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Definition
AAC (Advanced Audio Coding) is a lossy audio codec designed as the successor to MP3. It achieves better sound quality than MP3 at the same bitrate. AAC is the default audio format for Apple devices, YouTube, and most streaming services.
AAC was standardized in 1997 as part of the MPEG-2 and later MPEG-4 standards. It was designed to improve upon MP3's compression efficiency using more advanced psychoacoustic models, better handling of frequencies above 16 kHz, and support for up to 48 channels (compared to MP3's 2 channels). At typical bitrates, AAC produces noticeably better quality than MP3.
Apple adopted AAC as the standard format for iTunes, iPod, iPhone, and all Apple devices. YouTube, Spotify, and most major streaming platforms use AAC for audio delivery. The format is also the standard audio codec in MP4 video containers — when you watch a video in MP4 format, the audio track is almost always AAC.
AAC comes in several profiles: AAC-LC (Low Complexity — the standard profile), HE-AAC (High Efficiency — designed for low bitrates, used in streaming), and HE-AAC v2 (adds parametric stereo for even better efficiency at very low bitrates). For general music encoding, AAC-LC at 192-256 kbps provides excellent, transparent quality.