of images
Principal the codecs of compression of images is:
See also: audio Format
Principal the audio codecs is:
- PCM, without loss but noncompressed
- FLAC, free format of rights, compression without loss
- Ogg Vorbis, free format of rights, compression with loss
- ADPCM, weak compression with loss
- MPEG-1/2 To bush-hammer 3 (MP3)
- MPEG-2 left 7 (AAC)
- Sony ATRAC dolby AC3
- LCconcept SDR
- Windows Audio Media (WMA)
- Musepack (MPC)
- Speex for the voice
- G.711 for the voice (standard telephony or ISDN, and VoIP), based on PCM
- G.723 for the voice (VoIP)
- G.726 for the voice (satellite telephony, and VoIP), based on ADPCM
- G.729 for the voice (telephony of quality, visionconférence, and VoIP)
- G.722 for the voice broad-band
- G.722.2 for the voice broad-band
- G.729.1 for the voice broad-band
- ILBC for the voice (VoIP)
Principal the video codecs is:
- MPEG-1, which is not used any more, unless having some VCD.
- MPEG-2, which is the codec used in particular by DVD video standard, the terrestrial Digital television (TNT, or DVB-T) and the numerical diffusion by satellites (DVB-S).
- implementations of MPEG-4 ASP, like DivX and XviD
- MJPEG, for the video assembly image by image
- MJPEG2000, evolution of MJPEG for the numerical cinema, the imagery of precision (medical, space…)
- implementations of MPEG-4 AVC/H.264: X264, ATEME.
- Theora, free codec and open source
- Tarkin, free codec, put outstanding at the profit of Theora
- Video DIGITAL or FD, used in many video cameras with band.
- H.261, H.263, H.263+, H.264 for the videoconference on IP (see H.323 and SIP).
With not confusing: Codecs, Standards and Marks
Certain manufacturers of electronic materials develop to them-even audio and/or video codecs. One will be able to quote the Avchd for example, which is in fact a derivative of the X264, him even derived from H.264; but having certain functionalities which are specific to the manufacturers who use it or the editor of the software.
Some codecs derived from a standard become them-even thereafter a standard; for example DivX, which stays a derivative of the H.263 and H.264 became sufficiently different, with the wire of time, so that it now constitutes a true codec with whole share.
All the codecs are not describe by standards, but become, sometimes and in fact, the standards.
The originality between two codecs is sometimes fuzzy and badly defined. Often resulting from a Fork, two versions of the same codec can succeed, in the final analysis, with completely incompatible codecs between them (although functioning on the same principle).
Database on the codecs
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