Prestel, all of which ran on GEC Computers. They were sold to the PTTs of other countries, including Australia, Austria, Belgium, Italy, Hungary, Hong Kong, Germany, Netherlands, New Zealand, Singapore and Yugoslavia. 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 television 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. It was also demonstrated at the 1982 Worlds Fair in Knoxville, Tennessee. The original Prestel system, designed for cost effectiveness and simplicity, employed a rudimentary graphic capability known as serial mosaics. Through juxtaposition of the special mosaic characters, crude but recognizable graphic representations could be made on the screen. This graphic scheme had its limitations. To change colors between two mosaic graphic characters or between any two characters in general, a color change command was required. This command signal, however, physically occupied a blank space on the screen.The French sought to overcome this limitation when they joined the videotex world in the mid-1970s. They called their system Antiope. While based on the same mosaic graphics that were employed by the British, Antiope added a new feature, parallel attributes, or the ability to change the color from one cell to another without the need for a blank space. At approximately the same time, the Canadians adapted standard computer graphic commands into a set of functions called alphageometrics. These alphageometric functions did away with the block mosaic graphics used by the British and French and replaced them with drawing instructions, such as: DRAW LINE, DRAW ARC, DRAW POLYGON, etc. Through use of these geometric commands much higher resolution could be achieved than with the mosaic commands. This alphageometric scheme was integrated into the Canadian videotex system which the Canadians referred to as "Telidon". Having developed Prestel as a way of maximising usage of existing telephone lines, the British Post Office and subsequently British Telecom sought to "provide only the framework" for the service, delegating the provision of information to information providers. Nevertheless, considerable investment was required in the infrastructure to provide such a service. With information providers needing to pay rental