SRT — SubRip Format

SRT (SubRip) is the most widely used subtitle format in the world. Created by the SubRip OCR program, it stores subtitles as plain text with simple numeric timestamps. Every SRT file follows the same basic structure: a cue number, a start and end timestamp separated by an arrow (-->), one or more lines of subtitle text, and a blank line to separate cues.

SRT timestamps use the format HH:MM:SS,mmm with a comma before the milliseconds. The format is supported by virtually every media player, including VLC, MPC-HC, Plex, Kodi, and YouTube. Its simplicity and universal compatibility make it the default choice for subtitle distribution.

SRT is a text-based format, so files are small and can be edited with any text editor. However, SRT does not support styling, positioning, or effects. For advanced formatting, ASS or VTT are better options.

VTT — WebVTT Format

WebVTT (VTT) is the standard subtitle format for HTML5 video players. It was developed by the W3C and is natively supported by all modern browsers including Chrome, Firefox, Safari, and Edge. VTT is similar to SRT in structure but uses a decimal point instead of a comma in timestamps: HH:MM:SS.mmm.

Unlike SRT, VTT supports optional styling and positioning through cue settings. You can specify text alignment (left, center, right), vertical position (top, bottom), and line number. VTT also supports multiple lines per cue and vertical text for East Asian languages.

VTT files use the WEBVTT header and support comments and styling via a STYLE block. For web-based video content, VTT is the recommended format. See the full VTT guide →

ASS — Advanced SubStation Alpha

ASS (Advanced SubStation Alpha) is a powerful subtitle format that supports rich formatting and layout control. Originally developed for fansubbing, ASS allows precise control over font family, size, color, bold, italic, underline, and strikethrough — all on a per-cue basis.

ASS also supports positioning with pixel-level accuracy, rotation, scaling, and even simple animations. Karaoke effects, where words are highlighted in sync with the music, are a signature ASS feature. The format uses named styles defined in a [V4+ Styles] section, which can be reused across many cues.

The trade-off is complexity. ASS files are larger than SRT files and are not supported by all media players. VLC, MPC-HC, and PotPlayer handle ASS well, but many smart TV apps and streaming platforms do not. For simple subtitles, SRT or VTT is usually sufficient.

SSA — SubStation Alpha

SSA (SubStation Alpha) is the predecessor of ASS. It uses the same basic structure but with fewer styling options. SSA files define styles in a [V4 Styles] section and support basic font formatting, positioning, and colors. ASS extended this with additional features like drawing commands and more flexible styling.

Most SSA files can be used as ASS files without modification, as ASS is backward compatible. In practice, SSA has been largely replaced by ASS, but older subtitle collections may still use the .ssa extension. SyncFlow does not parse SSA/ASS natively, but the tools for timing correction can be applied to converted SRT files.

SUB/IDX — VobSub Format

SUB/IDX is a bitmap-based subtitle format extracted from DVDs. Unlike text-based formats (SRT, VTT, ASS), VobSub stores subtitles as raster images. The .sub file contains the bitmap image data, and the .idx file contains the timing index and language metadata.

Because subtitles are images, VobSub preserves the exact font and styling used in the original source. However, the files are much larger than text-based formats, the text cannot be searched or edited, and the format is not suitable for web video. VobSub is primarily used for DVD backups and archival purposes.

If you have a SUB/IDX file and need text-based subtitles, you would need to run the images through OCR software to extract the text — separate from SyncFlow's current feature set.

Format Comparison Table

Choosing the Right Format

Selecting the right subtitle format depends on your playback environment and requirements:

• For maximum compatibility — use SRT. It works on every platform, every media player, and every streaming device. If you are distributing subtitles for a broad audience, SRT is the safest choice.
• For web video — use VTT. It is the native format for HTML5 video and works in all modern browsers. VTT also supports basic positioning for better integration with web video players.
• For styled subtitles — use ASS. If you need custom fonts, colors, positioning, or karaoke effects, ASS provides the most control. Confirm your target audience's media player supports it.
• For archival — use SUB/IDX if you need pixel-perfect preservation of the original DVD subtitles. For most other cases, text-based formats are preferred for their editability and small file size.

SyncFlow works with SRT and VTT files. You can load either format and export in either format, making format conversion part of the sync workflow. For a detailed look at the VTT format, see the VTT file guide.