AI Subtitle Studio

How to convert SUB/IDX (VobSub) subtitles to SRT

DVD subtitles come as VobSub: a .sub file holding bitmap images and an .idx file holding the timing and palette. Like Blu-ray SUP, they are pictures — not text — so they need OCR to become an SRT you can edit, translate or play anywhere.

Understanding the .sub + .idx pair

VobSub subtitles always travel in pairs. The .idx (index) file is a small text file describing languages, timestamps and the color palette; the .sub file contains the actual compressed subtitle images ripped from the DVD (dvd_subtitle codec). Both files are needed for conversion — keep them together with identical names.

VobSub images are standard-definition (720×480/576), so OCR is slightly more error-prone than with Blu-ray PGS — which makes an automatic cleanup pass after conversion especially useful.

From DVD rip to clean SRT

AI Subtitle Studio detects VobSub tracks embedded in MKV/AVI rips as well as sidecar .sub/.idx pairs sitting next to the video, and the Extract screen saves the track as a .sub/.idx pair — that extraction step does not produce text on its own. Take the pair to the Convert screen and click Convert: the app's bundled OCR engine (graphics card via DirectML on Windows when available, otherwise the CPU) reads the images into timed text, and the Clean Up tool can then repair misreads, spacing issues and leftover hearing-impaired annotations — all locally, with no external OCR software, and no internet connection after the recognition model's one-time download.

Convert SUB/IDX to SRT step by step

  1. Add your video or subtitle pair Drop the MKV/AVI with the embedded VobSub track — or the folder containing the .sub/.idx pair — into Extract (or straight into Convert if you already have the pair).
  2. Extract the VobSub track Tick the dvd_subtitle track on the Extract screen and click Extract. Embedded tracks are pulled out as a .sub/.idx pair — not yet an .srt.
  3. Convert it to SRT on the Convert screen Click Convert next to the finished track (or add the pair on the Convert screen), then click Convert. Each subtitle image is recognized into text with its original show/hide times.
  4. Fix OCR errors automatically Open the result in Tools → Clean Up with Fix OCR errors on to correct classic OCR misreads and produce a polished, editable .srt.

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Frequently asked questions

Do I need both the .sub and the .idx file?
Yes. The .sub holds the images and the .idx holds timing and palette data — conversion is not possible with only one of them. Keep both files in the same folder with the same base name.
Does extracting a VobSub track automatically give me an SRT?
No. Extraction only pulls the raw .sub/.idx image pair out of the video — you then run it through OCR separately on the Convert screen to get a text .srt.
Why does my TV not show .sub/.idx subtitles?
Many modern players and TVs only support text subtitles like SRT. Converting VobSub to SRT with OCR makes them work everywhere.
Can I choose which language to convert?
Yes — an .idx can index multiple language streams, and the app lets you pick the track you want before OCR.
Is the conversion offline?
Completely. Extraction, OCR and cleanup all run locally on AI Subtitle Studio's own bundled engine; your files never leave your computer.

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