How It Works: Old Style TVs
Topics: TV, LCD, Plasma, CRT
The world is moving on from our beloved old clunky televisions to the young, snazzy LCD and plasma screens of the future. But since, in the words of Joni Mitchell, you don’t know what you’ve got til it’s gone, we take one last loving look at how our old tellies worked.
Like the atomic bomb, the television is an example of ingenious science producing a wonderful product that has dramatically changed the world. Just like the a-bomb, its functioning is fascinating and its evils are opposed with gusto.
No one person can claim to be the sole parent of television. It is an example, rather, of expertise from many fields combining to create a collaborative product.
In any electrical circuit through which a current flows, there is a negatively charged end called the anode and a positively charged one called the cathode. In a standard television the cathode ray tube fires electrons from a cathode which are attracted by an anode towards a flat, phosphor-coated screen. Electrified copper windings create a magnetic field that steer the electrons towards a given place on the screen.
By modifying the voltage traveling through the copper windings, the magnetic field can be altered to guide electrons to any position of the screen. The voltage is changed so that the electrons systematically hit every area of the screen, moving the continuous beam in a pattern called ‘raster scan’.
The electron beam travels across the screen similar to the way you would read a book: left to right, then quickly back, slightly down and so on over the entire screen. Then the process repeats itself.
The electron beam moves very quickly, and, in a progressive scan television, a relatively rare type of TV in which the entire screen is updated rather than only half, it manages to hit every portion of the screen 60 times per second. In most TVs, however, the electron beam incorporates a little speed reading known as interlacing and alternates between odd and even rows with each pass over the screen.
When the phosphors on the screen are hit by the electrons, they emit light. In a black and white television, there is only one type of phosphor, but in a colour television there are three different phosphors (emitting red, blue, or green light) and three separate electron beams, one for each colour of phosphor.
Depending upon which phosphors are illuminated, a picture is produced on the screen. As it is updated, the picture changes to give the illusion of motion and the glow of the world’s beloved tube.
Andy worked for four years studying ducks (no stop laughing, he really did). He went into his PhD thinking he was going to save the world (albeit from ducks) and now spends him time lovingly preening Null Hypothesis, the Journal of Unlikely Science!
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