Advanced Audio Coding (AAC)
AAC is the default audio codec for YouTube, Apple ecosystems, and most HLS/DASH streams. Distinguish ADTS-framed AAC from AAC inside MP4 boxes.
What It Defines
Lossy audio codec used in MP4/M4A containers (`mp4a` track) and streaming (HLS, DASH). ADTS framing adds a 7–9 byte header per frame for raw `.aac` streams; in MP4, sample description atoms carry codec config. MIME types: `audio/aac`, `audio/mp4`.
Canonical (Normative)
Convenient (Practical)
Related References
Joint ISO/IEC technical committee (JTC 1) publishes many of the binary file formats the web depends on: JPEG, PDF, MP4/ISOBMFF, ZIP (ISO/IEC 21320), and office document containers. Specs are often available via national body mirrors; some are freely readable online.
Related Specs
MP3 remains the most universally recognized audio format. ID3 tags (metadata envelope) prepended/appended to the frame stream are a classic header-envelope pattern.
MP4 is the dominant video container on the web and in MSE players. Box hierarchy, codec vs container distinction, and fragmenting (fMP4 for HLS/DASH) are essential for media engineering.
HLS is the dominant streaming format for live and VOD on Apple devices, CDNs, and most broadcast workflows. M3U8 text manifest + segmented media is the canonical chunked-file envelope pattern.
DASH is the open-standard counterpart to HLS for adaptive video. MPD manifest structure + fMP4 segment layout is how Netflix-style streaming works outside the Apple ecosystem.
Opus is the codec for real-time audio on the internet. WebRTC mandates it; Discord, Zoom, and most VoIP apps use it. Understanding bitrate, FEC, and frame sizing directly impacts audio quality and bandwidth.