MicroDVD subtitles
MicroDVD stores text cues using numbered frame boundaries, so their elapsed times depend on the associated frame rate.
MicroDVD is a text subtitle format using numbered frame positions, often with the extension .sub. Despite the name, it is different from DVD bitmap subpictures and the binary .sub file in a VobSub pair.
A cue can look like this:
{300}{375}The platform is empty.|The last train has left.
The braces contain the starting and ending frame numbers. The vertical bar separates displayed text lines.
The same numbers give different times
Using zero-origin frame boundaries and the indicated constant rate:
| Rate used for the numbers | Start at frame 300 | End at frame 375 | Interval |
|---|---|---|---|
| 25 fps | 12.000 s | 15.000 s | 3.000 s |
| 24 fps | 12.500 s | 15.625 s | 3.125 s |
The conversion is time = frame number / frame rate. A wrong rate changes every cue's interpretation by a factor; a single fixed offset cannot correct the whole file.
MicroDVD files can supply an initial frame-rate declaration, for example:
{1}{1}25.000
That is interpreted as a rate declaration by supporting readers rather than as a caption saying “25.000”. Files without usable rate information need the matching video's timing context or an explicitly supplied rate. Different reader fallback choices can explain different playback results; a default chosen by one tool is not evidence of the author's intended rate.
The suffix is only a starting clue
A readable file containing {number}{number} records is text. A VobSub .sub contains binary subpicture data and normally accompanies an .idx. Their matching suffix does not make them interchangeable.
For an Emby collection containing a frame-based legacy subtitle, a time-based conversion should use the rate associated with the subtitle's original edition. Merely reading the output video's advertised rate can be wrong if its timing was changed during a prior conversion. Frame-based timing develops that relationship.