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Subtitle formats explained

Brief introduction to the different subtitle types and formats.

While distributing content, subtitles can appear in one of 3 types:

Hard Subtitles

Also known as hardsubs or open nsubtitles. The subtitle text is irreversibly merged in original video nframes, and so no special equipment or software is required for nplayback. Hence, very complex transition effects and animation can be nimplemented, such as karaoken song lyrics using various colors, fonts, sizes, animation (like a nbouncing ball) etc. to follow the lyrics. However, these subtitles ncannot be turned off unless the original video is also included in the ndistribution as they are now part of the original frame, and thus it is nimpossible to have several variants of subtitling, such as in multiple languages.

Prerendered Subtitles

Also known as closed captions. Subtitles are nseparate video frames that are overlaid on the original video stream nwhile playing. Prerendered subtitles are used on DVD and Blu-rayn (though they are contained in the same file as the video stream). It isn possible to turn them off or have multiple language subtitles and nswitch among them, but the player has to support such subtitles to ndisplay them. Also, subtitles are usually encoded as images with minimaln bitrate and number of colors; they usually lack anti-aliased font rasterization. Editing the text from prerendered subtitles is virtually impossible, unless the subtitles are converted to a soft format (such as SRT) by means of an OCR-capable subtitle converting program, such as SubRip. Changing their style and color is also not possible while in this format.

Soft Subtitles

Also known as softsubs or closed nsubtitles. In this case, the subtitles are separate instructions, usually a specially marked up textn with time stamps to be displayed during playback. It requires player nsupport and, moreover, there are multiple incompatible (but usually nreciprocally convertible) subtitle file formats.n Text rendering quality can vary depending on the player, but is ngenerally higher than prerendered subtitles. Also, some formats nintroduce text encodingn troubles for the end-user, especially if very different languages are nused simultaneously (for example, Latin and Asian scripts).

Soft subtitles are easily editable by dedicated software such as DivXLand Media Subtitler, this tool being capable of creating and editing subtitles in more than 30 formats, such as SubRip (SRT), SAMI, SubStation Alpha, and others.

Common Subtitle Formats (for video players)

Name	Extension	Type	Text Styling	Metadata	Timings	Timing Precision
AQTitle	.aqt	Text	Yes	Yes	Framings	Dependent on Frame
Gloss Subtitle	.gsub	HTML/XML	Yes	Yes	Elapsed Time	10 Milliseconds
JACOSub	.jss	Text w/markup	Yes	No	Elapsed Time	Dependent on frame
MicroDVD	.sub	Text	No	No	Framings	Dependent on Frame
MPEG-4 Timed Text	.ttxt (or mixed with A/V stream)	XML	Yes	No	Elapsed Time	1 Millisecond
MPSub	.sub	Text	No	Yes	Sequential Time	10 Milliseconds
Ogg Writ	N/A (mixed with audio/video stream)	Text	Yes	Yes	Sequential Granules	Dependent on Bitstream
Phoenix Subtitle	.pjs	Text	No	No	Framings	Dependent on Frame
PowerDivX	.psb	Text	No	No	Elapsed Time	1 Second
RealText	.rt	HTML	Yes	No	Elapsed Time	10 Milliseconds
SAMI	.smi	HTML	Yes	Yes	Framings	Dependent on Frame
Spruce subtitle format	.stl	Text	Yes	Yes	Sequential Time+Frames	Sequential Time+Frames
Structured Subtitle Format	.ssf	XML	Yes	Yes	Elapsed Time	1 Millisecond
SubRip	.srt	Text	Yes	No	Elapsed Time	1 Millisecond
(Advanced)SubStation Alpha	.ssa or .ass (advanced)	Text	Yes	Yes	Elapsed Time	10 Milliseconds
SubViewer	.sub	Text	No	Yes	Elapsed Time	10 Milliseconds
Universal Subtitle Format	.usf	XML	Yes	Yes	Elapsed Time	1 Millisecond
VobSub	.sub + .idx	Image	N/A	N/A	Elapsed Time	1 Millisecond
XSUB	N/A (embedded in .divx container)	Image	N/A	N/A	Elapsed Time	1 Millisecond

There are still many more uncommon formats. Most of them are text-based and have the extension .txt.

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