Subtitle Formats Compared: SRT vs ASS vs VTT vs SUP

TL;DR: The first split is text versus image. SRT, ASS, and VTT are text you can edit, translate, and convert freely; SUP and SUB/IDX are pictures of text that need OCR before you can do anything with them. Among the text formats, pick SRT for maximum compatibility, ASS when you need styling, and VTT for the web.

Text formats vs image formats — the split that matters most?

Before comparing individual formats, there is one distinction that decides what you can actually do with a subtitle file, and it is not about features or compatibility — it is about whether the file contains words or pictures.

Text formats — SRT, ASS/SSA, and VTT — store the actual words of each line along with its timing. Because the text is right there in the file, you can open it in any editor, search and replace, feed it to a translator, and convert it to another text format without loss of meaning. Image formats are the opposite: SUP (Blu-ray PGS) and SUB+IDX (DVD VobSub) store each subtitle as a rendered picture of the words. The player overlays that picture on the video; there is no text inside to read.

That difference is why a format converter can turn an SRT into a VTT in a second, but nothing can turn a SUP into an SRT on its own — the SUP has no text to convert. You have to run OCR to read the words out of the pictures first.

Text formats (SRT, ASS, VTT) hold the actual words, so they are editable, translatable, and convert to one another freely; image formats (SUP, SUB/IDX) hold pictures of the words, so they cannot be edited or translated until OCR turns them into text.

SRT — the universal default?

SubRip (.srt) is the plainest subtitle format there is: a numbered list of cues, each with a start and end timecode and a line or two of text. No fonts, no colors, no positioning — just words and timings. That simplicity is exactly why it wins.

SRT is the most widely supported subtitle format anywhere: every video player, every subtitle editor, YouTube, and virtually every platform reads it. It is the safest choice for editing, translation, and moving subtitles between tools, because whatever you hand it to will understand it. For a deep look at the format itself — the exact structure, timecodes, and common pitfalls — see the SRT Subtitle Format Guide.

SRT is the best default for editing, translation, and portability — reach for something else only when you specifically need styling or the web.

ASS / SSA — when you need styling?

Advanced SubStation Alpha (.ass, and its older sibling SSA) is text plus rich styling. On top of the words and timings, it carries fonts, colors, outlines, exact on-screen positioning, and karaoke-style effects. It is the standard format in anime and fansub communities precisely because the look of the subtitles matters there — typesetting signs, songs, and multiple speakers each get their own style.

Choose ASS when the appearance is part of the job. The trade-off to know: converting ASS to SRT drops every bit of the styling, because SRT has nowhere to store fonts, colors, or positions — you keep the words and timings and lose the rest. If you need to preserve the look, keep the file as .ass.

Pick ASS/SSA when fonts, colors, positioning, or effects are essential to the result; accept that flattening it to SRT throws the styling away.

VTT (WebVTT) — for the web?

WebVTT (.vtt) is the subtitle format built for the web. It is the caption format for HTML5 <track> elements and web video players, and it is close enough to SRT that the two convert cleanly — VTT just adds web-oriented features like cue positioning and styling hooks that browsers understand.

Choose VTT when the subtitles will live in a web player or be embedded as captions in a web page. For almost everything off the web, SRT is the simpler and more portable choice; VTT earns its place specifically in the browser.

Use VTT for HTML5 video and web-embedded captions; for anything outside the browser, plain SRT is usually the easier path.

SUP and SUB/IDX — image formats you have to OCR?

These two are the image formats, and they behave nothing like the text ones. SUP (.sup) is the Blu-ray image subtitle track (PGS); SUB+IDX is the DVD image subtitle track (VobSub), where .sub holds the pictures and .idx holds their timing index. In both, every subtitle is a bitmap — a picture of the words rendered onto a transparent layer that the player draws over the video.

Because there is no text inside, you cannot edit, translate, or convert these files the way you would an SRT. A format converter has nothing to work with. Before you can do anything with a SUP or SUB/IDX file, you have to OCR the images into text — typically into an SRT — and only then can you edit or translate it. On a Mac, GeekLink does this OCR locally: it reads the words out of the image subtitles and exports a clean, correctly timed SRT, with nothing uploaded. If your subtitles are burned directly into the picture rather than in a track, the same OCR approach applies — see The 4 Kinds of Subtitles for how to tell them apart.

SUP and SUB/IDX are pictures, not text, so OCR to SRT is the mandatory first step before any editing, translation, or conversion.

Which format should I use? (quick decision)

Here is the whole comparison in one table. Read across from what you are trying to do to the format that fits.

Format	Text or image	Styling	Best for
SRT	Text	None	Editing, translation, portability
ASS	Text	Rich	Anime, styled subs
VTT	Text	Basic	Web video
SUP	Image	n/a	Blu-ray, needs OCR
SUB+IDX	Image	n/a	DVD, needs OCR

In practice, most people want SRT: it opens anywhere and is the easiest to edit and translate. On a Mac, GeekLink exports SRT (and bilingual subtitles) directly from speech recognition or OCR. For the image formats — SUP and SUB/IDX — OCR them to SRT first with GeekLink, and from there they behave like any other text subtitle.

Default to SRT; use ASS when the look matters and VTT for the web; for SUP and SUB/IDX, OCR to SRT first and then treat them as text.

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