A codec (syllable word from English coder, German encoder, and decoder, German decoder) is a pair of algorithms that digitally encodes and decodes data or signals.
A codec encodes a data stream or signal for transmission, storage or encryption, or decodes it for playback or processing. Codecs are used in video conferencing, streaming media and video editing applications.
As with conventional telephony, VoIP initially captures speech in analog form with a microphone. This analog information is then transferred by a converter into a digital format and converted into corresponding audio binary formats via codecs. Depending on the codec used, the data can be compressed to different degrees.
Most codecs use a procedure in which - similar to MP3 files - unimportant information for the human ear is omitted. This reduces the amount of data and thus the bandwidth required for transmission. However, if too much information is omitted, the voice quality suffers.
The various codec methods control audio compression with different levels of efficiency. Some are specially designed to achieve low bandwidth at all costs. The required bandwidth and voice quality therefore vary depending on the codec. To ensure that the data is correctly converted back into speech after transport, the receiver must use the same codec as the sender.
A large number of different codecs are used in international telephone networks; while fixed and mobile telephony works with a few G-series codecs from ITU-T (for example G.711 or G.726), there is a wide variety of mobile phones in the access area to mobile networks. To enable communication between subscribers whose terminals use different codecs, conversion to the other format, transcoding, is necessary. For telephony over the Internet, IP telephony , it is still a major technical hurdle to master all procedures and their transcoding.
The supported codecs also depend on the hardware and firmware version.
List of common codecs.
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