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Zencoder Guide to Closed Captions


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“On January 12, 2012, the FCC adopted rules requiring captioned programs shown on TV to be captioned when re-shown on the Internet.” — …

“On January 12, 2012, the FCC adopted rules requiring captioned programs shown on TV to be captioned when re-shown on the Internet.” —

Providing accessible media is not just a federal requirement, but it's good business; 48M people in the US (up to 15% of your site's visitors) are deaf or have some degree of hearing loss.

Learn about making your video accessible to a broader audience and in compliance with the 21st Century Communication and Video Accessibility Act (CVAA).

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  • 1. Zencoder Guide to Closed Captioning FOR WEB, MOBILE, AND CONNECTED TVAbout ZencoderZencoder is the performance leader in reliable, fast, high-quality cloud-based videoencoding.   Our service makes it easy to deploy Internet video on virtually any Internet-connected device, including web, mobile and connected devices.  Our simple yet powerfulAPI means that you can get up and running quickly, while our industry-leadingperformance provides the fastest turnaround.To learn more about Zencoder, visit http://zencoder.comor contact us at
  • 2. Captioning is coming to Internet video. Legislation goes FCC Rule Rolloutinto effect in the US during 2012 and 2013 that mandates Datesclosed captioning on certain categories of online content 9-30-12: Prerecorded TV(see side-bar text for specific dates). But even apart from programming that hasthis legislation, closed captioning is a good thing for not been edited foraccessibility and usability, and is yet another milestone as Internet distribution.Internet video marches towards maturity. 3-30-13: Live and near live programming that was recorded within 24If you want to publish video for web, mobile, and hours of broadcast onconnected TV delivery, what do you have to know about television. 9-30-13: Prerecordedclosed captioning? This guide outlines the basics: how programming that isclosed captions work, formats you need to know about, edited for Internetand how to enable closed captions for every device. distribution.How closed captions work. The first thing to understand is how closed captions are delivered, stored,and read. There are two main approaches today.1. Embedded within a video: CEA-608, CEA-708, DVB-T, DVB-S, WST. Thesecaption formats are written directly in a video file, either as a data track orembedded into a video stream itself. Broadcast television uses thisapproach, as does iOS.2. Stored as a separate file: DFXP, SAMI, SMPTE-TT, TTML, EBU-TT (XML),WebVTT, SRT (text), SCC, EBU-STL (binary). These formats pass captioninformation to a playeralongside of a video,rather than beingembedded in the videoitself. This approach isusually used by browser-based video playback(Flash, HTML5). A screenshot of a video encoded with captions -1-
  • 3. What about subtitles? Are they the same thing as closed captions?In some cases, captions and subtitles may be interchangeable, but there arethree main differences:1. Goals. Closed captions are an accessibility feature, making video availableto the hard of hearing, and may include cues about who is speaking or aboutwhat sounds are happening: e.g. “There is a knock at the door”. Subtitles arean internationalization feature, making video available to people who don’tunderstand the spoken language. In other words, you would use captions towatch a video on mute, and you would use subtitles to watch a video in alanguage that you don’t understand. (Note that this terminological distinctionholds in North America, but much of the world does not distinguish betweenclosed captions and subtitles.)2. Storage. Historically, captions have been embedded within video, andsubtitles have been stored externally. (See CEA-608, below.) This makes senseconceptually, because captions should always be provided along with a video;100% accessibility for hard-of-hearing is mandated by legislation. Whereassubtitles are only sometimes needed; a German-language video broadcast inGermany doesn’t need to include German subtitles, but that same videobroadcast in France would.3. Playback. Since captions are passed along with the video and interpreted/displayed by a TV or other consumer device, viewers can turn them on and offat any time using the TV itself, but rarely have options for selecting alanguage. In these situations when subtitles are added for translationpurposes, they are generally hard subtitles (see below) and thus cannot bedisabled. However, when viewing DVD/Blue-Ray/VOD video, the playbackdevice controls whether subtitles are displayed, and in which language. A screenshot of a video with subtitles -2-
  • 4. Formats and standards.There are dozens of formats and standards for closed captioning and subtitles.Here is a rundown of the most important ones for Internet video.CEA-608 (also called Line 21) captions are the NTSC standard, used by analogtelevision in the United States and Canada. Line 21 captions are encodeddirectly into a hidden area of the video stream by broadcast playout devices. Ifyou’ve ever seen white bars and dots at the top of a program, that’s Line 21captioning(1). A screenshot of a video with Line 21 captionsAn SCC file contains captions in Scenarist Closed Caption format. Thefile contains SMTPE timecodes with the corresponding encoded captiondata as a representation of CEA-608 data.CEA-708 is the standard for closed captioning for ATSC digital television(DTV) streams in the United States and Canada. There is currently no standardfile format for storing CEA-708 captions apart from a video stream.(1) For examples and detailed breakdown, check out: -3-
  • 5. TTML stands for Timed Text Markup Language. TTML describes thesynchronization of text and other media such as audio or video(2). TTMLexample:<tt xml:lang="" xmlns=""> <head> <styling xmlns:tts=""> <style xml:id="s1" tts:color="white" /> </styling> </head> <body> <div> <p xml:id="subtitle1" begin="0.76s" end="3.45s"> Trololololo </p> <p xml:id="subtitle2" begin="5.0s" end="10.0s"> lalala </p> <p xml:id="subtitle3" begin="10.0s" end="16.0s"> Oh-hahaha-ho </p> </div> </body></tt>DFXP is a profile of TTML defined by W3C. DFXP files contain TTML thatdefines when and how to display caption data. DFXP stands for DistributionFormat Exchange Profile. DFXP and TTML are often used synonymously.SMPTE-TT (Society of Motion Picture and Television Engineers – Timed Text(3))is an extension of the DFXP profile that adds support for three extensions(4)found in other captioning formats and informational items but not found inDFXP: #data, #image, and #information.SMPTE-TT is also the FCC Safe Harbor format – if a video content producerprovides captions in this format to a distributor, they have satisfied theirobligation to provide captions in an accessible format. However, video contentproducers and distributors are free to agree upon a different format.SAMI (Synchronized Accessible Media Interchange) is based on HTML and wasdeveloped by Microsoft for products such as Microsoft Encarta Encyclopediaand Windows Media Player. SAMI is supported by a number of desktop videoplayers._______________(2) For more information, see W3C TTML Recommendations at dfxp/(3) For more info on the SMTE-TT Standard, refer to files/st2052-1-2010.pdf(4) You can read more about the three extensions at -4-
  • 6. EBU-STL is a binary format used by the EBU standard, stored in separate .STLfiles.EBU-TT is a newer format supported by the EBU, based on TTML. EBU-TT is astrict subset of TTML, which means that EBU-TT documents are valid TTMLdocuments, but some TTML documents are not valid EBU-TT documentsbecause they include features not supported by EBU-TT.SRT is a format created by SubRip, a Windows-based open source tool forextracting captions or subtitles from a video. SRT is widely supported bydesktop video players.WebVTT is a text format that is similar to SRT. The Web Hypertext ApplicationTechnology Working Group ( has proposed WebVTT(5) as thestandard for HTML5 video closed captioning. WebVTT example:WEBVTT00:00.76 --> 00:03.45<v Eduard Khil>Trololololo00:5.000 --> 00:10.000lalala00:10.000 --> 00:16.000Oh-hahaha-hoHard subtitles (hardsubs) are, by definition, not closed captioning. Hardsubtitles are overlaid text that is encoded into the video itself, so that theycannot be turned on or off, unlike closed captions or soft subtitles. Wheneverpossible, soft subtitles or closed captions are generally be preferred, but hardsubtitles can be useful when targeting a device or player that does notsupport closed captioning._______________(5) For more information on WebVTT standard, refer to -5-
  • 7. Captioning for every device.What formats get used by what devices and players?Flash video players can be written to parse external caption files. Forexample, JW Player supports captions in SRT and DFXP format.HTML5 captions are not yet widely supported by browsers, but that willchange over time. There are two competing standards: TTML, proposedby W3C, and WebVTT, proposed by WHATWG. At the moment, Chromehas limited support for WebVTT; Safari, Firefox, and Opera are allworking on WebVTT support; and Internet Explorer 10 supports bothWebVTT and TTML. Example:<video width="1280" height="720" controls> <source src="video.mp4" type="video/mp4" /> <source src="video.webm" type="video/webm" /> <track src="captions.vtt" kind="captions" srclang="en" label="English" /></video>Until browsers support a format natively, an HTML5 player frameworklike Video.js can support captions through Javascript, by parsing anexternal file. (Video.js currently supports WebVTT captions.)iOS takes a different approach, and uses CEA-608 captions using amodified version of CEA-708/ATSC legacy encoding. This means that,unlike Flash and HTML5, captions must be added at the time oftranscoding. Zencoder can add captions to HTTP Live Streaming videosfor iOS.Android video player support is still fragmented and problematic.Caption support will obviously depend on the OS version and the playerused. Flash playback on Android should support TTML, though verylittle information is available. (If you have delivered captions to nativeAndroid video apps, please let us know!)Some other mobile devices have no support for closed captions at all,and hard subtitles may be the only option. -6-
  • 8. Roku supports captions through external SRT files.Some other connected TV platforms do not support closed captioningyet. But they will soon enough. Every TV, console, cable box, and Blu-Ray player on the market today wants to stream Internet content, andover the next year and a half, closed captioning will become arequirement. So Sony, Samsung, Vizio, Google TV, et al will eventuallymake caption support a part of their application developmentframeworks. Unfortunately, it isn’t yet clear what formats will be used.Most likely, different platforms will continue to support a variety ofincompatible formats for many years to come.Closed captioning for Internet video:2012 edition.The landscape for closed captioning will change and mature over time,but as of 2012, here are the most common requirements for supportingclosed captioning on common devices. • A web player (Flash, HTML5, or both) with player-side controls for enabling and disabling closed captioning. • An external file with caption data, probably using a format like WebVTT, TTML, or SRT. More than one file may be required – e.g. SRT for Roku and WebVTT for HTML5. • A transcoder that supports embedded closed captions for HTTP Live Streaming for iPad/iPhone delivery, like Zencoder. Zencoder can accept caption information in a variety of formats, including TTML, so publishers could use a single TTML file for both web playback and as input to Zencoder for iOS video.Beyond there, things get difficult. Other input formats may be requiredfor other devices, and hard subtitles are probably necessary for 100%compatibility across legacy devices. -7-