DVB stands for Digital Video Broadcasting is a consortium of broadcast industry companies. DVB technology delivers digital channels in a multiplex and relies on the viewer having a suitable digital receiver either as a standalone Set Top Box (STB) or built into their Digital TV set. The transmission method may either be digital terrestrial (DTT), satellite, or cable distribution. Not all receivers support DVB subtitles.
The DVB standard (EN 300-743) defines a bitmap based subtitling format. A DVB compliant stream can also carry Teletext data and Subtitles.
Subtitles are generated as a graphical bitmap and then encoded and compressed into the DVB delivery format). The subtitle data is multiplexed with the video, audio data and possibly other subtitle languages and then transmitted as part of the station’s output transport stream to the viewer’s decoder. Once a decoder receives the data the subtitle is re-constructed in the decoder’s memory. A user can select whether to display subtitles or not and, if available, which one of several languages. At the appropriate time the decoder will then display the subtitle on-screen.
Because the subtitles are delivered as a graphical bitmap, unlike the character code based Teletext and US style Closed Captions formats, there is no restriction on the languages that can be supported. This is an advantage when new or special characters need to be added such as the Euro symbol or new Chinese characters.
It is also possible to create special characters or logos but there are some restrictions on image size and colour depth.
Because the DVB multiplex can carry multiple subtitle streams multiple languages can be supported.