Saturday, March 23, 2013




Videotex is a term for any information system which uses a computer to transmit data to users' screens. The main examples are:

1 Teletext-this sends out information with ordinary television signals. The customer has an adapted television set and the information is displayed as pages of text. This system is not interactive. The user cannot send messages back to the computer.

2 Viewdata is an interactive information system where the telephone is used to link users to the host computer. Normally the user has a modem and a visual display terminal or a microcomputer. The user can send messages back to the computer.

Notes: 1 The term 'viewdata' is not generally used outside the United Kingdom. In other countries viewdata systems are simply called 'videotex'.

2 The term 'page' has several meanings in computing. For videotex a page is a screenful of information that can be sent as a unit. For some systems the user may pay for each page of data that is viewed.

An information provider is an organization which makes the information available for users to access.

The provider has to:

1 Divide the information into pages.

2 Produce a reference system so that users can find the pages easily.

Electronic mail is a system which allows users to send messages to one another using a viewdata network.

Information retrieval is the process of recovering selected information from large files or data bases.

An information retrieval system is a computing system which stores large quantities of data so that users can recover selected information from it.


Teletext systems are not interactive (see Fig 1). To use such a system you need:

1 An adaptation to your television set to receive the data.

2 A simple keypad to select the appropriate page.

A page is selected by keying the appropriate three-figure number on the keypad.

The same data is sent out repeatedly on television frequencies. All the user can do is to operate the keypad to select which page to look at. There is no means of sending data back to the computer.

Each television channel could send out up to 800 teletext pages. In practice they send fewer than that.


Fig 1 A teletext system

Examples of teletext systems

1 The BBC system-called CEEFAX.

2 The IBA system-ORACLE.

Examples of information available from CEEFAX or ORACLE

1 Up-to-date news.

2 Latest sports results.

3 Weather information.

4 Details of radio and TV programmes.

5 Stock exchange prices.

6 Money exchange rates.