What is Closed Captioning?
When creating a video the main aim is to pass certain information. The viewers should be able to understand the information that you are trying to portray. There is no better way of passing this information by the use of subtitles and captions. Captions are additional texts or audio to the video that make it comprehensible. There are two types of captions it can either be closed or open. In the case of open captions the viewer is liable to see the caption and has no control on its deactivation where as it is the exact opposite for closed captions. Closed captioning allow the viewer to have the power of choice meaning he can be able to deactivate it if he wishes to. Closed captioning can sometimes be abbreviated as CC. Closed captioning was initially formed so as to assist the deaf and those that might have hearing problems but they also tend to help any foreigners watching the video.
How Does Closed Captioning Work?
There are several formats of closed captioning depending on how it is read, it can be embedded within the video or stored separately. The common formats are CEA-608 CSC, CEA-708, TTML, DFXP, SMPTE-TT, SAMI, EBU-STL EBU-TT, SRT and WebVTT. The first two formats are embedded in the video while the rest are stored separately. Choose a format that will be simple, faster and compatible with the device that the programmer intends to use.
The CEA-608 CSC for closed captioning is among the oldest formats. CEA is an acronym for consumer electronic association. It is usually embedded in the video as a signal which when played on a video device displays the captioned texts. It was initially formed for the purpose of the analogue television. It mainly appeared as dots and dashes at the top along line 21 on the vertical video index hence the name line 21. A decoder reads and displays the caption text. There are usually two fields in the vertical blanking interval line 21. The first field will include CC1 and 2 and TT1 and 2 while the second field includes CC3 and 4 and TT3 and 4. CC1 carries the English version while CC3 carries the Spanish captions. One of the downfalls of CEA-608 is that it can only be encoded in English, Spanish, Dutch, German, Italian, French and Portuguese. The texts are usually white in a black box and the color of the text cannot be changed. It is straight forward when it comes to extracting and encoding of the line 21 caption.
CEA-708 was later developed digital closed captioning as opposed to CEA-608 which was used for analogue television. The main difference between the 608 and 708 is that it is possible to change the color and font of the captions in 708. Another advantage is that you can be able to omit the background box or make it appear translucent. The only problem is that you cannot change the outlay of the captions only the programmer can be able to do that. For example if it is a popup it cannot be changed to roll on the screen. This technology does not limit languages since it can read any alphabet in the world. In digital television there is no vertical blanking interval thus the 708 is usually embedded in MPEG-2 video streams. It usually transmits the packets in picture order. This form of transmitting is called DVTCC transport stream.
TTML format for closed captioning stands for timed text markup language. It mainly deals with the delivery of captions over the internet through a streaming process. Most online video players use this format to display captions. A TTML document usually follows three profiles, DFXP presentation, DFXP transform and DFXP full. DFXP is an acronym for distribution format for exchange profile. The presentation profile is used for video players while the transformation profile is used for video editing in the case that they may want to convert the captions. This type of format mainly deals with time encoding since it is stored separately from the video.
SMPTE-TT standing for society of motion picture and television engineer – timed text is another form of formatting closed captioning. This format was pushed by the FCC standing for federal communications commission so as to offer internet services for people with disabilities. Its main advantage is that it can be used in the modern studios to make clear and better captions to be transmitted online. It also has the ability to convert CEA-608 data. It is also part of the TTML just like DXFP. It was included in TTML so as to allow the conversion of older formats like the CEA-608 for the purpose of storage or redistribution. It is the main protocol used in online viewing though it is slowly being outdated due to the coming up of new simpler modernized formats.
SAMI on the other hand which stands for synchronized accessible media interchange was developed by Microsoft for the purpose of closed captioning. It is much simpler to use and has no licensing charges making it easier than the other formats. SAMI captions are stored separate from the video as simple text files. It is much easier to edit, maintain and incorporate different languages using this format. The construction of the SAMI format is more or so equivalent to the HTML format the only difference might be the parameters used this makes it easier to use. One of the main advantages of SAMI is that it allows the use of different fonts and colors that may suite the programmer.
The next type of format for closed captioning is the EBU-STL EBU-TT format. EBU-TT and EBU-STL mainly coexist since the first one tends to follow up on the latter. EBU-TT tends to use an XML format. It allows the making and saving of subtitles and online distribution. Its main advantage is that it allows live encoding. It mainly constraints the TTML features so that it can be able to incorporate its videos to broadcast and web applications. It uses the issue of time based encoding so accuracy is an important issue in this sector.
SRT is a form of closed captioning format which uses the Sub rip program. Sub rip software removes the subtitles and their timing from a video. It is mainly a windows program. It usually saves the program with a .srt extension. When using SRT format to input subtitles it is important to take note on time. SRT like most of the other formats is time based encoding. The format of its timing is just like that of a digital watch hours, minutes, seconds, milliseconds respectively. It is inclusive of mainly four parts the number of the subtitle sequence, time in order of appearance and disappearance, subtitle text then closed by a blank line. This is the most common type of formatting used in captioning.
WebVTT which stands for Web Video Text Tracks provides caption for the video using a line based text. It is very similar to SRT. When using WebVTT for closed captioning the encoding should start with the phrase WEBVTT followed by an optional blank line. In its timing it uses commas instead of a full stop as in the case of the SRT format. It is much simpler to use the text editor that comes with the operating system. When saving your captions make sure that it has the extension .vtt.
Closed Captioning and Devices
To be able to create captions for devices that are flash enabled the computer must have java and a flash player installed. It is easy to create captions for devices that use a windows platform since it recognizes all captioning formats. A flash player is a type of program that enables the viewer to be able to watch videos online. Using the SRT format for this case will work perfectly. In the case of creating captions for HTML5 devices it will be better to use the TTML or WebVTT formats so that the video can be viewed. In the case for an android platform it will depend if the device is java enabled, if so the TTML format might work.
For iOS it using the older caption formatting types will be easier. Try and use the CEA-608 and CEA-708 formats. Most devices these days are usually java and flash enabled so choosing to use the SRT or TTML format will keep the programmer on the safer side if he does not know what type of platform the viewer will be using.