Overview
ISO/IEC 14496-30:2018 defines how timed text (subtitles, captions and other visual overlays) is carried inside files based on the ISO base media file format (ISOBMFF) - the foundation of MP4 (ISO/IEC 14496-14) and Motion JPEG 2000 (ISO/IEC 15444-3). The standard specifies storage, layout and timing behaviour for timed text tracks so they can be used alongside audio/video, delivered by streaming servers (hint tracks), and integrated into MPEG‑4 systems.
Key topics and technical requirements
- Scope: Carriage of specific timed text and subtitle stream forms in ISOBMFF files; does not preclude alternate carriage methods (e.g., 3GPP Timed Text).
- Timed Text Formats: Formal support and mapping rules for TTML (Timed Text Markup Language) and WebVTT (Web Video Text Tracks), including sample entry and sample formats, codec parameters, and conversion (import/export) guidance.
- Layout rules: How track width/height and the track_size_is_aspect_ratio flag determine sizing; use of track header matrix translation values (tx, ty) for positioning; stacking order via the layer field (equivalent to z-index).
- Timing semantics: Delivery of text samples at sample decode time, rendering at composition time with sample duration; treatment of internal timing values embedded in payloads (start/end times, durations) and their relationship to sample composition intervals.
- Track and sample definitions: Track format, sample format and sample entry specifications required for interoperability within ISOBMFF-based players and tools.
- Resources & associations: Language handling, shared resources across samples, and rules for associating timed text tracks with visual tracks.
- Annex A: Informative guidance on captioning embedded in other media stream types.
Applications and who uses it
ISO/IEC 14496-30:2018 is essential for:
- Media player and device manufacturers implementing MP4 playback with native subtitle/caption support.
- Streaming platforms and CDNs packaging subtitles in MP4 containers and using hint tracks for delivery.
- Content creators and post‑production houses authoring TTML/WebVTT-based subtitles to be multiplexed into ISOBMFF files.
- Standards engineers and codec implementers ensuring interoperability of timed text tracks across ecosystems.
Practical uses include embedding subtitles/captions in MP4 files, synchronized text overlays for accessibility, and converting between external WebVTT files and in‑file timed text tracks.
Related standards
- ISO/IEC 14496-12 (ISOBMFF)
- TTML 1.0 (W3C)
- WebVTT (W3C)
- 3GPP TS 26.245 (3GPP Timed Text)
Keywords: ISO/IEC 14496-30:2018, timed text, subtitles, ISOBMFF, MP4, TTML, WebVTT, captions, timed text tracks.