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

1. Zencoder Guide to Closed
Captioning
FOR WEB, MOBILE, AND CONNECTED TV
About Zencoder
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encoding. Our service makes it easy to deploy Internet video on virtually any Internet-
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API means that you can get up and running quickly, while our industry-leading
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2. Captioning is coming to Internet video. Legislation goes
FCC Rule Rollout
into effect in the US during 2012 and 2013 that mandates Dates
closed 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 has
this legislation, closed captioning is a good thing for not been edited for
accessibility 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 24
If you want to publish video for web, mobile, and hours of broadcast on
connected TV delivery, what do you have to know about television.
9-30-13: Prerecorded
closed captioning? This guide outlines the basics: how
programming that is
closed captions work, formats you need to know about, edited for Internet
and 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. These
caption formats are written directly in a video file, either as a data track or
embedded into a video stream itself. Broadcast television uses this
approach, 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 caption
information to a player
alongside of a video,
rather than being
embedded in the video
itself. This approach is
usually used by browser-
based video playback
(Flash, HTML5).
A screenshot of a video encoded with captions
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3. What about subtitles? Are they the same thing as closed captions?
In some cases, captions and subtitles may be interchangeable, but there are
three main differences:
1. Goals. Closed captions are an accessibility feature, making video available
to the hard of hearing, and may include cues about who is speaking or about
what sounds are happening: e.g. “There is a knock at the door”. Subtitles are
an internationalization feature, making video available to people who don’t
understand the spoken language. In other words, you would use captions to
watch a video on mute, and you would use subtitles to watch a video in a
language that you don’t understand. (Note that this terminological distinction
holds in North America, but much of the world does not distinguish between
closed captions and subtitles.)
2. Storage. Historically, captions have been embedded within video, and
subtitles have been stored externally. (See CEA-608, below.) This makes sense
conceptually, because captions should always be provided along with a video;
100% accessibility for hard-of-hearing is mandated by legislation. Whereas
subtitles are only sometimes needed; a German-language video broadcast in
Germany doesn’t need to include German subtitles, but that same video
broadcast 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 off
at any time using the TV itself, but rarely have options for selecting a
language. In these situations when subtitles are added for translation
purposes, they are generally hard subtitles (see below) and thus cannot be
disabled. However, when viewing DVD/Blue-Ray/VOD video, the playback
device controls whether subtitles are displayed, and in which language.
A screenshot of a video with subtitles
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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 analog
television in the United States and Canada. Line 21 captions are encoded
directly into a hidden area of the video stream by broadcast playout devices. If
you’ve ever seen white bars and dots at the top of a program, that’s Line 21
captioning(1).
A screenshot of a video with Line 21 captions
An SCC file contains captions in Scenarist Closed Caption format. The
file contains SMTPE timecodes with the corresponding encoded caption
data 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 standard
file format for storing CEA-708 captions apart from a video stream.
(1) For examples and detailed breakdown, check out: http://nootropicdesign.com/
projectlab/2011/03/20/decoding-closed-captioning/
-3-

5. TTML stands for Timed Text Markup Language. TTML describes the
synchronization of text and other media such as audio or video(2). TTML
example:
<tt xml:lang="" xmlns="http://www.w3.org/ns/ttml">
<head>
<styling xmlns:tts="http://www.w3.org/ns/ttml#styling">
<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 that
defines when and how to display caption data. DFXP stands for Distribution
Format 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 in
DFXP: #data, #image, and #information.
SMPTE-TT is also the FCC Safe Harbor format – if a video content producer
provides captions in this format to a distributor, they have satisfied their
obligation to provide captions in an accessible format. However, video content
producers and distributors are free to agree upon a different format.
SAMI (Synchronized Accessible Media Interchange) is based on HTML and was
developed by Microsoft for products such as Microsoft Encarta Encyclopedia
and Windows Media Player. SAMI is supported by a number of desktop video
players.
_______________
(2) For more information, see W3C TTML Recommendations at www.w3.org/TR/ttaf1-
dfxp/
(3) For more info on the SMTE-TT Standard, refer to www.smpte.org/sites/default/
files/st2052-1-2010.pdf
(4) You can read more about the three extensions at codesequoia.wordpress.com/
-4-

6. EBU-STL is a binary format used by the EBU standard, stored in separate .STL
files.
EBU-TT is a newer format supported by the EBU, based on TTML. EBU-TT is a
strict subset of TTML, which means that EBU-TT documents are valid TTML
documents, but some TTML documents are not valid EBU-TT documents
because they include features not supported by EBU-TT.
SRT is a format created by SubRip, a Windows-based open source tool for
extracting captions or subtitles from a video. SRT is widely supported by
desktop video players.
WebVTT is a text format that is similar to SRT. The Web Hypertext Application
Technology Working Group (WHATWG.org) has proposed WebVTT(5) as the
standard for HTML5 video closed captioning. WebVTT example:
WEBVTT
00:00.76 --> 00:03.45
<v Eduard Khil>Trololololo
00:5.000 --> 00:10.000
lalala
00:10.000 --> 00:16.000
Oh-hahaha-ho
Hard subtitles (hardsubs) are, by definition, not closed captioning. Hard
subtitles are overlaid text that is encoded into the video itself, so that they
cannot be turned on or off, unlike closed captions or soft subtitles. Whenever
possible, soft subtitles or closed captions are generally be preferred, but hard
subtitles can be useful when targeting a device or player that does not
support closed captioning.
_______________
(5) For more information on WebVTT standard, refer to dev.w3.org/html5/webvtt/
-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. For
example, JW Player supports captions in SRT and DFXP format.
HTML5 captions are not yet widely supported by browsers, but that will
change over time. There are two competing standards: TTML, proposed
by W3C, and WebVTT, proposed by WHATWG. At the moment, Chrome
has limited support for WebVTT; Safari, Firefox, and Opera are all
working on WebVTT support; and Internet Explorer 10 supports both
WebVTT 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 framework
like Video.js can support captions through Javascript, by parsing an
external file. (Video.js currently supports WebVTT captions.)
iOS takes a different approach, and uses CEA-608 captions using a
modified version of CEA-708/ATSC legacy encoding. This means that,
unlike Flash and HTML5, captions must be added at the time of
transcoding. Zencoder can add captions to HTTP Live Streaming videos
for iOS.
Android video player support is still fragmented and problematic.
Caption support will obviously depend on the OS version and the player
used. Flash playback on Android should support TTML, though very
little information is available. (If you have delivered captions to native
Android 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 captioning
yet. But they will soon enough. Every TV, console, cable box, and Blu-
Ray player on the market today wants to stream Internet content, and
over the next year and a half, closed captioning will become a
requirement. So Sony, Samsung, Vizio, Google TV, et al will eventually
make caption support a part of their application development
frameworks. Unfortunately, it isn’t yet clear what formats will be used.
Most likely, different platforms will continue to support a variety of
incompatible 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 supporting
closed 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 required
for other devices, and hard subtitles are probably necessary for 100%
compatibility across legacy devices.
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