Relationship between VBI and teletext

Teletext is an information broadcast system developed in the UK in the 7O age of the last century. It mainly uses the TV signal field invisibility period (VBI) some lines in (or all valid lines of the TV signal can be used) to transmit text and data information. Text-and-text television broadcasting can spread news, sports, weather, TV program announcements, subtitles, and other information to the public at a low cost. Because of its rich information and graphics, teletext is even considered the predecessor of the Internet. It also has its own advantages over the Internet, such as a large number of users, the increase in the number of users will not affect the receiving speed, cheap Receiving Terminal. Over the past 30 years, image and text TV technologies and services have been popularized and popularized in many countries and regions around the world, especially in Europe. Currently, all set-top boxes exported to Europe have VBI
Teletext function. Some set-top boxes also provide the OSD teletext function. The basic teletext is composed of a group of pages, each page is numbered by three digits, each page is composed of 24 rows and 40 columns of characters and graphics. These pages are repeatedly sent to the user end.

1. What is VBI?
1. VBI is short for vertical blanking interval. It is called the reverse stage in Chinese. A TV program is called a main signal. In the 625/25 TV signal, the number of lines that can be used is from 6th to 22nd, and the number of lines between 318th and 335th can be inserted at most 17 data lines per game.
2. VBI data broadcast is an information transmission method that inserts data of various formats in a computer into the reverse of a TV program and broadcasts the data together with the TV signal. VBI teletext is a type of VBI data broadcast. Its receiving device is a TV set with the function of graphic TV.

Ii. Composition of Graphic Data

1. data packets. Each packet is fixed to 45 bytes. There are three basic types of packages: page header package, common line package, and extension package.

L page header package (package address y = 0): contains the page number and control information, plus 32 display characters including time. It corresponds to the first line displayed.

L direct display of the normal line package (package address y = l-25): contains 40 byte (625 line TV signal) corresponding to a line of 40 display characters. Vertical position of the line corresponding to the package address

L extension packages (package address y = 26-31): these packages are not directly displayed. They are used to provide enhanced performance for advanced decoder or special data services.

2. Page. Each page corresponds to the graphic information of a screen. Each page can contain up to 25 lines, and each line can contain up to 40 letters, numbers, or graphical symbols, it also has character color, blinking, and high-magnification attributes.

3. magazines. Teletext generally uses magazine to organize information, up to 8 different magazines, 100-199 (M = 1), 200-299 (m = 2 ), 300-399 (M = 3), 400-499 (M = 4), 500-599 (M = 5), 600-699 (M = 6 ), 700-799 (M = 7), 800-899 (m = 0), each magazine can have up to 100 page numbers, each page contains several sub-pages, so that each magazine can have hundreds of pages of information. For example, from page 100 to page 199, its magazine number is 1, its content is news; from page 200 to page 299, its magazine number is 2, its content is sports.

3. Four levels of teletext

Ets300706 technical specifications define the following four levels of graphic TV:

L level 1 inlaid character, empty attribute, fixed color palette, 24 lines X 40 characters and other features;

L level 1.5 expands the character set based on Level 1;

L level 2.5 extends the language type, adds a color palette that can be redefined, and introduces non-space attributes. It allows many simple redefinition characters and provides panels for adding text and graphics;

L-level 3.5 extends the number and complexity of redefinition characters, attracting different Font types and commensurate spaces.

The presentation and application features of the above levels are backward compatible, so that users using low-level protocols and decoders are not affected by the Standard upgrade of the sender's image and text. this protects the interests of tens of millions of early users. We mainly introduce the most widely used 1.5 level.

4. teletext characters

The basic character sets of level 1 teletext include G0, G2, and national option sub-sets, and G0 and G2, respectively. A subset of country characters consists of character sets of 13 countries, each of which has 13 characters. It is used to replace the 13 characters of the G0 shadow part, consists of the page headers (packet x/0) the control characters C12, C13, and C14 determine which country character subset to use,

The teletext Character Set of level 1.5 adds characters defined with packet x/26. Used to define complex characters.