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coder-decoder (CODEC)

The artificial word codec stands for coder and decoder, but also for compression and decompression. Codecs are software- or hardware-based functional units that convert incoming analog signals into digital signals and, after digital signal processing( DSP) with data compression, convert the resulting data stream back into analog signals. Codecs handle signal conversion in both directions, unlike A/D converters and D/A converters, which convert signals from one signal domain to another.

All codecs work with sample-and-hold circuits. They sample the analog signals in discrete time and convert the sampled signals to digital signals in A/D converters. The signal conversion and the subsequent compression are usually subject to losses. These losses are caused by the reduction of the dynamic range of the analog signals and by the data compression of the digital signals. The lossy processes inevitably lead to a reduction in the quality of the audio and video signals when the digital data stream is converted back into analog signals. However, the data reduction reduces the transmission bandwidth and the required memory.

Since codecs are used in real-time applications for voice transmissions, audio and video, the sampling rates and transfer rates are important parameters. According to their application areas, codecs can be divided into voice codecs, audio codecs and video code cs.

Codecs in real-time applications

Structure of a codec

Structure of a codec

In voice, audio and video technology, there are various codecs standardized by the International Telecommunication Union( ITU), which are described in ITU Recommendations H.261, H. 263, H. 320, H. 321, H. 323 and others. In addition, there are many codecs standardized by other institutions that are used in compact discs( CD), DVDs, Blu-Ray discs, in video and Internet telephony( VoIP), in Internet radio and Internet television( IPTV), or in players and game consoles.

Among video codecs, a distinction is made between intra- frame codecs and inter-frame codecs. The former try to erase as much information as possible from a single frame; the codecs that use the inter-frame method compare two consecutive frames and determine the difference between the two. This method is mainly used for video.

Quality differences of the various algorithms

Since codecs work with different compression algorithms, there can be strong differences in quality in terms of picture or sound quality, which is expressed in terms of continuity in display and transmission.

The most important hardware codecs for video are Motion- JPEG, Px64 according to H.261 and H.320, and MPEG in its different variants. Software codecs include Cinepak, Indeo, DivX, Quicktime, Video for Windows and Windows Media Video(WMV). Important audio codecs are aacPlus, mp3pro, Ogg Vorbis, RealAudio, Windows Media Audio, QDesign and others.

Because codecs generally contain many patents and proprietary knowledge, few details about their algorithms and processes are often published.

Englisch: coder-decoder - CODEC
Updated at: 09.11.2013
#Words: 449
Links: word, decoder (DEC), compression, decompression, software (SW)
Translations: DE

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