Overview
ISO/IEC TR 14496-24:2008 - "Audio and systems interaction" - is a Technical Report that defines the desired joint behavior between MPEG‑4 Systems (notably the ISO Base Media / MP4 file formats) and MPEG‑4 Audio codecs. Its goal is deterministic handling of finite‑length audio when encoded to files and decoded again so that the decoded waveform has nothing “extra” at the start and nothing “missing” at the end (aside from normal codec distortions). The report focuses on timestamping, encoder/decoder interaction, and file‑format helper data (pre‑roll, edit‑list) to enable exact round‑trips and predictable synchronization with other media such as video.
Key topics and requirements
- Composition time stamps: Access unit timestamps must refer to the time of the first sample in the corresponding composition unit (decoded output block). Proper timestamp handling is essential for audio/video synchronization and random access.
- Encoder/decoder behavior: Encoders must produce normative access units and clearly associated timestamps. Decoders output one composition unit per access unit; Systems use edit lists and pre‑roll to determine the exact valid output samples.
- Pre‑roll and edit‑list (ISO Base Media File Format helper data): File syntax can include a pre‑roll group (to supply required startup access units) and edit‑lists (to specify which decoded samples to keep/discard). These constructs enable precise reconstruction of finite audio segments from compressed data.
- HE‑AAC and dual‑mode decoding: For codecs like HE‑AAC that can be decoded in backward‑compatible (AAC) or enhanced (AAC+SBR) modes, decoders operating in enhanced mode may introduce additional algorithmic delay (example: SBR adds 962 samples at the HE‑AAC output sampling rate). If such delay exists, the decoder must notify Systems so timestamps can be adjusted.
- Deterministic round‑trip: The Report demonstrates how combining encoder output, timestamps, and file helper data yields exact round‑trips from raw audio → encoded file → raw audio (modulo lossy distortion).
Applications and who uses it
- Multimedia codec implementers (AAC, HE‑AAC developers) ensuring interoperable encoder/decoder timing.
- File format and media tool developers (MP4/ISO Base Media File Format implementers) implementing pre‑roll and edit‑list handling.
- Streaming and playback systems that require precise audio/video synchronization, random access, and exact audio segment reconstruction.
- Content production and post‑production tools that edit in encoded domains and need deterministic behavior.
Related standards
- ISO/IEC 14496‑1 (Systems), 14496‑3 (Audio), 14496‑12 (ISO Base Media File Format), 14496‑14 (MP4 file format)
- MPEG‑4 codec and file format specifications (for exact syntax and normative requirements)
Keywords: MPEG‑4, HE‑AAC, AAC, composition time stamps, pre‑roll, edit‑list, ISO Base Media File Format, audio synchronization, encoder/decoder behavior, deterministic decoding.