Transcoding of subtitles is the process of reading subtitle data from a signal or data stream and convert those subtitles on the fly to one or more other transmission formats.
STU operates in transcode mode to perform this function. The transmission formats that STU reads include VBI data in the form of Line-21 captions and EBU teletext, as well as DVB streams with DVB Teletext or DVB Subtitling. A typical example of an STU transcode application is to read US Line-21 captions from an NTSC signal and transcode it into EBU Teletext for PAL output. Another example is to read EBU teletext from the VBI of an SDI signal played out from tape with VBI burned in and transcode this into DVB Subtitling to a DVB multiplexer.
STU is also capable of switching between file based Playout mode and Transcode mode. This can be accomplished in several ways like automation control or GPI. STU can also monitor an input signal for subtitle data and switch mode whenever subtitle data is found. A timeout switches STU back to file based playout. This is typically used when commercials carry subtitling in the VBI on tape, while the program material plays subtitles from file.