My Science School

 

               Computers that are connected to a telephone line incorporate a device called a modem. It turns signals into a form that can be transmitted along the telephone line. The name ‘modem’ comes from the term Modulator-Demodulator. The device modulates, or changes, the digital signal from a computer into an analogue signal, which is the type of signal that travels along telephone lines. The modem decodes or demodulates the signals it receives back so they can be read by the computer.

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               Mobile phones, which are properly called cellular phones, allow calls to be made wherever the caller happens to be. They are called cellular phones because a territory is divided up into a series of small areas, or cells, each with a small radio station. When a call is made, the telephone sends a radio message to the base station, which in turn passes it to a mobile phone exchange. Here the signal can be routed to the ordinary telephone system, or transmitted back to another mobile phone. Mobile phones use low-powered microwaves to send and receive messages to and from the base station.

               Television pictures, radio and telephone communications are bounced off satellites to cover the greatest possible area of the world. Satellites orbiting the Earth must travel at high speed to escape being brought down by the Earth’s gravity. As the Earth itself is spinning rapidly, there is a point above the Earth’s surface where the orbiting speed of the satellite can be matched with the rotational speed of the Earth. At this point —35,900 kilometres above the Earth — the satellite appears to stand still and is said to be in a geostationary orbit.

Geostationary satellites can be positioned right over the areas where they are needed. They can also be used as spy satellites, because they remain constantly over a region of interest.

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               Television cameras break a picture into electrical signals, separating them into three colours (red, blue and green) and turning them into coded messages. Sound is recorded and coded at the same time. The coded pictures and sounds are transmitted by radio waves, electrical cables or optical fibres to the receiver. Inside a television receiver the signals travel to three electron guns — one for each colour. The electron guns emit streams of electrons, which are directed at a fluorescent screen. Magnets bend the electron streams so that they scan back and forth from top to bottom, exciting the phosphors in the screen and producing a colour image. They scan so quickly that our eyes see the images as a continuous picture.

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           Telecommunications is the passing of messages and information over long distances by means of electrical signals. These signals may be carried along wires, like telephone, telegraph and fax messages, or by radio waves.

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               Telephones transmit speech messages along wires by means of electrical signals. Telephones were invented as long ago as 1876.The handset of a telephone includes a loudspeaker and a very small microphone, which contains granules of carbon. When you talk into the microphone the sound waves of your voice cause a metal diaphragm to vibrate, and it presses against the carbon granules. The vibrations vary depending on the sounds. They change the very small amount of current flowing out along the wires to the receiver of another telephone.

               When the electric current carrying the signals reaches the receiver handset, the same variations in the current run through an electromagnet. This causes another diaphragm to vibrate in the earpiece, accurate reproducing the sound of the speaker’s voice.

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               We exchange ideas and messages by communication. Humans, as well as other animals, communicate visually and by sounds. Some animals also communicate by means of smells that convey messages. Humans differ from all other living things because we can communicate by means of symbols. These are compressed pieces of information, such as written letters or numbers that actually communicate very complex ideas.

 

 

 

 

 

 

 

 

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