Telstar 1962 . 2 1963. Intelsat I 6 1965 .
Orbita 1967 Molniya . , , Anik 1, 1972 . , 1976 .
, , , , , , .
- USB-, . , , .
, - . . , , .
The Advantages of Digital TV.
DTV has several advantages over analog one. Considerable quality improvement and digital TV signal interference immunity is reached when switching from analog to digital ground broadcasting.
Moreover, electric energy consumption capacity of a digital transmitter in terms of one TV program is reduced several times in comparison with analog TV. It allows to cut electricity total costs.
Stable and high-quality perception of TV programs is available both by a fixed and indoor antenna.
Digital and analog signals react differently to interference. Digital TV is more tolerant of noise and interference than analog.
Common problems of analog television are ghosting of images, noise from weak signals, and many other potential problems which degrade the quality of the image and sound, although the program material may still be watchable.
With digital television, the audio and video must be synchronized digitally, so reception of the digital signal is nearly complete.
Digital channels take up less bandwidth, so digital broadcasters can provide more digital channels in the same space, and a smaller range of channels can carry an all-digital set of television stations.
|
|
|
Digital technology provides high-definition television service and other non-television services such as multimedia or interactivity.
DTV permits special services such as multiplexing (more than one program on the same channel), electronic program guides and additional languages (spoken or subtitled). The sale of services may provide an additional revenue source.
Digital technology provides the possibility of expansion in the number of TV and_ radio programs on one frequency channel.
.
DTV . - .
, . .
.
- . , .
, , , , .
, , .
, , - .
, , .
DTV , ( ), ( ). .
- .
A Miraculous beam
In 1950s a new principle of electromagnetic wave generation was discovered simultaneously by Soviet and American scientists. Quantum generators called lasers produce a very concentrated beam of light which is much more intense than ordinary light and millions of times brighter than the Sun.
The word LASER is made up of the initial letters of a phrase describing the device function - light amplification by stimulated emission of radiation.
In a laser the electrons are excited to a higher energy level. As the electrons return to their
In a laser the electrons are excited to a higher energy level. As the electrons return to their normal state, they emit energy. This energy is emitted as light which can be seen.
The light emitted from a laser is unique in several ways. Unlike most light sources in which the light is emitted in all directions, laser light comes out in a straight narrow beam.
|
|
|
It's the high concentration of light energy over a very small area that makes the laser so useful. A laser is not a source of energy. The energy from the source of supply is only converted by a laser into an intense narrow beam of light energy.
There are many types of lasers which utilize both solid, liquid and gaseous materials, but they all operate almost in the same way.
Luminescent crystals, luminescent glass, a mixture of several gases and finally - semiconductors are used in lasers.
Lasers have found many practical applications. They are now used for many scientific, industrial and medical purposes.
But in the field of communications lasers have found the most intensive application. A single laser beam is able to carry simultaneously a thousand million telephone conversations or a thousand million television programs.
1950- . , , .
- .
.
. , . , .
, . , , .
, . . .
, , , .
, , , , - , .
. , .
. .
EARLY DAYS OF TV
In its early stages of development television employed a combination of optical, mechanical and electronic technologies to capture, transmit and display a visual image. By the late 1920s however those employing only optical and electronic technologies were being explored. All modern television systems relied on the latter although the knowledge gained from the work on electromechanical systems was crucial in the development of fully electronic television.
The first images transmitted electrically were sent by early mechanical fax machines including the pantelegraph developed in the late nineteenth century. The concept of electrically powered transmission of television images in motion was first sketched in 1878 as the telephonoscope shortly after the invention of the telephone.
In 1884 Paul Gottlieb Nipkow a 23-year-old university student in Germany patented the first electromechanical television system which employed a scanning disk, a spinning disk with a series of holes spiraling toward the center for rasterization. The holes were spaced at equal angular intervals so that in a single rotation the disk would allow light to pass through each hole and onto a light-sensitive selenium sensor which produced the electrical pulses. As an image was focused on the rotating disk each hole captured a horizontal "slice" of the whole image.
|
|
|
On March 25, 1925 Scottish inventor John Logie Baird gave the first public demonstration of televised silhouette images in motion at Selfridge's Department Store in London. On June 13, 1925 Charles Francis Jenkins transmitted the silhouette image of a toy windmill in motion over a distance of five miles from a naval radio station in Maryland to his laboratory in Washington using a lensed disk scanner with a 48-line resolution.
, , . 1920- , , . , , .
, Pantelegraph . 1878 telephonoscope .
1884 23- , , . , , . .
25 1925 Selfridge . 13 1925 - 48- .
