HDTV useful information
I've nicked this from elsewhere (typos and all) in the hope that it might be useful and not be too much forum drift for SD's patience.
High definition TV (HDTV) is the generic term that is used to describe any television signal that has a higher resolution than the standard TV signal that has been broadcast for nearly 40years. The term HDTV actually covers a range of different standards.
A standard UK television signal comprises 625 lines of information from the top of the screen to the bottom (just 576 of these lines can be seen by the viewer), and 768 lines from left to right. This means that the finer details - especially on newer large-screen TVs - tend to become blurred. HDTV increases the number of lines of information, either to 720x1280 or to 1080x1920. This means that finer details can be represented within the picture.
HDTV can also differ from a standard television broadcast by the way in which the lines of information are refreshed (updated) on the screen. In the standard TV, and in the current 1080x1920 format, the horizontal lines of the picture are renewed alternately - i.e, lines 1,3,5,7...etc. are refreshed to the bottom of the screen, then lines 2,4,6,8.. etc are refreshed. Since UK TV pictures are broadcast at 50hz, this means that every line of the picture only changes at half that rate - 25 times per second. This system is known as interlacing and is usually depicted by the suffix "i".
The alternative system is calle dprogressive scanning, and entails updating each line of the picture in sequence. This means that the whole picture refreshes 50 times per second - double the rate of interlacing. Progressive scanning can give the viewer the perception of higher resolution than an interlaced picture. Progressive scan formats have the suffix "p".
Currently, Sky's HD broadcasts use the 720x1280 progressive scan system (known as (720p), whereas the BBC's HD output is at 1080x1920 interlaced (1080i).
In important factor for HD viewers is that most "HD-ready" TVs can only support 720 line signals (most LCD and plasma TVs currently have 768 lines top-to-bottom). Indeed, the "HD-ready" logo can be applied to any TV that has a minimum of 720 horizontal lines. The small number of plasma TVs that support 1080x1920 are currently priced at the high end of the market, although prices may fall as production increases. Viewers with 768x1024 TVs can still view 1080i HD pictures, but some of the lines of information need to be combined in order to display the picture.
Further, I recently read that ITV & BBC Freeview transmissions are made using a different system, with the ITV type being much more prone to pixellation/breakup.