audio codec (AC)
Audio codecs(AC) are software or hardware functional units in which analog audio signals are digitized, the uncompressed digital signals are compressed according to a specific compression algorithm, and these are digitally processed, decompressed and decoded. Codecs are used on the transmitting side for coding and compression, and on the receiving side for decompression and decoding. The conversion and compression reduce the amount of data for transmission without noticeably affecting the audio quality.
Audio codecs differ in their performance, sound quality, upper frequency limit, bit rate, compression rate, and resulting file size. In the interest of frequency economy and to occupy as little storage capacity as possible, audio codecs are designed with the lowest possible bit rates. However, the sound image of most audio codecs deteriorates at low bit rates. This disadvantage is remedied by Spectral Band Replication( SBR), which is used in aacPlus and mp3Pro. In addition to this technique, masking is also used in many audio codecs. In addition to the lossy audio codecs, there are the lossless ones, which include Free Lossless AudioCodec( FLAC), Linear Pulse CodeModulation( LPCM), and WindowsMedia Audio(WMA Lossless).
The International Telecommunication Union( ITU) has standardized various audio codecs such as the H.323, G.722 and others for various data rates. The audio codecs standardized by other institutions can be used license-free or against license fees, depending on the marketing concept. Royalty-free audio codecs include Ogg Vorbis, Lame Ain't an MP3 Encoder( LAME) and Windows Media Audio (WMA), while those requiring licensing include MP3, MP3Pro, Advanced Audio Coding( AAC+ and aacPlus (AAC+ ).
Audio codecs are used in music players and multimedia players, voice communications, Internet telephony, music downloads, streaming, digital radio, Internet radio and digital TV. Since audio codecs have many patents and company-specific know-how, many details of the codecs remain unpublished.