Closed Captioning

Closed captions or subtitling is also known as Line 21 captions as line 21 is the preferred video line the caption data is inserted onto (NTSC video line numbering only). With closed captioning the viewer has the choice whether or not to display the captions. The EIA-708 standard has been introduced to support Closed Captioning in a digital environment. Closed Caption subtitling should not be confused with the technically similar Teletext system. See here.

Because the Closed Caption system is character based the character set and font style available is fixed and can not be changed. The character set is limited to the Latin alphabet (A-Z) plus a limited number of accented characters. This makes the system suitable only for English and the other main European languages.

Details

Closed Caption is a code based technology; this means that a number is used to represent each character in a caption. So for example A = 65, the coding is based on the ASCII standard used in many early computers.

Two characters are encoded on a single line in one field of video. Normally this is on Line 21 (NTSC numbering); hence the alternative name of Line 21. This gives a character rate of 60 characters per second although this is normally divided into two streams of 30cps known as CC1 and CC2. It is important that this low data rate is not exceeded and this limitation is one of the complexities of a Closed Caption system.

Because the Closed Caption system is character based the character set and font style available is fixed and can not be changed. The character set is limited to the Latin alphabet (A-Z) plus a limited number of accented characters. This makes the system suitable only for English and the other main European languages.

To display Closed Captions on a receiver it must be equipped with a suitable decoder and the user must select CC to be on. The vast majority of NTSC receivers are capable of decoding and displaying Closed Captions.

Closed Captions are also used on VHS tapes in both PAL and NTSC formats although for PAL a special decoder is required as most PAL receivers do not support Closed Captions.

Basic Character Set                                            Extended Character Set.