How a CRT TV Works

Because they are positively charged, the electrons flow towards the positively charged anodes. One (the 'accelerating anode') speeds the electrons up and fires them towards the screen at the other end of the vacuum chamber. The other anode (the 'focusing anode') forces the electrons into a tight beam so that they are not randomly scattered about the screen.

The screen itself is coated with a phosphor that reacts to the stream of electrons by emitting visible light.

This simple structure would only produce a single white dot in the centre of the TV screen, so steering coils are added to the cathode ray tube. These can move the electron beam both vertically and horizontally and make it possible for the beam to move across the screen, exciting the electrodes on a horizontal line from left to right, then skipping back to the left of the screen and exciting the next line. This is known as a 'raster scan'.

In this way a frame is built up line by line, before the electron beam is returned to the top of the screen and the next frame is created.

A single frame is usually presented as two fields - the electron beam first traces out the even-numbered lines of the frame, then the odd-numbered lines. This 'interlaced' image still appears as a whole picture due to persistence of vision (the way an image remains on the retina for a split second, allowing a following image to 'overlap' it), but does create a flickering effect.

To combat this, progressive scanning sees both fields traced at the same time. The resulting image is clearer, more detailed and does not suffer from flicker. Progressive scanning is usually used on computer monitors, where flicker would be distracting, but it is also available on televisions.

To produce colour images the electron gun fires three distinct beams and the phosphors on the TV screen are arranged in groups of three, each group making up a single pixel. The phosphors used in this arrangement give off red, green and blue light and a 'shadow mask' screens each pixel.

By modifying the strength of each electron beam, the qualities of each sub-pixel can be modified, creating the combinations necessary to form the millions of colours needed for a believable colour image.

In a PAL picture the process is repeated to create a 576-line image with 50 fields (that is, 25 frames), while the NTSC system creates a 480-line image with 60 fields per second.