AAC (Advanced Audio Coding) is the default audio codec for video, the sound in most MP4 files on YouTube, Netflix, Apple, and OTT platforms. Designed as MP3's successor, it delivers better quality at the same bitrate and scales through a family of profiles. AAC-LC (Low Complexity) is the everyday workhorse at 128-256 kbps stereo; HE-AAC adds Spectral Band Replication for efficient low bitrates; HE-AAC v2 adds Parametric Stereo for even lower; xHE-AAC (USAC) unifies speech and music with built-in loudness control. Near-universal hardware decoding makes AAC the safe default for any video service, even where Opus would be more efficient.
