Teletext Tutorial (1)
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Teletext Tutorial (1) |
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 Teletext is a system which uses broadcast television to provide an information broadcasting capability. A teletext service consists of a number of pages, each page consisting of a screen of information. These pages are transmitted one at a time utilising spare capacity in the television composite video signal. When the complete service has been transmitted, the cycle is repeated, although the broadcaster can choose to transmit some pages more frequently if required. A domestic television set equipped with a suitable teletext decoder can display any one of these pages at a time. The viewer selects the page for display by means of a remote handset. The service is one way; the user is unable to request a page directly and can only instruct the decoder to search for a particular page in the teletext data stream. There will usually be a delay before the requested page appears in the transmission cycle. When the page is detected, the decoder captures and displays the information contained in the page. Thus the more pages within the service, the longer the access item. For this reason, broadcasters usually adjust the size of their services to obtain a cycle time of around 30 seconds, and therefore an average access time of 15 seconds. A teletext service is divided into up to eight magazines; each magazine can contain up to 100 pages. Magazine 1 comprises page numbers 100 to 199, magazine 2, numbers 200 to etc. Each page also has an associated sub-page which can be used to extend the number of individual pages within each magazine.
A teletext display consists of letters, numbers, symbols and simple graphic shapes. In addition, there are a number of control codes which allow selection of graphic or text colours and other display features known as attribute. Each character or graphic shape occupies a defined area of the screen called a character rectangle.
When the teletext system was being designed, computer displays usually used a minimum of 80 columns per row. However, the resolution of a domestic television receiver is insufficient to allow this number of characters to be displayed along one row. In addition, a person looking at a teletext display from an average television picture viewing distance would find it difficult to read the 80 column text. Thus it was decided to use 40 characters per row, and for a similar reason the number of rows was fixed at 24, although an extra row is used in the new enhancements to the system known as Fastext.
The characters available for display in the teletext system are the letters A to Z, a to z, numerals 0 to 9, the common punctuation marks and symbols, including currency signs, accents and a whole range of other character sets. Other graphic shapes (termed block graphics) are used to create simple pictures. To obtain block graphic shapes, a character rectangle is divided into a matrix of 2 x 3 smaller rectangles. Each small rectangle can be either illuminated or blanked, and there is a direct correspondence between the display of these small rectangles and the state of individual bits in the transmitted code. Coloured teletext displays were incorporated from the outset. These and various other display attributes such as flashing and double height, are invoked by a number of control codes. There are seven colours available - the basic primary colours red, green and blue, together with the three combinations of two of these colours, to make yellow, magenta and cyan, plus white, which is produced by combining all three primary colours. | |
Introduction