Subtitle formats explained
A subtitle format defines how timing, text or image content, and presentation information are encoded separately from their storage or delivery.
A subtitle format describes how a timed sequence is encoded: its text or image content, timing, and any presentation instructions. Its filename extension, its container, and the renderer used for playback describe other parts of the route.
SRT inside an MKV is still text subtitle content. PGS inside the same MKV is still graphics. Matroska packages the tracks; it does not turn one representation into the other.
| Layer | What it describes |
|---|---|
| Subtitle representation | Characters, styles, graphics, and format-specific events |
| File or container packaging | Where track descriptions and timed payloads are stored |
| Delivery | How that data reaches the player |
| Rendering and composition | How the content becomes visible over the picture |
The format families
Text subtitles retain characters that a renderer draws. Individual formats have separate explanations:
- SubRip / SRT: numbered text cues with start and end times.
- WebVTT: web timed text with cue settings and defined markup.
- SSA: an earlier script format with reusable subtitle styles.
- ASS: styled script events with positioning and timed visual instructions.
- TTML: XML timed text with a structured timing and layout model.
- MP4 timed text / tx3g: timed text samples within a container track.
- SAMI: HTML-like synchronized caption blocks.
- MicroDVD: text tied to numbered video frames.
IMSC constrains TTML features for particular delivery profiles. It is a profile relationship, not a filename-only classification.
Bitmap subtitles supply pre-rendered graphics:
- DVDSub / VobSub: DVD subpictures and their external-file packaging.
- PGS: Blu-ray presentation graphics assembled into timed display sets.
- DVBSub: broadcast subtitle services built from pages, regions and objects.
Broadcast caption systems also have service and command models that do not fit a simple “one sentence per file block” description. Their service identifiers and carriage must be understood alongside their character data.
Format changes have a purpose and a cost
Converting a simple text cue between two formats can retain its wording and timing. Converting an extensively positioned ASS sign into plain text can retain the words while discarding their placement and animation. Turning a bitmap into text additionally requires recognizing the letters, usually through OCR or manual transcription.
Rendering any of those representations into video preserves the displayed result in that particular output picture. It removes the separate subtitle choice from that version.
For Emby, a supported media container and a selected subtitle renderer are separate compatibility questions. The underlying playback route determines whether the track is delivered separately, converted into another subtitle representation, or incorporated into the video. A container extension alone cannot decide which work the selected subtitle requires.