Before the rise of 24-hour networks and readily accessible internet access, bored television viewers in the UK seeking entertainment at 3 AM would have to make do with CEEFAX.
The service was originally designed to offer subtitles for the deaf and hard of hearing but morphed over the decades into a fully-fledged information service.
Designed and developed by BBC research engineers, CEEFAX began transmitting in September 1974, utilising existing television transmissions to provide pages of written information on the receiver screen.
The information could be in the form of words, numbers or simple diagrams – or a mixture of all three.
The viewer could at any time dial a page of the latest news headlines, sports results, a weather map, or many other subjects, and this replaced the television picture within a few seconds.
Newsflashes or subtitles could be superimposed on the picture. The viewer could select another page or return to the television programme whenever they wished.
Although the picture and CEEFAX information were carried on the same television channel, a receiver with a special decoder was required for CEEFAX because the method of carrying the information was quite different from that in normal television transmission
There were a few unused, blank lines at the top of the television picture which were not visible on a correctly-adjusted TV set. BBC engineers succeeded in using some of these for the CEEFAX service; they found that they could put enough digital pulses on four of the spare lines in each complete picture to represent four rows of characters on the screen.
In this way, enough pulses could be sent to code a whole page of 24 rows of text in just under a quarter of a second.
One hundred pages could be transmitted in turn as a continuously recycling stream without interfering with the picture in any way. In the receiver, the CEEFAX decoder recognised and extracted the pulse groups from the television waveform and gathered them into a store.
They were then used to activate an electronic character generator in the receiver to display whatever page the viewer had selected.
The display used any combination of six colours and white, with upper and lower case letters, and the facility for flashing individual words for emphasis.
These techniques were used to produce very attractive and informative pages.
After a period of technical tests, which started in May 1974, the Home Office authorised an experimental service to run for two years from September 1974, in order to test the public reaction.
A small editorial team produced pages of up-to-the-minute information which was broadcast throughout the television day on BBC1 and the BBC installed a number of receivers at centres throughout the country to show CEEFAX to as many people as possible.
Receiver manufacturers began producing prototype receivers with CEEFAX decoders.