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Advanced Audio Coding (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. Efficient audio compression is achieved by a variety of perceptual audio coding and data compression tools, which are combined in the MPEG-4 AAC specification.

The MPEG-4 AAC standard incorporates MPEG-2 AAC, forming the basis of the MPEG-4 audio compression technology for data rates above 32 kbps per channel. Additional tools increase the effectiveness of AAC at lower bit rates, and add scalability or error resilience characteristics. These additional tools extend AAC into its MPEG-4 incarnation (ISO/IEC 14496-3, Subpart 4).

The MPEG-4 AAC patent license grants rights for multiple MPEG-4 AAC Object Types, including AAC LC (Low Complexity), AAC LTP (Long-Term Prediction), AAC Scalable, and ER AAC LD (Low Delay).

A copy of the MPEG-4 Audio standard can be purchased from the ISO online store (search for "14496-3").

Because of its exceptional performance and quality, Advanced Audio Coding (AAC) is at the core of the MPEG-4, 3GPP and 3GPP2 specifications and is the audio codec of choice for Internet, wireless and digital broadcast arenas. AAC provides audio encoding that compresses much more efficiently than older formats, such as MP3, yet delivers quality rivaling that of uncompressed CD audio.

AAC was developed by the MPEG group that includes Dolby, Fraunhofer (FhG), AT&T, Sony and Nokia—companies that have also been involved in the development of audio codecs such as MP3 and AC3 (also known as Dolby Digital). The AAC codec in QuickTime builds upon new, state-of-the art signal processing technology from Dolby Laboratories and brings true variable bit rate (VBR) audio encoding to QuickTime.


Because of its incredible quality at a wide range of data rates and ratification as an industry standard, AAC audio is gaining wide adoption in the marketplace. For example, all of the music sold in the iTunes Music Store uses AAC, for playback on desktop or iPod. AAC has also been adopted by the major standards organizations including the ISO MPEG (MPEG-4), 3GPP and 3GPP2, DVB, as well as XM satellite radio. As a result of its support for this technology, the audio you create with QuickTime is interoperable with a wide range of devices and media players.

When compared side-by-side, AAC proves itself worthy of replacing MP3 as the new Internet audio standard. Take a look at these AAC advantages over MP3:

Improved compression provides higher-quality results with smaller file sizes
Support for multichannel audio, providing up to 48 full frequency channels
Higher resolution audio, yielding sampling rates up to 96 kHz
Improved decoding efficiency, requiring less processing power for decode
The Data Speaks for Itself
In numerous comparison tests, AAC comes out on top. Check out these impressive results:

AAC compressed audio at 128 Kbps (stereo) has been judged by expert listeners to be “indistinguishable” from the original uncompressed audio source.*
AAC compressed audio at 96 Kbps generally exceeded the quality of MP3 compressed audio at 128 Kbps. AAC at 128 Kbps provides significantly superior performance than does MP3 at 128 Kbps.*
AAC was the only Internet audio codec evaluated in the range “Excellent” at 64 Kbps for all of the audio items tested in EBU listening tests.*