Ravnur Media Services (RMS) supports Apple Tx3g captions for HLS playback. This simplifies the way accessibility is retained during content migration or re-encoding.
In this article:
About Tx3g captions
Tx3g is Apple’s implementation of MPEG-4 Timed Text, commonly used to carry subtitles and closed captions inside MP4 files. It’s been a standard part of Apple-centric workflows for years, powering captions in content prepared for iTunes, iOS devices, and many legacy streaming pipelines. That means a huge amount of existing video libraries - especially those built around Apple tooling - already rely on Tx3g for subtitles and accessibility metadata.
How RMS handles Tx3g captions
RMS ensures that all caption data is preserved and converted automatically when you ingest your content into the platform.
When you upload a video that contains the Tx3g caption track(s), RMS:
Automatically detects the embedded Tx3g stream and all languages.
Extracts the captions into a sidecar WebVTT file.
Stores the WebVTT file in the same asset container as the encoded renditions.
Updates the HLS manifest to reference the WebVTT caption track(s).
Because the manifest is updated to reference the WebVTT file, players can automatically discover and display captions from the manifest - no extra wiring, no manual file handling, no re-authoring subtitles.
Tx3g captions are preserved end-to-end through the RMS encoding pipeline and remain available to viewers without additional configuration on your part. You keep your existing accessibility investments, your viewers keep their captions, and your team gets a simpler workflow for streaming delivery.
Important:
RMS does not support Tx3g captions for MPEG-DASH playback. Use HLS playback instead.