Advanced Audio Coding

From VideoLAN Wiki
Revision as of 07:01, 11 September 2005 by Ciscokid (talk | contribs) (from wikipedia (GNU Free Documentation License))
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to navigation Jump to search

Advanced Audio Coding (AAC) is a lossy data compression scheme intended for audio streams.


AAC is a wideband audio coding algorithm that exploits two primary coding strategies to dramatically reduce the amount of data needed to convey high-quality digital audio. First, signal components that are perceptually irrelevant and can be discarded without a perceived loss of audio quality are removed. Next, redundancies in the coded audio signal are eliminated.

Why AAC was designed

AAC was designed as an improved-performance codec relative to MP3 (which was specified in MPEG-1) and MPEG-2 Part 3 (which is also known as "MPEG-2 Audio" or ISO/IEC 13818-3).

AAC ISO Standard

AAC, which was first specified in the standard known formally as ISO/IEC 13818-7, was published in 1997 as a new "part" (distinct from ISO/IEC 13818-3) in the MPEG-2 family of international standards.

Codec improvements

The codec design was further improved in MPEG-4 Part 3, known formally as ISO/IEC 14496-3, with the addition of Perceptual Noise Substitution (PNS) and a Long Term Predictor (LTP).

Bifurcations in the AAC standard

Although the AAC codec specified in MPEG-2 Part 7 and the AAC specified in MPEG-4 Part 3 are somewhat different, they are both informally known as AAC (for clarity it is best to refer specifically either to MPEG-2 AAC or to MPEG-4 AAC).

AAC's improvements over MP3

Some of its advances:

  • Sample frequencies from 8 Hz to 96 kHz (official MP3: 16 Hz to 48 kHz)
  • Up to 48 channels
  • Higher efficiency and simpler filterbank (hybrid → pure MDCT)
  • Higher coding efficiency for stationary signals (blocksize: 576 → 1024 samples)
  • Higher coding efficiency 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.

Modular Coding

AAC 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 specific set of tools they want use for a particular application. The standard offers four default profiles:

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.

Products that support AAC

iTunes and iPod

In April, 2003, Apple Computer brought mainstream attention to AAC by announcing that its iTunes and iPod products would support songs in MPEG-4 AAC format (via a firmware update for older iPods), and that customers could download popular songs in a protected version of the format via the iTunes Music Store. AAC has now become so associated with Apple hardware and software that people are commonly of the mistaken belief that AAC expands to "Apple Audio Codec." Optionally, a digital rights management scheme (named FairPlay) can be employed in tandem.

Apple has added support for VBR encoding of AAC tracks in iTunes v5.0 (released 9/7/2005).


Sony PSP has added AAC support with version 2.0 firmware update, which was released in August 2005.


See also

Some External links