(High Definition Television)

Standard television works at either 525 or 625 transmitted lines of resolution. To give time for the electronic beam to reset iteself in a CRT television, 45 of these lines are transmitted empty. This is where we get the term 480i (interlaced). The interlacing means that only half the image is transmitted at one time, i.e. first lines 1,3,5,7 etc. are transmitted then lines 2,4,6,8 etc. This looks fine on a standard size television but it's deficiencies show up on larger screens where smooth lines become jagged because the lines are slightly out of step.

There are two possible solutions to this problem, which can be used seperately or together. Firstly the signal can be transmitted progressively i.e. lines 1,2,3,4,5 etc. which is where you get 480p or you can increase the number of scan lines i.e. 720,1080 or both so we get 480i (standard), 480p (progressive), 576i and p, 720p, 1080i and 1080p.