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    Understanding Closed Captioning Formats: What You Need For Your Player

    By: Aylin Dunham
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    With so many different caption file formats it can be difficult to know which ones you’ll need to get your video to display closed captioning. The CaptionSync Support Center provides a list of captioning formats available, but you need to know where your video will be played in order to choose from this list. This depends mainly on the platform or player you intend to use.

    When you play a video, the player will read the caption file if it supports closed captioning. Each player or platform has different file requirements for displaying captions. Most web media requires caption data to be held in a “side-car” file, meaning captions are in a separate file from the video. All players have slightly different settings when it comes to making caption file work.

    Rather than give you a long list of formats, we’ve picked several popular platforms and players to let you know which formats they accept. If you don’t find what you need here, the CaptionSync Support Center has details on many additional video players and caption file formats.


    Most commonly you’ll need an .SRT File. CaptionSync can send this file directly to your YouTube account when you submit the URL to your YouTube video on CaptionSync. This process, along with other output files are described in our support center.


    Caption files can be added to Vimeo during the video file upload or at any time. Vimeo accepts .SRT files as well as a few others.

    Ensemble Video

    Ensemble Video is integrated with CaptionSync which automatically makes the correct output formal available. You are also able to upload your own caption files by requesting an .ensemble.xml caption file, which is a variant of the DFXP caption file format. Follow these instructions to see how it’s done.

    TechSmith Camtasia

    For accessible, searchable and compliant videos in Camtasia from Techsmith you will need an .smi or .srt caption. See our tutorial on how to caption for Camtasia Studio 7.1 or later.


    For quick captioning with Blackboard Collaborate you can submit a list of Collaborate archive URLs and CaptionSync will generate captions for you that are encoded into the video file in M4V/MP4 video format. This format will allow you to view captions on any computer or mobile device with iTunes or Quicktime. View our tutorial.

    JW Player

    For JW Player you will generally need a .DFXP, .SRT, or .VTT file. Guidelines on JW Player caption files can be found here, or on our Support Center here.


    You can show caption files in Flowplayer as an external text file or embed them inside a mp4 container. Using CaptionSync’s Video Encoding you can also create subtitled videos.

    Windows Media Player

    Generally you use .SMI files with Windows Media Player but there are four different methods of implementing them you can read about here.

    QuickTime Player

    There are several ways to associate captions with your Quick Time videos. It is recommended that you use an .QT.XML.


    For Captions with RealPlayer add .rt.smil and .rt caption files, just place them in the same directory as your media file.

    Software Packages (iSubtitle, Handbrake, DivX, PS3, VLC, and MPlayer)

    Software packages, including open-source and freeware packages for captioning and subtitling, often require a .ASS file, also known as the Advanced SubStation Alpha format. 

    To view closed captioning formats for other players, look for it on our support page.

    Also, we note that we provide sample media files on the CaptionSync Support Center, and we can provide sample closed caption files in desired formats on request. Just log in to your CaptionSync account and open a support request or initiate a chat if you have any questions.