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Alexander Graham Bell biography
|Jazyk:||Počet slov:||3 202|
|Referát vhodný pre:||Stredná odborná škola||Počet A4:||11.1|
|Priemerná známka:||2.97||Rýchle čítanie:||18m 30s|
|Pomalé čítanie:||27m 45s|
By the early 1990s, however, press associations and broadcast media transmitted both text and pictures electronically via satellite.
B. Facsimile Reproduction
Facsimile telegraph systems, which send and receive images and text, have been rendered largely obsolete by facsimile transmission, commonly referred to as fax.
IV. Telegraph Carrier Media
The electrical impulses that make up telegraph messages may be carried through wire circuits or may be broadcast as radio waves.
When Morse invented the telegraph, the only way that a message could be carried from one point to another was by wires strung directly from the transmitting device to the receiver, regardless of the distance. The wire could carry only one message at a time, and reamplification and signal correction devices had to be set up at regular points along the line. By utilizing carrier currents, which are alternating currents of a number of different frequencies, a single pair of wires can simultaneously transmit hundreds of messages, for each frequency represents a transmission channel (see Carrier Wave; Frequency). The various channels are combined at the sending station into the carrier current transmitted by the telegraph wires. At the receiving end the carrier current is passed through electrical filters, each of which transmits only a particular frequency to an appropriate receiving device. Thus, a great number of individual channels may be obtained with only one electrical circuit.
V. Microwave Transmission
The use of microwave radio transmission for long-distance telegraphic communication all over the world grew to be of major importance after World War II ended in 1945 (see Radar). The first commercial microwave radio link in telegraphy began operation between Philadelphia and New York City in 1947. It was followed in 1948 by a three-way network linking New York City, Washington, D.C., and Pittsburgh. The system then spread rapidly across the United States through the use of microwave relay antenna towers.
Microwave telegraphy is capable of carrying vocal, printed, graphic, photographic, and video communication almost instantaneously and in large quantities. It operates in the 4000-megahertz range of the commercial communications band. In this range, 40 voice bands are available in either direction, providing about 800 telegraph channels. The radio signals originating at the broadcast source are relayed to their destination by a series of parabolic reflector antennas mounted at the top of tall masts.
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