DVD subtitles: DVDSub and VobSub
DVD subtitle subpictures are compressed indexed graphics, while VobSub packages related subtitle data into external SUB and IDX files.
DVD-Video subtitles use subpictures: compressed indexed graphics with controls for placement, colour, transparency and display timing. Media tools commonly call the decoded subtitle format DVDSub.
VobSub is closely related external-file packaging. A typical pair uses a binary .sub for subtitle data and a text .idx for indexing and presentation information.
| Form | Where its information lives |
|---|---|
| DVD subtitle in disc content | Subpicture data plus the disc's associated stream and palette information |
| External VobSub | Binary SUB data with its IDX companion |
| VobSub track inside Matroska | Timed payloads plus the track's VobSub configuration |
These are related representations of subpicture content, not three interchangeable filenames.
The subpicture carries pixel codes and controls
The compressed picture uses run-length-coded pixel values. In a conventional DVD subpicture, four pixel-code values select colours through a mapping into the DVD palette, with separately controlled transparency. The decoded lettering therefore depends on both the pixel runs and the control/palette information.
A correct outline can appear in the wrong colour if the palette is missing or misinterpreted. Treating the pixel codes themselves as grayscale values would also produce the wrong image.
The controls establish the displayed area and its changes over time. The binary payload is not a string containing the sentence's letters. Rendering it is decoding graphics; getting an editable sentence from it requires recognition.
The IDX provides the way into the SUB
An index entry can look like:
timestamp: 00:00:12:500, filepos: 0000000200
The timestamp identifies a presentation position. The hexadecimal filepos identifies a location in the binary SUB data: 0x200 = 512 bytes from its start. This is an illustrative entry, not a complete pair or proof that a subtitle packet exists at that offset.
The IDX can also provide palette, display size and language-stream information. Sending the binary SUB alone can deprive a consumer of the information needed to find or interpret the chosen stream.
When muxed into Matroska, those roles are reorganized. Relevant configuration goes into the track's codec-private information; the container carries the timed payloads. The pair is not simply attached as two unrelated documents.
Emby's external-subtitle naming documentation covers identification of a SUB/IDX pair. The format question is what data the selected decoder receives after that identification.
The subtitle's display plane can be resized
DVD subpictures are associated with a standard-definition display plane and its presentation geometry. A stored 720-pixel-wide plane does not by itself establish its displayed aspect ratio. The relevant disc and output geometry must be respected.
A high-definition output can scale the decoded bitmap. That does not recover a higher-resolution font outline or rewrite the layout as newly shaped text. Preserving the original visual content and improving its sampling on a larger output are different tasks.