HTML and CSS Reference
In-Depth Information
(or even .ogg ). The MPEG-4 container uses Advanced Audio Coding (ACC) for its
audio codec. Since it is in the MPEG-4 container, it generally uses the file extension
.m4a to differentiate it from .mp4 . ACC is the successor to the MP3 format that is
widely used in digital music (for portable music players such as the Apple iPod). ACC-
encoded audio may be freely played over the Web (barring any licenses needed for the
content itself), but it is not completely patent free. A license is needed for developing
tools for encoding and decoding ACC (if you're curious, license fee costs are listed here: ) .
Note The WAV audio format can also be used and is sometimes included in the list
of fallback content. WAV is an uncompressed audio format, which means the files are
relatively large compared to Ogg Vorbis and ACC. MP3 may also be used but MP3 has
stricter licensing issues surrounding it than ACC, so it is best avoided in favor of ACC.
Both WAV and MP3 are not supported universally across all major browsers either, so
there is nothing to gain from their use on that front.
Captions track
HTML5 adds a great new feature that opens up the accessibility of videos. The track
element has been added to the HTML specification to define a method of adding sub-
titles and related timed text tracks to the video and audio elements. By “added,” I
mean it has been added to the specification, because web browser makers are still work-
ing toward implementing the element's functionality. The general idea is that a text file
is marked up with textual information that is then loaded with the video or audio and is
usually timed to be presented at points throughout the playback of the media resource.
The track element would then be placed in the video or audio element and be used
to load the timed text file. The track element defines five attributes: kind , src , la-
bel , srclang , and default .
Note If you want to provide closed captioning in your video today, check out the
Captionator project ( ) . This
project uses JavaScript to emulate functionality of the track element so that it can be
used while the web browsers catch up with its implementation.
The kind attribute determines what kind of text track is being added (see Table 5-4 );
if omitted, the default is to identify the loaded text file as being of the subtitles kind.
The src attribute defines the address of the text file to load. The actual format for the
text file is not defined, but it will likely be a format called Web Video Text Track (We-
bVTT). The file extension is .vtt . There are other formats, however, such as Timed
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