Frame or scan rates. Who uses what?
A picture is constructed of lines, each line constructed of elements of light of varied luminance. In constructing a picture, each of these lines is drawn on the screen sequentially, one after the other, until the screen is filled. The process then begins again from the top. The rate at which this happens, that is the number of lines needed to fill the screen and the amount of time needed to do it is known as the scan rate. The scan rate is often tied to the AC power frequency, though obviously the advent of microprocessor control means that this is not as important as it used to be. In the UK and a fair chunk of Europe, that frequency is 50Hz, while in the US, it is 60Hz (Hz means “Hertz” or cycles per second). There is a formula which is then used to work out the optimum number of lines per frame. I shall not bore you with it here, but as you could probably guess, this means that there is a difference between UK and US frame sizes. In fact the US uses 525 lines per frame, while the U
