In common with the Ceefax and ORACLE teletext services provided by the BBC and ITV television companies, the system used a modified television to display information in a non-scrolling window of 40x24 text characters, with some simple graphics, conforming to the 1981 CEPT1 standard. Unlike the limited data available on Ceefax and Oracle, Prestel offered an extensive range of information that had been supplied both by a Prestel department at the Post Office and by third-party Information Providers. The range of IPs was wide, including the government and Parliament. This data was entered on a central Update Computer, "Duke", located in London, and then mirrored onto a number of satellites (mirrored computers known as IRCs) "Dryden", "Kipling", "Derwent", "Enterprise", "Dickens", "Keats", "Bronte", "Eliot" and "Austen" (among others) that were located throughout the country. Access was open to all users except for a number of CUGs (Closed User Groups) membership of which was provided to a controlled userbase, usually by a paid subscription. Mail was handled by a machine known as "Pandora". They were all GEC 4000 series machines.
Whilst the teletext services were provided free of charge, and were encoded as part of the regular television transmissions, Prestel data was transmitted via telephone lines to a set-top box terminal, and while this enabled interactive services and a crude form of e-mail to be provided, it also involved purchasing a suitable terminal, and paying both a monthly subscription and the cost of local telephone calls. On top of this, some services (notably parts of Micronet800) sold content on a paid-for basis. Each Prestel screen carried a price in pence in the top right-hand corner. Single screens could cost up to 99p.
The original idea was to persuade consumers to buy a modified television set with an inbuilt modem and a keypad remote control in order to access the service, but no more than a handful of models were ever marketed and they were prohibitively expensive. Set-top boxes were pioneered by the Nottingham Building Society for its customers, who could make financial transactions via Prestel. The access situation improved as home computers became more commonplace, and by the late 1980s it was possible to use a machine such as a BBC Micro or Atari computer, equipped with a 1200/75 baud modem and some simple software, to access the Prestel service. Even the more games-orientated Sinclair ZX Spectrum has a large number of users via a low-cost modem called the VTX-5000. It was possible to buy downloadable content such as simple games. This would be encoded in a series of pages that with body text that was not human-readable but encoded the content in blocks of rather less than 1 kilobyte at a time. The header and footer of these pages was normal, however, so users could watch the pages appearing one after another to build up the downloaded file. To charge for content, the final pages of the downloaded file were charged at 99p each until the total charged was within 99p of the total price, after which one page would be charged at the balance of the total price and subsequent pages were free.
Because the communication over telephone lines did not use any kind of error correction protocol it was prone to interference from line noise which would result in garbled text. This was particularly problematic with early home modems which used acoustic couplers because most home phones were hard-wired to the wall at that time.
However, it was still an expensive proposition, and as a result, Prestel only ever gained a limited market penetration among private consumers achieving a total of just 90,000 subscribers, with the largest user groups being Micronet800 with 20,000 users and Prestel Travel with 6,500 subscribers.
GEC Computers produced a number of national variants of Prestel which were sold to the PTTs of other countries, such as Italy, Austria, Yugoslavia, Hungary, Germany, Singapore, [incomplete list]. Italy was the largest system with 180,000 subscribers. The Singapore system had a notable technology difference in that pages were not returned over the modem connection, but were returned using teletext methods over one of four TV channels reserved specially for the purpose, which had all scan lines encoded in teletext format. This higher bandwidth enabled use of a feature called Picture Prestel which was used to carry significantly higher resolution pictures than were available on other Prestel systems.
In 1990, BT introduced a new commercial model which effectively killed the domestic usage of the service. Finally in 1991 it was decided that BT should move away from providing Value Added Services and should focus on network provision. Consequently the various consumer and business services were run down or sold off with such services as Prestel Travel and BTIS (BT Insurance Service) becoming private networks services for third party providers.
The 1984 hacker intrusion into the (very likely unused) Prestel mailbox of the Duke of Edinburgh garnered the network some unfavourable press, particularly when the simplicity of its security measures became apparent. The subsequent failure to successfully prosecute the intruders contributed to the introduction of the Computer Misuse Act 1990.
The Prestel name and equipment was eventually sold by British Telecom, and purchased by a private company, Financial Express, in 1994 and renamed New Prestel. During this period, the platform software was redeveloped onto a SCO Unix and Linux x86 platform away from the massive mainframes that filled a room. Additionally, the Citiservice financial data product was successfully redeveloped in house after being outsourced to Datastream during the time with BT.
The dial-up viewdata service was run down as the Internet gained in popularity.
In contrast to the demise of the British system, the French equivalent of Prestel, Teletel/Minitel, which used the slightly superior CEPT2 standard, received substantial public backing when millions of Minitel terminals were handed out free to telephone subscribers (causing Alcatel huge financial problems). As a consequence the Teletel network became very popular in France, and remains well used, with access now also possible over the Internet.
A closed access videotex system based on the Prestel model was developed by the travel industry, and continues to be almost universally used by travel agents throughout the country: see Viewdata. The Prestel technology was also sold abroad to several countries, and in 1984 Prestel won a UK Queen's Award for Industry both for its innovative technology and use of British products (it largely ran on equipment provided by GEC).