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Subtitles are timed to appear on screen at specific times.
Depending on the video format and the subtitle display method, the timing can be encoded in either Timecode or Timestamp.
TIMESTAMP
Example (SRT): 00:00:04,204 –> 00:00:10,143
This method encodes time in milliseconds. Timestamp has the advantage of being timecode independent and is used in subtitling systems that operate in parallel to the video display. One common usage for SRT timestamp title is to display closed captioning on YouTube videos. The timecode of the uploaded video is irrelevant to the timing of the subtitles, the subtitles appear at a specific global time ensuring compatibility across a wide-range of video formats.
Formats that use timestamp: SRT, SCC
TIMECODE:
Example (AVID DS): 10:01:07;27 10:01:09;02
This method is used for timing subtitles within editing applications. The video playback happens on a timeline at a specific frame rate. The timecode tells the editing application where to put the text box in-point and out-point on the edit timeline. The disadvantage of this format is that it is not compatible across different frame rate timelines. Let’s say you have an 29.97fps video transcoded to 25fps, it is unlikely that the subtitles will transfer correctly from one timeline to another. Some editing application can handle such transfers though various strategies, but overall, it is better practice to retime the subtitles to the new timecode using a subtitling program. There are some free online tools that can do this as well.
Formats that use timecode: AVID DS, video editing programs
FCP XMLS:
FCP XMLs (both FCP7 and FCPX) use a combination of timestamp and timecode to import and export title information from the editing application. The title start time and duration are encoded in milliseconds using a time-fraction that referrers to the frame-rate of the sequence (which is written in the header of the file).
For example, for a 24pfs timeline, the header info would be : frameDuration=”1/24s”
and each title would have specific timing information like: title duration=”76/24s” and start=”596/24s”
FCP then converts this information into timecode on import.
