National Captioning Canada can deliver closed captions and subtitles for prerecorded videos in any number of ways, but more often than not, our clients need what’s called a “sidecar” caption file.
This means that we will be sending them a caption file that contains all the necessary timing and textual information while being separate from the video itself. “Sidecar” might bring to mind visions of modified motorcycles, and if you think about it, that’s kind of what a sidecar caption file does: the caption file rides alongside the other program elements, balancing out access to the content.
For television broadcasts, a sidecar file is most often embedded into the signal so that TV viewers at home can toggle captions on or off. When it comes to web videos, a sidecar caption file is typically uploaded to the platform/player along with the video so that viewers can toggle captions on or off using their individual browsers.
But what form do these sidecar files take? Here’s a quick rundown of our top five most common sidecar subtitle and caption file types.
If there was a caption file popularity contest, the leading vote-getter and envy of all the other caption files would no doubt be a Scenarist closed caption (.SCC) file.
Unlike its fellow OG caption file, the aptly named .CAP file, the .SCC has adapted to the world of content beyond traditional broadcast, and it’s got our top spot on this list because it can also be used for delivery to many web players. It’s also compatible with most video editing software. In addition to the baseline caption text and timing information necessary for all closed caption files, .SCCs can also contain style information such as positioning, italics, and music notes – which are valuable tools in the captioner’s arsenal to visually convey audio elements, such as dialogue spoken off-screen and lyrics being sung. For all of its well-established attributes, one drawback of the .SCC file is that no matter the destination, it requires decoding. If you were to open it up and look under the hood (not to mix automotive metaphors), you’d see a bunch of numbers and letters arranged in a language only robots speak.
A SubRip (.SRT) file is by far the most common sidecar file for prerecorded web videos, and is preferred by many video hosting platforms (including YouTube) thanks to its adaptability. It can be used for both closed caption and subtitle workflows.
Elegant in its simplicity, the .SRT file can be opened up and read by a human. It is a basic text file that contains sequential numbering for each caption, its in-time and out-time, and the text content of the caption itself. When uploaded with its corresponding video, web players use the timing information to sync the captions so they display properly for end users. Some video players can decode formatting information in .SRTs (italics, positioning, etc.), but not most.
.SRTs are also an immensely popular subtitle deliverable because they can contain characters in almost any language and can sometimes be used in video editing software for rendering.
The evolution of the .SRT, the WebVTT has been on the rise in popularity due to its use in HTML5 media playback. It has been one of the “it” formats that everyone agrees to use, and like all of its predecessors, it has seen other “it” formats speed on by, sitting in the sidecar of some cool new bike. But prerecorded captions aren’t a race; they’re a road trip, and the .VTT is the friend who brings all the good snacks.
On the surface, .VTTs look almost identical to the .SRTs; when you look closer, they are much more feature-rich, with more universal support of stylistic elements like italics and movement of text. Some players can use .VTTs, while others like Vimeo prefer them.
Coming in fourth, we have one file type that contains multitudes, the Extensible Markup Language (.XML) file.
XML-based caption files are so popular because they can come in many formats – the most common variations being .TTML, .DFXP, and SMPTE-TT. Each has its own unique quirks that are more compatible with various video players and editors, all of which are too complex to dive into here. Most of these variations can be used for multiple languages (which makes them quite useful for localization and subtitling), but support of the other stylistic features we’ve touched on varies between .XMLs.
Last but not least, the .ITT, or iTunes Text file, is a sidecar file developed for use on—you guessed it—iTunes. While media playback on Apple devices has evolved over the years, the captioning and subtitling world honors the legacy of the pioneering media player by whole-heartedly embracing the .ITT. Like other non-broadcast formats on this list, the .ITT offers a human-readable combination of text and timing, with the same broad support of many stylistic features and multi-language characters that make text-based sidecar files so appealing.
Remember: sidecar files are just along for the ride. The player is driving, so while features like placement and italics may be possible in a caption or subtitle file, whether they show up where you expect them depends on the player bringing them there, and some players just don’t take the path you expect.
Because they’re rebels.
Capture File (.CAP)
Advanced Authoring Format (.AAF)
MacCaption Caption file (.MCC)
Spruce Subtitle File (.STL)