The first successful television transmissions occured between 1928 and 1935. Because the pictures in this system were composed of only 30 lines, small details could not be reproduced. This system was know as a low definition system. Several hundred lines are needed to give definition comparable to 35 mm film.
The first high definition system was introduced by the "BBC" in 1936. This system consisted of 405 lines. In 1964, they introduced a second high definition set. This was a 625 line system that was better suited to many other broadcasting authorities, so it enabled an even higher picture resoulution than before.