Optical Receiver
The optical receiver is like the sailor on the deck of the receiving ship. It takes the incoming digital light signals, decodes them and sends the electrical signal to the other user's , or (receiving ship's captain). The receiver uses a photocell or photodiode to detect the light.
Advantages of Fibre Optics
Why are optic-optic systems revolutionizing telecommunications? Compared to conventional metal wire (copper wire), optical fibres are:
Less expensive - Several miles of optical cable can be made cheaper than equivalent lengths of copper wire. This saves your provider (cable TV, Internet) and you money.
Thinner - Optical fibres can be drawn to smaller diameters than copper wire.
Higher carrying capacity - Because optical fibres are thinner than copper wires, more fibres can be bundled into a given-diameter cable than copper wires. This allows more phone lines to go over the same cable or more channels to come through the cable into your cable TV box.
Less signal degradation - The loss of signal in optical fibre is less than in copper wire.
Light signals - Unlike electrical signals in copper wires, light signals from one fibre do not interfere with those of other fibres in the same cable. This means clearer phone conversations or TV reception.
Low power - Because signals in optical fibres degrade less, lower-power transmitters can be used instead of the high-voltage electrical transmitters needed for copper wires. Again, this saves your provider and you money.
Digital signals - Optical fibres are ideally suited for carrying digital information, which is especially useful in computer networks.
Non-flammable - Because no electricity is passed through optical fibres, there is no fire hazard.
Lightweight - An optical cable weighs less than a comparable copper wire cable. Optic-optic cables take up less space in the ground.
Flexible - Because fibre optics are so flexible and can transmit and receive light, they are used in many flexible for the following purposes:
Medical imaging - in bronchoscopes, endoscopes, laparoscopes
Mechanical imaging - inspecting mechanical welds in pipes and engines (in , , , )
Plumbing - to inspect
Physics of Total Internal Reflection
When light passes from a medium with one (m1) to another medium with a lower index of refraction (m2), it bends or away from an imaginary line perpendicular to the surface (normal line). As the angle of the beam through m1 becomes greater with respect to the normal line, the refracted light through m2 bends further away from the line.
At one particular angle (critical angle), the refracted light will not go into m2, but instead will travel along the surface between the two media (sine [critical angle] = n2/n1 where n1 and n2 are the indices of refraction [n1 is greater than n2]). If the beam through m1 is greater than the critical angle, then the refracted beam will be reflected entirely back into m1 (total internal reflection), even though m2 may be transparent!
In physics, the critical angle is described with respect to the normal line. In fibre optics, the critical angle is described with respect to the parallel axis running down the middle of the fibre. Therefore, the optic-optic critical angle = (90 degrees - physics critical angle).
In an optical fibre, the light travels through the core (m1, high index of refraction) by constantly reflecting from the cladding (m2, lower index of refraction) because the angle of the light is always greater than the critical angle. Light reflects from the cladding no matter what angle the fibre itself gets bent at, even if it's a full circle!
Because the cladding does not absorb any light from the core, the light wave can travel great distances. However, some of the light signal degrades within the fibre, mostly due to impurities in the glass. The extent that the signal degrades depends upon the purity of the glass and the wavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium optical fibres show much less signal degradation -- less than 10 percent/km at 1,550 nm.
Signals
Digital signal
A is a signal in binary code (a sequence of ones and noughts and can be sent using sound as well).
A may be a series high and low voltages sent along a metal cable or a series of flashes of light sent down an .
Analogue signals are a signal that changes continuously – for example a varying voltage or current or a sound wave are all analogue signals.
A simple “modem” which sends data from your computer down the phone line may use two notes, one high and one low, representing a “1” and a “0”, so that a sequence of these notes represents a stream of “1”s and “0”s.
Flashes of light are another example of a , where “on” is a “1” and “off” is a “0”.
An is relatively simple to “hear” or understand, whereas a must be “decoded” in some way before it is understandable.
The advantages of digital signals:
Optical fibres can transmit digital signals (and optical fibres are cheap).
Digital signals can be compressed so more channels can be transmitted along the same fibre
The electronic circuitry for digital electronics is cheap
It’s easy to combine several types of information on once channel (e.g. video and audio) by . Video phones would be impossible with analogue systems as there wouldn't be enough channels available to send all the information for eveyone's phonecalls.
Digital signals are more secure – it is easier to encrypt digital signals making internet shopping less risky for examole.
Can connect several different users to the same link – such as video conferencing.
Digital signals suffer less from because any errors can be detected and corrected using regnerators. Therefore the signal you receive is more easily decoded and, on television, you will see a clearer picture or hear a clearer sound, for example.
The electronics can make corrections to the signal if it has been corrupted during transmission which means that the image or sound is complete.
The disadvantages of digital signals:
Digital signals need more ‘bandwidth to transmit the same information
The and have to synchronise very carefully so that the information makes sense. If you start counting half way through a digital number, you might receive 10110001 whereas you should have received 1110110001 – very different!
Storage
can be used to store analogue or digital information (DAT or digital audio tape).
Magnetised particles in the tape store the encoded information.
The tape recorder has a record head which magnetises particles in the tape into a particular pattern to represent the sound (and images)
The playback head senses the direction that the particles are magnetized and generates a matching varying current. This is decoded by the electronics in the and as sound (and images)
Compact Discs
The sound (or video) information can be recorded or stored as digital code on a CD () or DVD (digital versatile disc) . A diode is reflected off the surface of the disc. A photo diode senses the reflected light and so the digital code is read, decoded into an .
Vinyl Records
Sound (analogue information) can be recorded directly on a as a series of lumps. These generate an analogue electrical signal which matches the analogue sound wave.
Transducers
A is a that changes sound into electrical signals (varying current in a wire).
This electrical signal can either be sent directly to an and played back through a or it can be encoded into digital form and sent as a phone message.
A moving coil works like a working backwards. The sound make the diaphragm move. Because the wire is moving near to a magnet, this induces a current in the wire. This effect is called electromagnetic induction.
A is a that takes electrical signals and turns them into sound.
It has a large cone (diaphragm) that vibrates to form the sound.
The alternating electric current in the coil of wire generates its own magnetic field. The coil's magnetic field reacts to the magnetic field of the magnet and so the coil moves back and forth. This is called the Motor Effect.
The moving coil makes the cone move.
History of Communication
The Beginning of Electronic Communications
In 1825, British inventor (1783-1850) exhibited a device that laid the foundations for large-scale electronic communications: the electromagnet. Sturgeon displayed its power by lifting nine pounds with a seven-ounce piece of iron wrapped with wires through which the current of a single cell battery was sent.
In 1830, an American, (1797-1878), demonstrated the potential of Sturgeon's device for long distance communication by sending an electronic current over one mile of wire to activate an electromagnet which caused a bell to strike. Thus the electric telegraph was born. Samuel F.B. Morse (1791-1872), whose sketches of a "magnetized magnet" in operation are , successfully exploited Henry's invention commercially.
While a professor of arts and design at New York University in 1835, Samuel Morse proved that signals could be transmitted by wire. He used pulses of current to deflect an electromagnet, which moved a marker to produce written codes on a strip of paper - the invention of Morse Code. The following year, the device was modified to emboss the paper with dots and dashes. He gave a public demonstration in 1838, but it was not until five years later that Congress (reflecting public apathy) funded $30,000 to construct an experimental telegraph line from Washington to Baltimore, a distance of 40 miles.
Six years later, members of Congress witnessed the sending and receiving of messages over part of the telegraph line. Before the line had reached Baltimore, the Whig party held its national convention there, and on May 1, 1844, nominated Henry Clay. This news was hand-carried to Annapolis Junction (between Washington and Baltimore) where Morse's partner, Alfred Vail, wired it to the Capitol. This was the first news dispatched by electric telegraph.
The message, "What hath God wrought?" sent later by "Morse Code" from the old Supreme Court chamber in the United States Capitol to his partner in Baltimore, officially opened the completed line of May 24, 1844. Morse allowed Annie Ellsworth, the young daughter of a friend, to choose the words of the message, and she selected a verse from Numbers XXIII, 23: "What hath God wrought?", which was recorded onto paper tape. Morse's early system produced a paper copy with raised dots and dashes, which were translated later by an operator.
Samuel Morse and his associates obtained private funds to extend their line to Philadelphia and New York. Small telegraph companies, meanwhile began functioning in the East, South, and Midwest. Dispatching trains by telegraph started in 1851, the same year Western Union began business. Western Union built its first transcontinental telegraph line in 1861, mainly along railroad rights-of-way.
In 1881, the Postal Telegraph System entered the field for economic reasons, and merged with Western Union in 1943.
The original Morse telegraph printed code on tape. However, in the United States the operation developed into sending by key and receiving by ear. A trained Morse operator could transmit 40 to 50 words per minute. Automatic transmission, introduced in 1914, handled more than twice that number.
In 1913 Western Union developed multiplexing, which it made possible to transmit eight messages simultaneously over a single wire (four in each direction). Teleprinter machines came into use about 1925. Varioplex, introduced in 1936, enabled a single wire to carry 72 transmissions at the same time (36 in each direction). Two years later Western Union introduced the first of its automatic facsimile devices. In 1959 Western Union inaugurated TELEX, which enables subscribers to the teleprinter service to dial each other directly.
Until 1877, all rapid long-distance communication depended upon the telegraph. That year, a rival technology developed that would again change the face of communication -- the . By 1879, patent litigation between Western Union and the infant telephone system was ended in an agreement that largely separated the two services.
Samuel Morse is best known as the inventor of the telegraph, but he is also esteemed for his contributions to American portraiture. His painting is characterized by delicate technique and vigorous honesty and insight into the character of his subjects.
In the 1870s, two inventors and Alexander Graham Bell both independently designed devices that could transmit speech electrically (the telephone). Both men rushed their respective designs to the patent office within hours of each other, Alexander Graham Bell his telephone first. Elisha Gray and Alexander Graham Bell entered into a famous legal battle over the invention of the telephone, which Bell won.
The and telephone are both wire-based electrical systems, and Alexander Graham Bell's success with the telephone came as a direct result of his attempts to improve the telegraph.
When Bell began experimenting with electrical signals, the telegraph had been an established means of communication for some 30 years. Although a highly successful system, the telegraph, with its dot-and-dash Morse code, was basically limited to receiving and sending one message at a time. Bell's extensive knowledge of the nature of sound and his understanding of music enabled him to conjecture the possibility of transmitting multiple messages over the same wire at the same time. Although the idea of a multiple telegraph had been in existence for some time, Bell offered his own musical or harmonic approach as a possible practical solution. His "harmonic telegraph" was based on the principle that several notes could be sent simultaneously along the same wire if the notes or signals differed in pitch.
By October 1874, Bell's research had progressed to the extent that he could inform his future father-in-law, Boston attorney Gardiner Greene Hubbard, about the possibility of a multiple telegraph. Hubbard, who resented the absolute control then exerted by the Western Union Telegraph Company, instantly saw the potential for breaking such a monopoly and gave Bell the financial backing he needed. Bell proceeded with his work on the multiple telegraph, but he did not tell Hubbard that he and Thomas Watson, a young electrician whose services he had enlisted, were also exploring an idea that had occurred to him that summer - that of developing a device that would transmit speech electrically.
Model of Alexander Graham Bell's Telephone
This model of Bell's first telephone (right) is a duplicate of the instrument through which speech sounds were first transmitted electrically (1875).
While Alexander Graham Bell and Thomas Watson worked on the harmonic telegraph at the insistent urging of Hubbard and other backers, Bell nonetheless met in March 1875 with , the respected director of the Smithsonian Institution, who listened to Bell's ideas for a telephone and offered encouraging words. Spurred on by Henry's positive opinion, Bell and Watson continued their work. By June 1875 the goal of creating a device that would transmit speech electrically was about to be realized. They had proven that different tones would vary the strength of an electric current in a wire. To achieve success they therefore needed only to build a working transmitter with a membrane capable of varying electronic currents and a receiver that would reproduce these variations in audible frequencies.
On June 2, 1875, Alexander Graham Bell while experimenting with his technique called "harmonic telegraph" discovered he could hear sound over a wire. The sound was that of a twanging clock spring.
Bell's greatest success was achieved on March 10, 1876, marked not only the birth of the telephone but the death of the multiple telegraph as well. The communications potential contained in his demonstration of being able to "talk with electricity" far outweighed anything that simply increasing the capability of a dot-and-dash system could imply.
Alexander Graham Bell's notebook entry of 10 March 1876 describes his successful experiment with the telephone. Speaking through the instrument to his assistant, Thomas A. Watson, in the next room, Bell utters these famous first words, "Mr. Watson -- come here -- I want to see you."
Radio owes its development to two other inventions, the telegraph and the telephone, all three technologies are closely related. (Read the history found on the and pages to better understand the roots of radio)
Few radio broadcasts travel through the air exclusively, while many are sent over telephone wires. In the 1860s, , a Scottish physicist, predicted the existence of radio waves, and in 1886 , a German physicist, demonstrated that rapid variations of electric current could be projected into space in the form of radio waves similar to those of light and heat.
, an Italian inventor, proved the feasibility of radio communication. He sent and received his first radio signal in Italy in 1895. By 1899 he flashed the first wireless signal across the English Channel and two years later received the letter "S", telegraphed from England to Newfoundland. This was the first successful transatlantic radiotelegraph message in 1902.
(Note: is now credited with having inventing modern radio; the Supreme Court overturned Marconi's patent in 1943 in favor of Tesla.)
Wireless signals proved effective in communication for rescue work when a sea disaster occurred. Effective communication was able to exist between ships and ship to shore points. A number of ocean liners installed wireless equipment. In 1899 the United States Army established wireless communications with a lightship off Fire Island, New York. Two years later the Navy adopted a wireless system. Up to then, the Navy had been using visual signaling and homing pigeons for communication.
In 1901, radiotelegraph service was instituted between five Hawaiian Islands. By 1903, a Marconi station located in Wellfleet, Massachusetts, carried an exchange or greetings between President Theodore Roosevelt and King Edward VII. In 1905 the naval battle of Port Arthur in the Russo-Japanese war was reported by wireless, and in 1906 the U.S. Weather Bureau experimented with radiotelegraphy to speed notice of weather conditions.
In 1909, Robert E. Peary, arctic explorer, radiotelegraphed: "I found the Pole". In 1910 Marconi opened regular American-European radiotelegraph service, which several months later, enabled an escaped British murderer to be apprehended on the high seas. In 1912, the first transpacific radiotelegraph service linked San Francisco with Hawaii.
Overseas radiotelegraph service developed slowly, primarily because the initial radiotelegraph set discharged electricity within the circuit and between the electrodes was unstable causing a high amount of interference. The high-frequency alternator and the tube resolved many of these early technical problems. The Navy made major use of radio transmitters -- especially Alexanderson alternators, the only reliable long-distance wireless transmitters - for the duration.
During World War I, governments began using radiotelegraph to be alert of events and to instruct the movement of troops and supplies. World War II demonstrated the value of radio and spurred its development and later utilization for peacetime purposes. Radiotelegraph circuits to other countries enabled persons almost anywhere in the United States to communicate with practically any place on earth.
Since 1923, pictures have been transmitted by wire, when a photograph was sent from Washington to Baltimore in a test. The first transatlantic radiophoto relay came in 1924 when the Radio Corporation of America beamed a picture of Charles Evans Hughes from London to New York. RCA inaugurated regular radiophoto service in 1926.
Two radio communication companies once had domestic networks connecting certain large cities, but these were closed in World War II. However, microwave and other developments have made it possible for domestic telegraph communication to be carried largely in part over radio circuits. In 1945 Western Union established the first microwave beam system, connecting New York and Philadelphia. This has since been extended and is being developed into a coast-to-coast system. By 1988 Western Union could transmit about 2,000 telegrams simultaneously in each direction.
The first time the human voice was transmitted by radio is debateable. Claims to that distinction range from the phase, "Hello Rainey" spoken by Natan B. Stubblefield to a test partner near Murray, Kentucky, in 1892, to an experimental program of talk and music by Reginald A. Fessenden, of Brant Rock, Massachusetts, in 1906, which was heard by radio-equipped ships within several hundred miles.
In 1915 speech was first transmitted across the continent from New York City to San Francisco and across the Atlantic Ocean from Naval radio station NAA at Arlington, Virginia, to the in Paris. There was some experimental military radiotelephony in World War I between ground and aircraft.
The first ship-to-shore two way radio conversation occurred in 1922, between Deal Beach, New Jersey, and the S.S. America, 400 miles at sea. However, it was not until 1929 that high seas public radiotelephone service was inaugurated. At that time telephone contact could be made only with ships within 1,500 miles of shore. Today there is the ability to telephone nearly every large ship wherever it may be on the globe.
Commercial radiotelephony linking North America with Europe was opened in 1927, and with South America three years later. In 1935 the first telephone call was made around the world, using a combination of wire and radio circuits.
Until 1936, all American transatlantic telephone communication had to be routed through England. In that year, a direct radiotelephone circuit was opened to Paris. Telephone connection by radio and cable is now accessible with 187 foreign points.
Radio Waves
Radio are long electromagnetic . Other electromagnetic include ultra violet, visible light, infra red and microwaves.
We use radio to carry information such as sound and video (not just radio programmes!)
Transmitted radio can reach the as ground, sky or space depending on their .
The ionosphere, between 45 and 250 miles (70 to 400 km) above the earth. can reflect radio .
Radio can be diffracted by buildings, mountains and the curvature of the Earth. The amount of depends upon the of the radio wave and the physical size of the obstacle.
can affect the quality of the received radio signal making it sound crackly or appear snowy or even show 'ghosts' on a television picture.
Sometimes you get crackling on radios or 'snowy effects' on television pictures - or the picture breaks up completely! This is caused by or - but what's the difference?
is the additional unwanted signals that get added to a signal and corrupts it making it difficult to understand.
happens when two signals arrive at the same place at the same time giving a very muddled message.
Sometimes a signal is made worse by :
is when a signal strength is reduced. Electrical signals are attenuated by the electrical resistance of the wires. Light signals are attenuated by being dispersed or absorbed in the glass of the . This makes the signal too weak to be decoded by the .
To improve a signal we can use a or a :
Regenerators and repeaters in electrical cables and communications add energy to the signal so that it arrives with enough power to be understood when it is decoded.
Modulation
means 'change'.
and are used in radio communications. It's the only way to add the sound wave to a radio wave and get it carried to the radio or television.
AM is . The of the is altered by adding the of the sound wave to it.
FM is . The of the signal is added to the of the , changing it.
AM signals have a greater range than FM signals. This is why local radio is FM - it only covers the small area near where you live. After that the dies away.
AM signals are more susceptible to than FM signals. This is why AM radio crackles more than FM radio. FM signals are often preferred as they are more likely to give you a clean signal without .
Sound, like your voice, is an example of an . As you speak, your voice contains lots of different frequencies at lots of different amplitudes.
Satellites
