Why big-screen is not always better in television

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

29 January 2006

Steve Kimani has bought a big screen TV (doesn’t say what size) and he is disappointed because the picture is not as clear as it was on his older 17-inch box. He adds,  “When it was tested at the shop it was good, but at home the picture appears blurred. I’ve tried many aerials but there is no change.”

No Steve, there is nothing wrong with your new television set. A TV forms a picture by scanning the screen continuously with a narrow beam of electrons. This beam makes many horizontal lines, but when viewed from some distance, they merge into one another to form a complete picture.

The total number of line on the screen is fixed and it depends on the TV standard used. In Kenya (and most countries in Africa and Europe), we use the PAL system that has 576 horizontal lines on the screen. Thus a small telly will have very thin lines and a bigger one will have broader lines. For example, the lines on a 28-inch screen are double the width of those on a 14-inch.

But thin are the lines? A television screen is rectangular in shape and the standard ratio of length to height is four to three (4:3). This ratio is the same for all ordinary TVs regardless of the picture system used. If the length is, say, 12 inches, then the height will be 9 inches. The quoted size of the screen is measured diagonally. (For some strange reasons, screen sizes have “refused” to go metric.).

In our PAL system, 576 picture lines will be fitted on the screen from top to bottom. A 14-inch screen measures 11.2 inches (284 millimetres) by 8.4 inches (213 mm), therefore, each line will be about 0.4mm (213 divided by 576). That’s very thin. On a 28-inch TV, the thickness goes up to about 0.7mm – three quarters of a millimetre.

Obviously, the farther you are from an object, the smaller it appears. In fact, if an object is placed at a distance equivalent to 2,000 times its size, it will be seen as a point without any lateral size. Therefore, you have to be at least one metre away from a 14-inch screen for the picture to be clear and two metres away in the case of a 28-inch telly.

To understand this principle better, go to a long, straight stretch of road at night – about five kilometres long. (There is a good place down Mombasa road, past the Machakos turn-off.) When you look at the headlights of an on-coming car in the distance, they seem to merge into one dot. Then, as it gets closer, the two separate lights appear. This is the illusion applied on a TV.

If the screen is big, the lines are big; therefore you need to be farther to see a clear picture. Thus the solution to Steve’s problem is to move to a house with a bigger sitting room.