Subtitling can be the same language as the dialogue aimed at the hard of hearing people or translated to another language for the understanding of foreign language dialogue.
There are two ways to transmit subtitles in a TV signal, open and closed. Open subtitles, or in-vision subtitles, are part of the picture that everyone sees. Closed subtitles are transmitted as data in parallel with the picture and can be decoded and displayed for each individual viewer. There are many standards for transmitting closed subtitles, like teletext, Line-21 closed captioning, DVB subtitling etc.
Most countries have legislation demanding that a high percentage of the programs to be subtitled using closed subtitles to allow also hearing impaired viewers to follow the programs. Subtitles also allow viewers to follow a program when it is not practical to listen to the audio, for example in public areas like sports bars. The subtitles for hard of hearing can also help viewers with poor language skills better understand the language used in the program.
In order to show foreign language content it is necessary to translate the dialogue. The translated dialogue can be transmitted using audio dubbing or subtitles. In most countries with a high proportion of imported programs subtitling is preferred. To display a single language the subtitles can be added to the picture prior to transmission. This method is good when all viewers understand the language of the subtitles or the original audio of the program. In order to service multiple language audiences such as for satellite transmissions to multiple countries it is required to transmit multiple languages in parallel with the video and to have the viewer select the language that he wishes to display in his TV set or set-top box.
Continue and read about subtitle workflow