As much as we at Ai-Media like to consider ourselves as WordNerds, we also pride ourselves in being TechNerds. This is the first in a series of tech-related blogs which will explore what happens when the microphones are switched off and the Dragon* is put to sleep.

If you’ve ever thought about how those words appear on screen in time with the dialogue, this blog will explore how all those words and timecodes are stored in a caption file, and how different file types affect what information can be stored in them.

A caption file is quite simple (no, really). A caption file is, in the most basic terms, a transcript of what is said in a video or audio file, which has been split into chunks of 76 characters or less, and given a “time in” and a “time out”. Depending on the file format, you can also add colours and control the positioning of the captions.

EBU STL (try saying that 3 times in a row)

EBU STL is a widely used broadcast standard file. It supports colouring of the background and foreground, as well as positioning. It’s a binary file, meaning if you open it in Notepad you see a bunch of gobbledygook (see below), but worry not, when streamed to your television it is truly beautiful!


Traditionally, this file type was only used for television broadcast. However, more recently it has been supported by YouTube and other online video players – meaning you can now have coloured and positioned captions online!



SRT files are the most common caption file used by online video players such as YouTube and Vimeo. They are considered the most basic of all caption file formats. They are essentially a text file with very specific formatting, allowing them to be opened and edited by most programs.


Because of their basic structure, traditional SRT files do not support any colouring or positioning – meaning all white captions at the bottom of the screen.srtyoutube


Some online video players prefer the .XML extension. There are many different .XML extension file types and the online players can be quite picky when it comes to which one they will accept. The .XMLs also can support different things between different versions, so some can support colouring while others do not.

XMLs are a little bit more complex compared to SRTs, but can still be opened and edited by most word processors.xml

Other supported closed caption file types

DVD Text Exports

  • Apple Studio Pro
  • QTtext
  • Spruce
  • Sonic
  • SCC

XML Files in the following formats

DFXP Export

DFXP export is based on the W3C Timed Text (TT) Authoring Format 1.0 – Distribution Format Exchange Profile (DFXP)

  • PAC Files and SVT PAC Files (.PAC)
  • L21 SMPTE2052 TTML

For Line21 there is an SMPTE2052 TTML export that includes 608 tunnelling and metadata options.

  • SIF Files
  • Closed EBU Files (different .STL format)
  • Net Link Files (.NLK)
  • Real Text format Files (.RT)
  • SAMI format Files (.SMI)
  • Scantitling Files (.890)
  • Screen EBU Files (different .STL format)
  • Spruce Text Export (different .STL format)

More supported file types

  • Adobe Encore FAB
  • Avid STL
  • CapMaker Plus
  • Cavena 890
  • Cheetah CAP
  • DVD Studio Pro STL
  • EDL
  • Final Cut Pro
  • Spumux
  • VobSub

What about audio/video file types?

We can create captions from almost any audio or video file type by utilising our multiple codecs and transcoders that transform the files provided into file types that are supported by our software suite.



*While we may or may not own a pet dragon, in this instance, Dragon refers to voice recognition software used by our live captioners.


Written by Namik Sedlarevic, Offline Operations Manager

namik headshot

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