A playout subtitle system is based on subtitle files that are separate from the video and audio material. While the video material is stored on a master tape or in a video server the subtitle files are stored on a separate subtitle server.
The files are only linked to the video media by a common asset number, normally via the file name. The individual subtitles are synchronised via the original tape time code. This will have been used at the time the subtitle file was authored and it is vital that it remains unchanged at transmission time.
Because the video media and the subtitle file are only combined at the time of playout, the subtitle playout system needs to have the correct file loaded and to know the tape time code. This is achieved by having a station automation input and a matching tape (or server) time code input for each channel.
Automation systems vary but in most cases a playlist will be available that shows a number of upcoming events and this allows the subtitle system to preload the matching subtitle files and to raise an alarm if a file is not present.
When a subtitled event is put on-air by the station automation the channel time code for that event is fed to the transmission system and the subtitle file is played out to the attached output devices such as character generators, Teletext VBI inserters, DVB encoders etc.