My Science School

Printing is the process of making many copies of a document or a picture. Printing is normally done on paper, but it can also be done on fabrics, and sheets of plastic or metal. Books are printed on a machine called a printing press. The text and pictures start as patterns on a plate. In the press, the plate is inked so that these areas become ink-covered, and are pressed on to the paper via a rubber-covered drum. A fast printing press can make several prints a second because each print is made by one simple operation.

One of the first methods of printing was wood-block printing; where the images to be printed were carved in reverse into wooden blocks. The blocks were then inked and pressed onto paper to make a print. Ink from the raised areas was transferred on to the paper. Simple block printing is still used for hand-printed textiles.

In an early printing press a screw was turned to press the paper firmly down on to inked type.

Two of the most important inventions in printing were moveable metal type (which allowed words and paragraphs to be built up from individual metal blocks with letters on them) and the printing press. In Europe, these were both developed in the fifteenth century by the German printing pioneer Johannes Gutenberg. They allowed books to be printed in large quantities, whereas before each book had to be hand-copied.

On a printed colour page, the text is normally solid black ink, while the pictures are made up of tiny dots of coloured - and black - ink.

Most colour printing is done with just four colours of ink: cyan, magenta, yellow and black. By printing dots in varying sizes, the first three colours combine to create almost any other colour. In practice, all three mixes to create brown, so black is used to darken some areas.

In a modern printing press, printing is carried out using a sheet of metal called a printing plate, rather than with individual blocks of type. The shape of the letters that make up the text, together with the dots that make up the pictures, appear as patterns on the surface of the plate. The plates are prepared using photographic and chemical processes.

Type and pictures for a book, magazine or leaflet are nowadays usually designed and laid out on a computer using desktop publishing software. The files from the computer may then be sent to the printer, which uses them to make four printing plates, one for each colour of ink on the printing press.

For most publications, the paper needs to be printed on both sides. Some presses can do this but on others the paper has to be sent through the press twice. Several pages of the final book or magazine are normally printed on each sheet of paper. The sheets then go for print finishing, where machines fold, collate (sort), staple or sew, and trim the sheets to create the finished product.

BROADCASTING

There are thousands of different television channels around the world, broadcasting entertainment, news, information and sport. Television programmes are created at television stations. Each station normally broadcasts several separate channels. Some programmes, such as news and sports, are broadcast live, which means the viewers see action as it happens. Most programmes are recorded on videotape and broadcast at a later date. Some programmes are a combination of live and recorded action.

A television camera electronically divides an image of the scene it is pointing at into hundreds of narrow horizontal lines, each made up of hundreds of small dots of colour. It creates an electrical signal that represents the colours of all the dots. It repeats this process 25 or 30 times a second to create a continuous picture signal.

Pictures from the cameras in a television studio, and from cameras at outside broadcasts, such as sporting events, are fed to a control room, where they appear on screens. Here, live pictures from cameras, pictures from videotape (such as short news reports) and computer graphics are mixed to create the signal for the pictures that will be broadcast. Sound from studio microphones or audio tape is also added.

There are several ways of broadcasting signals. In each case, the signal is modulated before it is sent, with different channels using different carrier signals. The receiver tunes in to the signal from the channel the viewer wants to watch. Many signals now travel in digital rather than analogue form. This allows many more channels to be broadcast, and eliminates the interference that often makes pictures sent using analogue signals fuzzy.

In terrestrial television, the signal goes to a transmitter where it is turned into a radio signal that is spread out in all directions. The signal can be detected by an aerial of any receiver within range of the transmitter. In cable television, the signal travels from a cable television station through a network of underground cables that link directly to receivers connected to the network. In satellite television, the signal is beamed by microwaves to a satellite high above the Earth. The satellite detects the signal with its own aerial and re-transmits it so that it can be picked up by receivers on the Earth’s surface. In webcasting, television and video pictures are transmitted over the Internet. The pictures are first converted into a digital video format and then made available on a website.

In closed-circuit television (CCTV), signals are not broadcast at all. Instead, they go directly from the camera to a receiver. CCTV is used for security systems, with the pictures being recorded as well as viewed.

Terrestrial television signals are broadcast from transmitters at the top of tall masts, often on hill tops, and detected by aerials placed high up on roofs. This gives the signals a clear route from transmitter to aerial. But in mountainous areas the signals are often blocked by hills. This is not a problem with satellite television, where the signals come down from a satellite high in the sky to small aerial dishes aimed accurately at the satellite.

Interactive television is television in which the viewer can send information back to the television station, normally via a telephone line. The combination of digital television and a telephone line also allows viewers to access the Internet.

Television broadcasting satellites sit directly above the Earth’s equator, about 36,000 kilometres up, in an orbit called a geostationary orbit. They orbit at the same rate as the Earth turn, which means they always stay above the same point on the Earth’s surface. Television signals are beamed to them by microwaves from dish aerials at a ground station. The satellite transmits a broad signal beam which covers a wide area when it hits the surface. Any receiver in the area can detect the signal.

TELEVISION

The word “television” means transmitting images of a moving scene (and the sound to go with them) from the scene to a different place. A simple television system needs a television camera at the scene, a way of transmitting the images, and a television receiver where the images appear. A television camera takes 25 or 30 electronic photographs (called frames) of a scene every second and creates an electric signal that represents the colours in the frames. The receiver uses this signal to display the frames in quick succession on a screen, which creates the illusion of smooth movement because our brains merge the frames together.

The first working television system was demonstrated in 1926 by the British pioneer John Logie Baird. It created a small, wobbly image of a dummy’s face. Baird’s system used a mechanical camera and receiver. These contained large spinning discs with a spiral pattern of holes in them which divided the light from the scene into narrow lines and rebuilt it into an image. Although Baird’s system was used for experimental broadcasts, it was made redundant in the 1930s by systems that used fully electronic cameras. These were based on a device called iconoscope, developed in the USA by Vladimir Zworykin. The first broadcasting service started in Britain in 1936, with one or two hours of programmers are being shown each day.

HOW A TV SET WORKS

A television receiver (or set) builds each frame of the moving television picture line by line, using the electrical signal originally created by the camera to control the colour of the dots along the lines. In most television receivers, the picture is created by a cathode-ray tube.

The cathode-ray tube in a television receiver has a narrow neck and a flat base that forms the screen. The air is pumped out to create a vacuum. At the back of the tube are three guns that fire beams of electrons (for red, green and blue light) at the screen. Electromagnets make the beams scan quickly across the screen line by line, while the picture signal controls their strength. Where the beams hit a special coating on the inside of the screen, it gives off light. Just behind the TV screen is a plate with holes in it called a shadow mask. It ensures that the beams hit the screen only behind filters of their own colour. Filters make the light produced by the three beams appear red, blue or green. These add together to form the picture colours.

RADIO

The word “radio” means communicating with radio waves, which are part of the electromagnetic spectrum. Radio has a huge range of applications. It is used in the telephone network for mobile telephones and links in the network, for broadcasting, for two-way radio communications as used by the emergency services, and for remote control of machines. “Radio” also means the media of radio, in which music and speech from radio stations are transmitted by radio waves, and are picked up by radio receivers.

The existence of radio waves was confirmed in 1888 by the German physicist Heinrich Hertz, but it was the Italian Guglielmo Marconi who, in 1896, was the first to make long-distance radio transmissions. In 1901 he transmitted Morse code across the Atlantic. Two-way radio communications using Morse code began in the early twentieth century, and radio broadcasting began in the 1920s.

On their own, radio waves do not carry any information. To make an electrical signal, such as one representing sound, into a radio signal, the electrical signal is used to shape another signal, called the carrier wave. The shaped carrier signal is sent to a transmitter, where it creates radio waves.

The shaping process is called modulation. So that radio signals from different transmitters do not interfere with each other, they are sent using carrier waves with different frequencies. The whole family of radio waves is divided into sections called wavebands. Each waveband is reserved for a different form of communication. A radio receiver detects radio waves of the right frequency and demodulates them to get back the original electrical signal.

COMMUNICATIONS NETWORKS

Telephone calls, fax messages, e-mails, computer data - and often radio and television signals as well - all travel from place to place through a complex, worldwide telecommunications network. All the different forms of telecommunications are turned into signals that can travel through the network. They always travel between two points in the network (for example, two telephone receivers or fax machines), and are directed through it by electronic circuits at telephone exchanges. This sort of telecommunications network is called a switched network. Radio and television networks, where signals are sent to many receivers from one transmitter, are known as broadcast networks.

All the telecommunications devices (telephones, fax machines and home computers) connected to normal telephone lines are linked by the lines to a local telephone exchange. Each line has its own unique telephone number which the exchange uses to find it. All the local exchanges in one area are linked to a main exchange, which in turn is linked to other main exchanges to form a national network. Also linked into the network are special exchanges for mobile telephones and Internet service providers. Most information (speech, fax messages and computer data) travels to and from the local exchange in analogue form and between local exchanges in digital form.

There are several different ways of linking together telephone exchanges on a network. Some links are underground cables, either in the form of electrical cables or optical-fibre cables, in which signals are carried by light. Some links are made with microwaves. International links across oceans are made via satellites in orbit around the Earth, and through cables stretched across the sea bed.

THE INTERNET

The Internet is a vast computer network that stretches right around the world, made up of hundreds of millions of computers. Data can travel from any computer on the network to any other computer. The Internet began in the 1960s, when research agencies in the USA built their own communications network. Other organizations, such as universities, gradually joined. As home computers became cheaper and more popular in the 1990s, the Internet began expanding rapidly, with anybody being able to use it via a telephone line.

Internet use falls into two main areas - e-mail and the World Wide Web. With e-mail, it is possible to send a text message (with other data files, such as photographs, attached if needed) almost instantly to any other Internet user at their e-mail address. The World Wide Web (the “Web”) is a huge information-gathering system. It allows one computer connected to the Internet to ask for and copy files from another computer. The files are stored in standard form so that any computer can read them.

 

TELECOMMUNICATIONS

Telecommunications is the sending and receiving of information using electricity, radio waves or light. The information can be sound, television pictures or computer data (which itself can be numbers, words, sounds and images). Forms of telecommunications include the telephone, fax, two-way radio, television and radio broadcasting, and the Internet. Most of these forms of communication require transmitting and receiving machines, and a network to link them together.

The first telecommunications device was the telegraph. Messages travelled along wires from a sending device to a receiving device as pulses of electricity, using some sort of code that both the sender and receiver understood. Practical telegraph systems were developed in the first half of the nineteenth century, and were first used for railway signalling. Early systems needed several connecting wires, but the system that eventually became standard, developed in the USA by Samuel Morse, needed just one wire. A network of telegraph lines, including undersea cables across the Atlantic, was quickly established right around the world.

In the early 1900s the telegraph was automated so that machines turned the message into code and back again. The sender could type messages on a keyboard and they would be printed out at the receiver’s end. To send a telegraph message, people had to visit a telegraph office. The message arrived at another office and was delivered by hand to the recipient.

The next major step in the development of telecommunications was the invention of the telephone, which could transmit speech, allowing people far apart to talk to each other. The first telephone receiver (the part that you talk into and listen to) was patented in 1876 by Alexander Graham Bell. This device both turned the sound of the user’s voice into an electrical signal, and an incoming signal into sound, which meant that the user could not talk and listen at the same time.

When the telephone was invented, there was no telephone network to link telephones in different places, but one soon grew up. All the telephone lines in an area meet at a telephone exchange, where they can be connected to one another, or to a line to another area’s exchange. The first exchange, opened in 1878 in Connecticut, USA, had just 21 lines. Like all early exchanges, it was operated by hand. A subscriber had to tell the operator which line he or she wanted to be connected to. The automatic exchange, which allowed people to dial numbers, was invented in the USA by Almon Strowger, and started working in 1897. Meanwhile, complex telephone networks grew in large cities. It took longer for different cities and countries to be linked, and until the middle of the twentieth century, the telegraph was still used for long-distance communication.

HOW A TELEPHONE WORKS

All telephone receivers are linked to a telephone exchange by a telephone line. When you lift or turn on the receiver, electronic circuits at the exchange detect it and wait for a number to be dialled. As you dial the number, the receiver sends signals to the exchange, which uses them to make a connection to the line of the person you are calling. The exchange makes the other telephone ring, and when it is answered, it connects the two lines together.

When you speak into the receiver’s mouthpiece, the sound makes a thin metal plate called a diaphragm vibrate. This movement affects the strength of an electric current, creating an electrical copy of the sound, which is called a signal. The signal travels through the connections in the telephone network to the other receiver, where it operates a tiny speaker in the earpiece, recreating the sound.

The signal travels in digital form for most of its journey through the network.

ELECTROMAGNETIC RADIATION

Radio waves, microwaves, light and X-rays have different characteristics, but they are all forms of electromagnetic radiation. Together with other forms, they make up a family called the electromagnetic spectrum. These forms of radiation can also be thought of as waves moving through space, in the same way as waves move across the surface of water. They all travel at the speed of light. Forms of electromagnetic radiation can be grouped according to their wavelengths - the distance between one wave crest and the next.

In reality, the wavelength at the left-hand end of the spectrum is a million million million times the wavelength at the right-hand end.

RADIO WAVES

The longest waves of the electromagnetic spectrum are radio waves. They have wavelengths ranging from more than 100 kilometres down to less than a metre. Radio waves are produced when an electric current changes strength or direction. Radio waves are important in communications through air and space. Microwaves are high-frequency radio waves also used in communications. Some microwave frequencies can be used in cooking.

Electromagnetic waves have amplitude and a frequency. Amplitude is the height or strength of a wave. Frequency is the number of wave crests that pass a point every second. To make a radio wave carry sound, it has to be modulated. This can be done by modulating (varying) either the strength of a wave - amplitude modulation or AM - or the speed of a wave - frequency modulation or FM.

LIGHT

In the middle of the electromagnetic spectrum is a small group of waves that our eyes detect, which is called visible light. It has wavelengths of around a thousandth of a millimetre. Waves with slightly different wavelengths appear as different colours, which together make up the colour spectrum. Light and especially laser light is very important in modern communications. Where practical, it is used in place of electricity and radio waves, because it can carry far more information without problems of interference.

 

INFRARED AND ULTRAVIOLET

To the left of visible light on the spectrum is infrared (IR) radiation. This is the radiation you feel as heat from hot objects. It is one of ways in which heat energy travels. Infrared radiation is used for short-range communications, such as in television remote controls, video camera autofocus and remote locking in cars.

To the right of visible light on the spectrum is ultraviolet (UV) radiation. It carries more energy than visible light. Ultraviolet radiation from the Sun is mostly absorbed by the atmosphere, but it still causes tanning of the skin and sunburn.

X-RAYS AND GAMMA RAYS

To the right of ultraviolet radiation are two more forms of electromagnetic radiation - X-rays and gamma rays. They both have very short wavelengths (less than a millionth of a millimetre) and extremely high frequencies (more than a million million million cycles per second). This means that X-rays and gamma rays have extremely high energies, and they can pass right through some solids. This makes them useful for investigating what is inside solid objects, such as human bodies, or closed suitcases at an airport security checkpoint.

X-rays were discovered by the German physicist Wilhelm Rontgen in 1895. They have a wide range of applications. In medicine, they are used to see the structure of bones and other organs by placing the patient between an X-ray source and a photographic film or camera. X-rays and gamma rays are also used in radiotherapy for treating cancers. However, in high doses they can damage tissues. X-rays are given off by high-energy, distant objects in space. X-ray telescopes can detect them.