AAC (Advanced Audio Coding) — compressed audio format

Characteristics and technical overview

AAC achieves compression by removing audio information that is unlikely to be perceived by human listeners, using psychoacoustic models and transform coding techniques. Implementations vary: common profiles include AAC-LC (Low Complexity), HE-AAC (High Efficiency, which adds spectral band replication, SBR) and HE-AAC v2 (adds parametric stereo). Profiles trade complexity, delay and fidelity to suit applications from portable players to low-bandwidth streaming.

History and standardization

AAC was developed during the 1990s and was standardized as part of the MPEG-2 and later MPEG-4 families. The format was created by a group of contributors from research labs, consumer electronics companies and broadcasters who sought improved efficiency over earlier perceptual codecs. Over time, extensions such as SBR and parametric stereo were introduced to improve performance at lower bitrates, and various implementations and encoders have been optimized for different platforms.

Uses, containers and file types

AAC appears both as a raw elementary stream and inside container formats. Common file extensions include .aac for raw streams and .m4a, .mp4, .m4b when AAC is placed inside the ISO base media file format. The codec is popular for online music stores, streaming (radio and video services), mobile devices and digital broadcasting because of its favorable quality-to-size ratio.

Advantages, examples and limitations

• Generally provides better audio quality than older codecs like MP3 at equivalent bitrates, especially at low to moderate bitrates.
• Flexible: several profiles allow deployment in constrained environments (voice, streaming) and high-quality archives.
• Subject to patent and licensing regimes—implementers should check licensing for encoders and decoders.
• Perceived quality depends on encoder implementation; a high-quality encoder can compress CD-quality audio to a small fraction of its original size while maintaining perceived fidelity.

For further technical details and implementation notes, consult authoritative codec specifications and encoder documentation. Related formats and historical context can be explored through resources maintained by standards organizations and by vendors that deploy AAC in consumer products.