The Teletext Museum
Main indexTimelineGalleriesOnline
LinksFAQsTeletext ChatContact
TransmittersMHPTBSAstrohosts

BBC ENGINEERING INFORMATION
Information Sheet 4008(5), October 1975


Figure 1

CEEFAX is the BBC's 'dial-a-page' news and information service. It lets the viewer 'see facts' written in words and figures on the television screen. The service is an optional extra and to use it television receivers need a special, built-in decoder. Eventually, a receiver incorporating CEEFAX will cost relatively little more than a standard television set.

A television receiver equipped with a CEEFAX decoder has two extra controls that enable the viewer to switch from the ordinary programme and choose one of the 100, coloured, screen-size pages the CEEFAX magazine can provide. News headlines, sports results, weather, travel, Stock Market prices, consumer affairs - CEEFAX covers all of these and more. All the viewer does is to press a button to select the service then dial a three figure number for the page required.

CEEFAX was first made known in an announcement by the BBC on 23-Oct-1972 and experimental transmissions began in March 1973. After extensive collaboration with other interested organizations, a United Kingdom unified specification for such an information service was agreed and published. BBC test transmissions changed to the unified standard in April 1974 and an experimental service carrying many different pages, continually updated by a team of editors and researchers began, with the authorization of the Home Office, on 23-Sep-1974.


Figure 2 - The CEEFAX Weathermap:
like the other pages, it can have up to six colours along with white.

Specification of standards for information transmission by digitally coded signals in the field-blanking interval of 625-line television systems, published jointly by the British Broadcasting Corporation, the Independent Broadcasting Authority (IBA) and the British Radio Equipment Manufacturers' Association (BREMA), price 50p, post free. CEEFAX is the BBC's name for the information service; it is known as Teletext to BREMA and ORACLE to the IBA.

A CEEFAX PAGE

Figures 1 and 2 show what CEEFAX pages look like. Every page can have 24 rows of information with up to 40 characters  in every row [except tne page header which has only 32 characters]. The shape of the characters will depend on receiver design but the system is tailored to a 7 x 5 matrix.

The first row of every page is a special row called the 'page header'. The page header will show the page number, the date and the time to the second. The CEEFAX receiver recognises signals transmitted at the beginning of the header row so that it can tell when a page is beginning and which page it is. When the signals match the orders given by the viewer through his page selector the receiver will proceed to display the header row and the text which follows it.

TRANSMISSION

The information is transmitted as a series of coded electronic pulses. Figure 3 shows how the system operates. The pulses are added to two lines in the field blanking period of the television waveform (lines 17 and 18 in one field and 330 and 331 in the next). Each of these television lines carries the coded information for one row of the CEEFAX display. Thus with two lines available for each television field it will take

24 rows  ( 2 rows/field x 50 field/sec ) = 0.24 seconds

to transmit a full page of 24 rows. For good legibility and attractive layout, the average page will contain less than 24 rows of text and is therefore transmitted in less than 0.24 seconds. Complete pages are transmitted one after the other and it may take up to 24 seconds after pushing the selector buttons before the required page appears on the screen.


Figure 3 - CEEFAX basic arrangement

RECEPTION

Like ordinary television, CEEFAX can be disturbed by severe interference but the effect is to produce incorrect characters. However, good CEEFAX reception is possible wherever good television pictures can be received and field tests have shown that CEEFAX works equally well on uhf and vhf transmissions.


Figure 4 - CEEFAX receiver arrangement

EXTENDING A MAGAZINE

The system allows for many refinements. The amount of information that can be shown can be considerably extended by using the pages in the following ways.

CEEFAX can also be used to superimpose subtitles on the normal programmes. This could be useful for deaf viewers or could give an alternative language version of the programme sound. In a similar way the viewer can choose to have newsflashes automatically displayed as soon as they are transmitted.

BRIEF SPECIFICATION


Figure 5 - The organisation of CEEFAX data on lines 17, 18, 330 & 331.

THE CEEFAX WAVEFORM

  1. Pulses
  1. Pulse Grouping
    Active line time is divided as follows: [see also fig.4]
  1. Clock and identification pulses

  1. Data pulses
    Information carrying pulses fall into two groups; the addresses, which tell the receiver the number of the row and page being transmitted, and the characters. Each group uses different transmission codes.

  1. Pages and Magazines
    The specification of the system allows for eight magazines with up to 100 pages in each magazine.

Teletext Timeline | Teletext links

 Teletext Chat 

Subscribe...

For discussion about all aspects of teletext, from content right through  to complex technical matters.
If you have a question, this is the place to ask it...

The Teletext Museum is a collaboration withThe TV Room


Transdiffusion

EMC RMC TMC
PMC Bitstream Sounds On

mb21 by Mike Brown
Hosted by Astrohosts

Top