the bt prestel software development team developed a number of national variants of 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.