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Caption Format Acronyms, Explained

  • Jumble of letters with 'SRT' legible

    If you’ve just started looking into how closed captioning works, you’ve probably come across terms for caption file formats like SRT, SMPTE-TT, DFXP, WebVTT, etc. The list goes on.

    If you’re drowning in a sea of acronyms, here’s a breath of fresh air: a list of caption file formats broken down by name, compatibility, and use case.

    What does SRT stand for?

    SRT (.srt) stands for “SubRip Subtitle” file, and it’s the most common subtitle/caption file format. It is a text format that originated in the DVD-ripping software SubRip, hence the name.

    A caption frame in SRT consists of:

    1. A number indicating which subtitle it is in the sequence.
    2. The time that the subtitle should appear on the screen, and then disappear.
    3. The caption text.
    4. A blank line indicating the start of a new subtitle.

    SRT caption files are compatible with videos on media players, lecture capture software, and video recording software like:

    SRT caption file
    Excerpt of SRT caption file

    What does WebVTT stand for?

    Developed by the Web Hypertext Application Technology Working Group (WHATWG), WebVTT stands for “Web Video Text Tracks.” Modeled after the SRT format, it’s a relatively new, user-friendly caption format that consists of line numbers, timelines, and text with formatting options.

    WebVTT (.vtt) is similar to SRT format, except that it accommodates text formatting, positioning, and rendering options (pop-up, roll-on, paint-on, etc.). It has quickly gained in popularity because it is the caption format of choice for HTML5 text track rendering.
    WebVTT caption file
    WebVTT caption files are compatible with videos on cloud-based, HTML5 media players and video management systems like:

    What does SCC stand for?

    SCC (.scc) stands for “Scenarist Closed Captions.” It’s commonly used with broadcast and web video, as well as DVDs and VHS videos.
    SCC caption file
    SCC file data is based on closed captioning data for CEA-608. i.e., Line 21 or EIA-608 broadcast data; this used to be the standard transmission format for closed captions in North America.

    SCC caption files are compatible with:

    What does STL stand for?

    STL stands for “Spruce Subtitle File.” It was developed by Spruce Technologies primarily for used in DVD Studio Pro software. The STL format allows you to configure most subtitle settings and change the settings on a subtitle by subtitle basis.

    STL subtitle files consist of:

    1. Commands: These are preceded by the dollar sign ($). It is these commands that allow you to configure the various aspects of the subtitles, such as their font and position.
    2. Comments: These are preceded by a double slash (//). These allow you to add text comments throughout the subtitle file without affecting its import.
    3. Entries: These include the start and end timecode values and the text or graphics file for that subtitle clip.

    STL caption files are compatible with:

    What does DFXP stand for?

    DFXP (.dfxp) stands for “Distribution Format Exchange Profile.” It’s a timed-text format that was developed by W3C and mostly commonly used for Flash video captions.
    DFXP caption file
    DFXP is used by many online video providers, but typically in a limited role without full CEA-608 features (making that video non-compliant with the CVAA rules for TV content online).

    DFXP caption files are compatible with videos on media players, lecture capture software, and video management systems like:


    What does TTML stand for?

    TTML stands for “Timed Text Markup Language.” TTML a class of XML file that can come in various formats, including DFXP. TTML is often used interchangeably with the term DFXP, although there can be TTML files that are formatted slightly differently from DFXP. TTML files have the file extension (.ttml).

    TTML caption files are compatible with videos on media players, lecture capture software, and video recording software like:

    What does SMPTE-TT stand for?

    SMPTE-TT stands for the “Society of Motion Picture and Television Engineering – Timed Text.” It is a type of XML file developed by SMPTE.

    Television content providers like to use SMPTE-TT because it is compliant with FCC closed caption regulations for broadcasters, unlike other formats like DFXP. Another important difference of SMPTE-TT captions is that they reference video frames instead of video time. SMPTE-TT files end is extension .xml.

    4 features that set SMPTE-TT apart from DFXP/TTML:
    SMPTE-TT caption file

    1. The #image attribute can display .png images.
    2. The #data feature allows the player to pass CEA-708 data (the standard for captioning digital TV) through to the video player, as well as CEA-608 data (the line-21 standard for broadcast TV captioning).
    3. SMPTE-TT attributes traditionally associated with subtitles, including foreign-alphabet characters and some mathematical symbols.
    4. The #information feature tells the player whether to display the caption data with the original look and feel (preserve mode) or to take advantage of the more advanced display capabilities (enhance mode).

    What does QT stand for?

    QT stands for “QuickTime.” Developed by Apple, QT is used exclusively for QuickTime Pro video or audio files.

    QT is compatible with:

    What does CAP stand for?

    This is a common subtitle/caption file format for broadcast media.

    It was developed by Cheetah International to accommodate characters in many languages for international use. Cheetah files have the file extension .asc or .cap. This format is used in professional video editing systems.

    What does CPT.XML stand for?

    CPT.XML stands for “Captionate XML.” It’s an XML format originating in the caption-embedding software Captionate and used for encoding captions into Flash video.

    CPT.XML is compatible with:

    What does PPT.XML stand for?

    PPT.XML stands for “PowerPoint XML,” a customized TTML file that works with STAMP in PowerPoint.

    PPT.XML caption files are compatible with:

    What does EBU.STL stand for?

    EBU.STL stands for “European Broadcasting Union subtitles.” This is a common subtitle/caption file format for PAL broadcast media in Europe.

    EBU.STL captions are typically used professional video editing systems, like Avid.

    What does RT stand for?

    RealText (.rt) captions are timed-text file for RealMedia. Similar to SMIL markup, RealText is a very simple text file that consumes minimal bandwidth and streams quickly to RealOne Player.

    RealText caption files are compatible with media players:

    • RealPlayer
    • RealOne Player

    What does SAMI or SMI stand for?

    SAMI or SMI caption file
    Developed by Microsoft, SAMI — or SMI, as it is also known — stands for “Synchronized Accessible Media Interchange.” Used for Windows Media video or audio. SMI files end in either .sami or .smi extensions.

    (See example to the right)

    SMI caption files are compatible with video media players like:

    What does SBV or SUB stand for?

    SBV or SUB both stand for “SubViewer.” This is a very simple YouTube caption file format that doesn’t recognize style markup. It’s a text format that is very similar to SRT.

    Compatible with:

    Other Caption File Formats

    These formats are examples of customized or less frequently used caption file types. For many of these, a more common caption format may be substituted.

    • ADBE – Adobe
    • Apple XML – Apple XML Interchange Format
    • AAF – Avid
    • Avid DS – Avid
    • CCA – MacCaption
    • ONL – CPC 715
    • Crackle TT – Crackle Timed Text (variant of SMPTE-TT)
    • DECE CFF – Variant of SMPTE-TT with auxiliary PNG files
    • Evertz ProCAP
    • ITT – iTunes Timed Text
    • Matrox4VANC – Matrox for MX02
    • MCC – MacCaption
    • MCC V2 – MacCaption
    • Multiplexed SCC – Multiple CC
    • Rhozet – XML file
    • SonyPictures TT – Sony Pictures Timed Text XML
    • TIDLP Cinema – Texas Instruments DLP Cinema XML
    • WMP.TXT – Windows Media timed text file
    • LRC (.lrc) – No styling, but enhanced format supported.
    • Videotron Lambda (.cap) – Primarily used for Japanese subtitles.

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