Advanced Audio Coding (AAC) is a lossy data compression scheme intended for audio streams. AAC was designed to replace MP3. AAC, ISO/IEC 13818-7, is an extension of the MPEG-2 international standard, ISO/IEC 13818-3. It was further improved in MPEG-4, MPEG-4 Version 2 and MPEG-4 Version 3, ISO/IEC 14496-3.

Some of its advances:

  • Sample frequencies from 8 kHz to 96 kHz (official MP3: 16 to 48 kHz)
  • Up to 48 channels
  • Higher coding efficiency 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)

What this all means to the listener is better and more stable quality than MP3 at equivalent or slightly lower bitrates.

As anyone who has used several different MP3 encoders will tell you, each encoder performs differently and they produce output of sometimes wildly varying quality. AAC, on the other hand, takes a modular approach to encoding. Depending on the complexity of the bitstream to be encoded, the desired performance and the acceptable output, implementers may create profiles to define which of a specfic set of tools they want use for a particular application. The standard offers three default profiles:

  • Main Profile (MAIN)
  • Low Complexity Profile (LC) - the most widely used.
  • Sample-rate Scaleable (SRS) - also called Scaleable Sample Rate (MPEG-4 AAC-SSR)

Depending on the AAC profile and the MP3 encoder, 96 kbit/s AAC can give nearly the same or better perceptional quality as 128 kbit/s MP3.

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 iTunes Music Store. While this helps AAC, it only further splinters a market overcrowded with formats, leaving many people and companies confused over which formats to embrace. However, AAC requires a license due to the patents involved; the Ogg Vorbis format is free and has similar quality, so it may overtake AAC.

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