ASS and SSA subtitles explained: styles, effects and conversion
What ASS and SSA subtitle files can do that SRT cannot, why anime and fan subtitles use them, and what changes when you convert them.
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SubStation Alpha and Advanced SubStation Alpha
SSA, SubStation Alpha, is a subtitle format from the 1990s that added named styles to subtitles: a font, size, color, outline and screen position that many lines can share. ASS, Advanced SubStation Alpha, is its successor and the version in use today. Both are plain text, with a script information section, a list of styles and a list of events, one Dialogue line per caption.
ASS lines can include override tags in curly braces, such as {\i1} to start italics or {\an8} to place a line at the top of the screen. Typesetters use them to match signs in the video, animate text and create karaoke effects, which is why ASS is common for anime and in MKV video files.
When you need ASS
Choose ASS when the look of the subtitles matters: translated signs that follow the picture, two speakers shown in different places, or song lyrics with karaoke timing. Most desktop players and media servers can show ASS files, but many TVs, video platforms and editors cannot, and browsers cannot show them at all without extra software.
What converting to SRT or VTT keeps
When you convert an ASS or SSA file on this site, every Dialogue line keeps its timing and words. Line breaks written as \N, and italic, bold and underline, are carried over. Fonts, colors, outlines, positions, rotations and karaoke timing have no equivalent in SRT and are left out. Comment lines are skipped, and lines that are vector drawings rather than words are removed.
Signs and songs are often separate lines in an ASS file. After converting, they become ordinary captions at the bottom of the screen, so check the preview and remove lines you do not want with the subtitle cleaner or a text editor.
Converting to ASS
Converting an SRT, VTT or SBV file to ASS gives every caption one default style: a white sans-serif font with a black outline at the bottom center of a 1920 × 1080 frame. Open the file in an ASS editor such as Aegisub afterwards if you want to design your own styles.
Further reading: Aegisub: ASS override tags.