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Advanced audio coding

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AAC is an abbreviation for Advanced Audio Coding. It is a lossy data compression scheme intended for audio streams.

AAC was designed to replace MP3.

Some of its advances:

  • Sample frequencies from 8 kHz...96 kHz (official MP3: 16...48 kHz)
  • Up to 48 channels
  • Higher coding efficency for stationary signals (blocksize: 576 -> 1024 samples)
  • Higher coding efficency for transient signals (blocksize: 192 -> 128 samples)
  • Much better handling of frequencies above 16 kHz
  • More flexible joint stereo (separate for every scale band)

This result in better and more stable quality than MP3 at equivalent bitrates. 96 kbps AAC gives nearly the same perceptional quality than 128 kbps MP3.

AAC is part of the MPEG-2 international standard, ISO/IEC 1????-3. It's was further improved in MPEG-4, MPEG-4 Version 2[?] and MPEG-4 Version 3.

There are several complexity profiles possible:

  • Low Complexity Profile (LC)
  • Main Profile (MAIN)
  • ...

Mostly MPEG-2 LC is used.

In April 2003, Apple Computer brought mainstream attention to AAC by announcing that its iPod and iTunes products would support songs in AAC format, and that customers could download popular songs in this format via the internet.

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