Teletext Tutorial (3)
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Teletext Tutorial (3) |
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 Subtitling is an application of teletext that is fast growing in use as the number of TV sets with the teletext facility increases. In many countries, subtitling is the dominant method used to convey the spoken dialogue of foreign language programmes and it is also being used increasingly as a service to the deaf and hard of hearing. Although subtitling presupposes that the viewer has reading ability (a constraint for young children, the illiterate and those with poor sight), and although subtitling cannot convey all the information in the foreign dialogue, it does have certain advantages compared to sound dubbing. The most important advantages are: it is less expensive; In view of these advantages, subtitling is now regarded in some countries as an important component of the overall television programme service. It is to be expected that, with the increasing use of satellites transmitting to several countries, there will be a further demand for subtitles.
Two different methods are used to provide subtitles to the television audience. One, known as 'in-vision' or 'open' subtitling, involves the insertion of the text directly into the displayed picture prior to on-air transmission. In the second, know as 'closed' subtitling, the subtitles are transmitted as encoded data carried in a channel (e.g. teletext) accompanying the picture signal. For both methods the means of subtitle preparation is almost the same. Most of the systems in operation today use a personal computer with dedicated word-processing software, a video tape recorder, a time-code reader, a subtitle keyer and a character generator. The three main steps in the preparation of subtitles are: the dialogue is translated into text. the translated text is shortened to ensure that it is easily legible on the screen. The subtitles are tagged to indicate their timing relative to the programme. EBU vertical-interval or longitudinal timecode is often used for this purpose. In general, a two row subtitle with thirty or forty characters will be displayed for about 5 or 6 seconds. The data file containing all the subtitle information (text and time-code data) is stored in the PC. It can then be transferred to a floppy disk for distribution with the video tape, or perhaps recorded as ancillary data on a dedicated line of the Vertical Blanking Interval 9VBI), on the same tape as the programme.
Until recently subtitles for a television programme or film could be translated several times into different languages - or into the same language - according to the requirements of different distribution media (cinema, home video, television broadcast, cable television, etc.). Furthermore manufacturers of subtitling equipment have all used different data file formats. This has resulted in a duplication of effort and incompatibility of technologies. In 1991, the European Broadcasting Union (EBU) developed a standard subtitle exchange format for subtitle information to reduce these problems.
Transmission of subtitles by means of teletext is normally achieved using a dedicated page number in the teletext service. In the UK all four terrestrial channels use page 888. An inserter is used to place subtitle data into the Vertical Blanking Interval of the video signal using programme time code to synchronise insertion with the dialogue. The transmitted subtitle page is formatted so that the teletext receiver superimposes the titles over the picture. A control signal in the subtitle page informs the decoder that this is a subtitle page to be superimposed over the programme instead of the normal teletext only display.
Teletext can be used to simultaneously transmit multi-lingual subtitles. Many European countries have to cater for perhaps two or three spoken languages all of which need to be provided as subtitles. A separate teletext page is used for each language. Satellite broadcasters can face an even larger requirement having to simultaneously transmit up to ten sets of subtitles.
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Subtitling