The Television

Block Diagram AerialTuner Decoder (audio) Decoder (vision) Amplifier (audio) Amplifier (vision) Loudspeaker Picture tube

The Picture Tube The electron gun, which is at the narrow end of the tube, produces negative charges (electrons) and ‘fires’ them as an invisible beam. As they cannot travel very far in air, there is a vacuum (no air) inside the tube to allow them to reach the screen. A fluorescent coating on the screen produces a tiny spot of light when hit by the electrons.

Picture Tube continued This spot can be moved around the screen by deflecting the electron beam. In a TV picture tube, electromagnets are used. These are magnets powered by electricity and are coils of wire carrying current. One pair of coils is arranged to move the beam up and down (vertically) and another to move it from side to side (nearly horizontally).

Picture Tube continued Because the electrons in these tubes come from a part of the electron gun called the cathode, the tubes are sometimes called cathode ray tubes.

Picture Tube continued

Inside a Television

How a Television Picture is Formed A British television picture is built up from 625 lines. The more lines the better the definition, or clarity, of the picture. The electron beam begins at the top left corner and moves across the screen at a speed of about 7000 m/s.

continued On completing the line the beam is switched off, moves to the left side, before starting the next one. The lines which build up the picture are not built up in sequence but are done as lines 1, 3, 5, etc. followed by the other lines 2, 4, 6, etc. This technique is called ‘interlacing’.

continued This continues until all 625 lines are completed so forming one picture. The beam then returns to the top left corner to start on the next picture.

continued The TV signal from the aerial controls the brightness of the spot, by altering the number of electrons which travel from the electron gun to the screen. The greater the number of electrons, the brighter the spot.

The moving beam