Teletext is a common name for teletext, and it is a new type of TV additional information service system. It came out in 1970s and has been widely used in more than 50 countries. In recent years, the teletext developed by the relevant departments in China has also started the pilot service, and the CCST teletext technical specification compatible with Chinese and western languages has been formulated in 1992.

Teletext uses broadcast television or closed-circuit television system to send ordinary TV programs and additional graphic information at the same time. These additional contents include news flash, program forecast, meteorological service, traffic information, stock market, foreign exchange price comparison and other charts and texts, just like a "TV newspaper". This information is not transmitted through the new channel, but is attached to a blank part of the ordinary TV signal transmission channel, where no information is loaded and uploaded to the user.

As we all know, TV images are composed of scanning lines. Take the TV system in China as an example, a frame grating has 625 scanning lines. However, the image displayed on the screen usually consists of more than 500 scanning lines. The main reason is that 50 lines per frame are used to transmit non-video signals such as field synchronization and field blanking to ensure the normal reception of TV. In the process of field blanking signal transmission, it is a waste not to transmit information on the whole line. According to the principle of time division multiplexing, during the field blanking period of normal TV signals, teletext uses these wasted lines to additionally transmit useful teletext information in designated lines.

Unlike ordinary TV signals, the pictures transmitted by teletext are not analog signals, but digital information. Therefore, only users with dedicated decoders can receive this information. In addition, due to the limited additional transmission capacity of the channel, only a few lines of scanning information can be inserted into each frame of the ordinary TV screen, otherwise it will affect the normal reception of the ordinary TV of this channel. Therefore, the decoder will temporarily store the received information until the transmission of the whole page of graphic information is completed before displaying a complete information picture.

Teletext usually transmits 100 ~ 400 pages of information to users for remote selection and inquiry. When a certain type of information page is selected, the decoder grabs them from the information page chain that is transmitted circularly and displays them on the TV screen as long as it wants.

It can be seen that teletext is an information service system that can provide still pictures, with a wide range of contents and facing the public. In some countries, it has become a common electronic media to transmit public information. Of course, it also has some inherent shortcomings. The most important thing is one-way. Users can only choose to watch the information they want from the information page being transmitted, but they can't exchange information with the TV station in two directions, and they can't query and call all kinds of information pages stored by the TV station at will.

It is worth pointing out that many imported color TVs marked with teletext functions cannot receive teletext signals in China. Because of different standards, they can only receive and display western teletext signals, but not Chinese teletext signals. At present, domestic TV factories have purchased the Chinese and western graphic decoder technology developed by the scientific research institutions of the Ministry of Radio, Film and Television, and will soon produce TV sets with graphic decoders. After mass production, the decoder may only need two or three hundred yuan to be installed in the TV set. It is foreseeable that teletext will gradually enter China families.