HLS — Segmented WebVTT Caption Playlist
The subtitle rendition the master playlist references through EXT-X-MEDIA. Captions in HLS are segmented like media — each segment is a small WebVTT file covering its own time window, carrying X-TIMESTAMP-MAP to align with the MPEG-TS clock. Completes the master playlist's declared rendition group so a player can resolve the whole tree.
#EXTM3U
#EXT-X-VERSION:7
#EXT-X-TARGETDURATION:4
#EXT-X-PLAYLIST-TYPE:VOD
#EXTINF:4.000,
cap-00001.vtt
#EXTINF:4.000,
cap-00002.vtt
#EXTINF:4.000,
cap-00003.vtt
#EXTINF:2.000,
cap-00004.vtt
#EXT-X-ENDLIST
Specifications
- Protocol
- HLS
- Rendition Type
- SUBTITLES
- Segments
- 4
- Segment Format
- WebVTT
- Referenced By
- master.m3u8 EXT-X-MEDIA
- Language
- en
What is a .m3u8 file?
M3U8 (.m3u8) is a UTF-8 M3U playlist, most importantly the backbone of HTTP Live Streaming (HLS). An HLS media playlist lists timed media segments with #EXTINF tags and directives like #EXT-X-TARGETDURATION, and a master playlist points to alternate bitrate renditions.
How to use this file
Use an example .m3u8 file to test HLS parsers and players, segment and directive extraction, and adaptive-streaming tooling, or as a plain UTF-8 M3U playlist.
How to use this file for testing
“HLS — Segmented WebVTT Caption Playlist” is a deterministic Novus Examples fixture for Streaming manifests, Conversion testing, Video QA. HLS playlists and DASH MPDs covering multi-bitrate ladders, byte-range addressing, and segment templates. Served inline with permissive CORS under /files/stream/, so a player can load them from any origin without re-hosting.
Documented properties for this file: en. Compare results against paired or grouped companions on this page when present (clean↔damaged, searchable↔scanned, or format twins) so scores stay reproducible across runs.
Download the file once, keep the path stable in CI or local scripts, and treat the spec table as the contract: dimensions, seeds, field lists, and roles are intentional. Corrupt or invalid samples are labelled as such — expect parsers to fail loudly rather than silently accept them.
Media fixtures are short and synthetic by design. Prefer waveform or transcript ground truth in the same group when measuring ASR, trim, upscale, or sync tools; do not assume broadcast-quality masters.
Code examples
# manifests resolve their own segments, so pass the URL, not a local copy
ffprobe -v error -show_entries stream=codec_name,width,height captions-en.m3u8
ffplay captions-en.m3u8Related files
- webmAlpha Channel — Opaque VP9 TwinThe same codec and container with NO alpha plane, as the control for the alpha clip in this group. Useful for checking that alpha detection reads the pixel format rather than assuming every WebM is transparent — and for confirming a compositing bug is in the alpha handling rather than in the player.
- webmAlpha Channel — VP9 in WebMVideo with a real per-pixel alpha channel: VP9 yuva420p in WebM, the one alpha path browsers decode natively. Composite it over a page background and the transparency is genuine, not a chroma key. Roughly 90% of each frame is fully transparent. Two traps this file exists to expose. First, auto-alt-ref must be disabled at encode time or libvpx silently drops the alpha plane, producing a valid file with no transparency and no error. Second, WebM stores VP9 alpha in BlockAdditional and signals it with AlphaMode=1, so FFmpeg's NATIVE vp9 decoder reports pix_fmt yuv420p and decodes fully opaque — you must force `-c:v libvpx-vp9` to see the alpha at all. Probing this file with default settings and concluding it has no alpha is the expected mistake.
- assASS — Karaoke Timing and Override TagsAdvanced SubStation Alpha using the features that distinguish it from SubRip: per-syllable \k karaoke timings in centiseconds, a full V4+ style definition with primary and secondary colours, and an \an override that repositions a line. Converting this to SRT necessarily loses all of it, which makes it a good test of whether a converter warns about that or drops it silently.
- mp4Bitstream — All-Intra (Every Frame a Keyframe)Every frame is an I-frame, so any frame is an independent cut point. Much larger, and the shape editing and frame-accurate seeking want. Same picture and same codec as every other clip in this group — only the GOP and frame-type structure differ, so the effect on size and seekability is directly attributable.
- mp4Bitstream — Heavy B-Frames (8 Consecutive)Eight consecutive B-frames with a B-pyramid, so decode order and display order diverge sharply and DTS runs well behind PTS. The case that breaks naive timestamp handling and any code that assumes frames arrive in display order. Same picture and same codec as every other clip in this group — only the GOP and frame-type structure differ, so the effect on size and seekability is directly attributable.
- mp4Bitstream — Long GOP (24 Frames, One Per Second)One keyframe per second — the delivery default, and the setting that determines HLS/DASH segment boundaries, since a segment must start on a keyframe. Same picture and same codec as every other clip in this group — only the GOP and frame-type structure differ, so the effect on size and seekability is directly attributable.
Generated by generation/video_timedtext.py. Free for any use, no attribution required — license.