SMI to STL Subtitle Converter
Transform Synchronized Accessible Media Interchange subtitles into STL format instantly and privately in your browser.
Drag & drop your SMI file here
or click to browse
Supports .smi, .sami, .html files up to 50 MB
Free Online SMI to STL Converter
Our SMI to STL converter reads Synchronized Accessible Media Interchange input and produces Spruce Subtitle output entirely in your browser. Source SMI timing (whole milliseconds (Start=...)) is converted to STL timing (SMPTE HH:MM:SS:FF frame timecodes at 29.97 fps). SMI styling and markup are stripped, leaving valid, ready-to-use output. The conversion runs client-side, so your subtitle files are never uploaded to a server.
SMI to STL Code Syntax & Structure Inspection
Side-by-side code inspection illustrating header definitions, timestamp syntax, and cue payload formatting
SMI vs STL Technical Specifications
SMI
SMI (SAMI, Synchronized Accessible Media Interchange) is a Microsoft subtitle format built from HTML-like <SYNC> elements whose Start attribute is given in milliseconds. SMI supports multiple language tracks through CSS classes and was widely used for Windows Media Player captions and accessibility content.
STL
STL (Spruce Subtitle) is a text-based subtitle format used in DVD authoring and some broadcast workflows. Entries use comma-separated start and end timecodes followed by subtitle text, with the pipe character (|) for line breaks. The output is accepted by DVD authoring tools and Spruce-based workflows.
Format Differences
Conversion Troubleshooting & Gotchas
Practical solutions for player compatibility, character encoding, and timing synchronization
Subtitle timing drifts or appears out of sync on media players
Video files frequently differ in framerate (e.g., 23.976 fps vs 25 fps). If the timing drifts steadily, use our Subtitle Sync tool to adjust the global offset or framerate stretch.
Characters appear as garbled symbols or question marks (Mojibake)
Legacy files saved in Windows-1252, Shift-JIS, or GBK may render incorrectly. Our exporter standardizes output to clean UTF-8. Use our Subtitle Encoding Converter if the source file is corrupted.
Overlapping captions when multiple speakers talk simultaneously
Different formats handle overlapping dialogue differently. If your target player cannot render stacked cues cleanly, open the file in our Subtitle Editor to adjust individual cue timings.
Video editing software (Premiere, DaVinci, Final Cut) rejects the file
Professional NLE software enforces strict syntax rules. Our converter eliminates malformed line breaks and non-standard tags, producing clean, standardized files.
Features
Why Convert?
Common Use Cases
DVD Authoring
Produce STL subtitle files for DVD authoring tools that expect the Spruce format.
Broadcast Workflows
Generate STL for Spruce-based and legacy broadcast subtitle pipelines.
Frame-Accurate Timing
Output SMPTE frame-based timecodes, quantized to 29.97 fps STL timing.
Related Formats
SRT (SubRip)
The universal plain-text alternative: convert to SRT for maximum device compatibility.
VTT (WebVTT)
The web alternative: convert to VTT if your target is HTML5 video or YouTube.
SCC (Scenarist Closed Captioning)
The North American broadcast alternative: convert to SCC for CEA-608 caption delivery.
How It Works
- 1
Upload your .smi file by dragging it in or browsing.
- 2
The parser reads the Synchronized Accessible Media Interchange structure and extracts each cue's start, end, and text.
- 3
Source SMI timing (whole milliseconds (Start=...)) is converted to STL timing (SMPTE HH:MM:SS:FF frame timecodes).
- 4
SMI styling and markup are stripped, and cues are formatted to the STL specification.
- 5
The output .stl file is generated and ready to download.
More Subtitle Tools
Frequently Asked Questions
It reads a Synchronized Accessible Media Interchange file, parses each cue's timing and text, strips SMI markup, and writes a valid .stl file. Source SMI timing (whole milliseconds (Start=...)) is converted to STL timing (SMPTE HH:MM:SS:FF frame timecodes at 29.97 fps).
Absolutely. Every conversion runs client-side in your browser. No subtitle data, file contents, or metadata is ever transmitted to a server.
SMI styling and markup are dropped. Timing is quantized to 29.97 fps STL frames, so sub-frame precision from the source is not retained.
Yes, completely free with no sign-up, no watermarks, and files up to 50 MB each. All processing happens in your browser, so your files are never uploaded to a server.
Source SMI timing (whole milliseconds (Start=...)) is converted to STL timing (SMPTE HH:MM:SS:FF frame timecodes).
Yes. The converter is fully responsive and works in any modern mobile browser on iOS and Android.
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