Natural Motion — Television facts

The father of television

On Christmas Eve in 1883, the German physics student, Paul Nipkow, sat alone in his room in Berlin, wishing that he was with his family in distant Lauenberg. "If only we could see each other," he thought to himself.

"Then the basic idea began to emerge by itself: the principle of television," wrote Nipkow later in his memoirs. He took a pencil and paper and made sketch after sketch. The design that he came up with finally, was of a rotating disc containing a circle of holes. This 'Nipkow disc', which was used in all television sets up to 1936, was based on the idea that an image to be transmitted electronically must be divided up into separate dots.

The apparatus worked as follows: a round plate or disc which can rotate about its axis, and through which holes are drilled at fixed intervals, is used for transmitting the image of an object, e.g. a photograph. The holes of the disc form a slight inward spiral, each successive hole being slightly nearer to the centre of the disc. When the disc rotates, a beam of light entering the first hole strikes the photo and scans the first line, dot-for-dot. As soon as the first hole reaches the end of the photo, the second hole scans the second line, and so on until the hole nearest the centre of the disc is reached. Then it is the turn of the first hole once again. The entire photo is thus scanned with each turn of the disc.

Black lips, green eyes, golden hair

The first experimental television broadcast, on the 8th of February, 1928, was carried out with a picture of 30 lines. However, contours were not clearly defined with this number of lines, and there was a complete absence of detail. In order to make faces recognizable at all, lips had to be blackened and eyelids colored green. Hair even had to be powdered with gold dust so that it sparkled. On the other hand, contrasts which were too great also gave rise to problems, so white buttons and articles of clothing had to be made grey.

Later on, the number of lines was changed, first to 50, then to 90, and finally to 240. But with this development, the Nipkow disc had reached its limit because the more lines there were, the more holes were needed in the disc. And at more than 240 lines, the disc was subject to breakage. So in 1938, the Nipkow disc was replaced by the 'iconoscope' (image viewer), which comprised a signal plate with small photocells. These were scanned by a magnetically-controlled electron beam. With the help of an iconoscope, a picture could be built up out of 500 lines or more. But this piece of apparatus was also destined to be improved. The physicist Fritz Schroeter, working in Berlin, invented the conventional method of constructing television pictures using tubes, as well as a camera that could be used in the tube of the receiver. First the odd lines 1, 3, 5, etc., were scanned, followed by the even lines 2, 4, 6, etc., which were then placed between the odd lines.

25 million pixels per second

The European television picture is composed of 625 lines, each with around 800 dots or 'pixels'. This is about half a million pixels per picture. With a standard 50 Hz TV, the picture is refreshed about 25 times per second. This is equivalent to 12.5 million pixels per second. With a 100 Hz TV, there are twice as many pictures, and therefore about 25 million pixels per second are produced.

Electron beam

Television signals are currently split into a picture or video signal, and a sound or audio signal. The audio signal is amplified and fed to the loudspeaker, while the video signal controls the picture tube.

The inside of the screen has a fluorescent coating which glows as soon as it is struck by an electron beam. The beam is directed from the neck of the picture tube, which also contains devices to focus the beam of electrons and direct it over the screen. The beam forms a small dot on the screen, which traces horizontal lines from the top to the bottom of the screen, at very high speed.

The power of the electron beam is controlled by a device in the picture tube. This enables the brightness of the dot which traces the lines, to be varied. Due to the speed with which everything takes place, the speed at which the pictures change, and the large number of picture lines, the single dot forms what is seen by the human eye as moving pictures.

With color television, there are three electron beams: one for red, one for blue and one for green. The inside of a color picture tube contains strips of fluorescent material which glow when they are hit by the electron beam. These 'phosphors' are arranged in groups of three: so called 'triads'. One of the phosphor strips glows blue, the second glows red, and the third glows green. A picture tube with a screen size of 50 centimeters (measured diagonally) contains around 400 000 triads. These can only be seen individually at very close range. At the normal viewing distance they are not visible to the naked eye.

There are various sorts of picture tubes. Instead of being coated with three groups of colored strips, some are coated with patterns of lines made from three different phosphors.