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HOW TELEVISION WORKS?

A Simplified

Viewpoint

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INTRODUCTION

A television produces a series of tiny dots on a screen that, when seen as awhole, appear as an image. Older televisions rely on a cathode-ray tube toproduce images, and operate with an analog signal. As technology hasadvanced and broadcast signals transitioned from analog to digital, plasmaand LCD (liquid crystal display) televisions were created. These TVs are more

compact and have crisper pictures than their cathode-ray counterpartsbecause they use a thin grid of pixels to create images rather than a vacuumtube.

http://www.wisegeek.com/what-is-a-cathode.htmhttp://www.wisegeek.com/how-do-i-convert-analog-to-digital.htmhttp://www.wisegeek.com/what-are-pixels.htmhttp://www.wisegeek.com/what-is-a-vacuum-tube.htmhttp://www.wisegeek.com/what-is-a-vacuum-tube.htmhttp://www.wisegeek.com/what-is-a-vacuum-tube.htmhttp://www.wisegeek.com/what-is-a-vacuum-tube.htmhttp://www.wisegeek.com/what-is-a-vacuum-tube.htmhttp://www.wisegeek.com/what-are-pixels.htmhttp://www.wisegeek.com/how-do-i-convert-analog-to-digital.htmhttp://www.wisegeek.com/how-do-i-convert-analog-to-digital.htmhttp://www.wisegeek.com/how-do-i-convert-analog-to-digital.htmhttp://www.wisegeek.com/how-do-i-convert-analog-to-digital.htmhttp://www.wisegeek.com/how-do-i-convert-analog-to-digital.htmhttp://www.wisegeek.com/what-is-a-cathode.htm

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THE EYES AND THE BRAIN

Most kinds of television work from the same basic principle.

The tiny dots of light produced on the TV screen, called

pixels, flash according to a specific pattern provided by the

video signal. A person's eyes transmit this pattern to the

brain, where it is interpreted as a recognizable image. Thetelevision set refreshes these patterns hundreds of times per

second faster than the human eye can see which gives

the illusion of movement.

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FROM STUDIO TO VIEWER

Television content

is developed in a

studio from avariety of sources

Live televised

events Pre-recorded events

Combination of both

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SATELLITE T.V.

Satellite television is television delivered by the means of

communications satelliteand received by an outdoor antenna, usually a

parabolic mirror generally referred to as a satellite dish, and as far as

household usage is concerned, a satellite receiver either in the form of

an external set-top box or a satellite tuner module built into a TV set.

Satellite TV tuners are also available as a card or a USB stick to beattached to a personal computer. In many areas of the world satellite

television provides a wide range of channels and services, often to areas

that are not serviced by terrestrialor cableproviders.

Direct broadcast satellite television comes to the general public in two

distinct flavors - analog and digital. This necessitates either having an

analog satellite receiver or a digital satellite receiver. Analog satellite

television is being replaced by digital satellite television and the latter is

becoming available in a better quality known as high-definition television.

http://en.wikipedia.org/wiki/Televisionhttp://en.wikipedia.org/wiki/Communications_satellitehttp://en.wikipedia.org/wiki/Satellite_dishhttp://en.wikipedia.org/wiki/Set-top_boxhttp://en.wikipedia.org/wiki/Personal_computerhttp://en.wikipedia.org/wiki/Terrestrial_televisionhttp://en.wikipedia.org/wiki/Cable_televisionhttp://en.wikipedia.org/wiki/High-definition_televisionhttp://en.wikipedia.org/wiki/High-definition_televisionhttp://en.wikipedia.org/wiki/High-definition_televisionhttp://en.wikipedia.org/wiki/High-definition_televisionhttp://en.wikipedia.org/wiki/High-definition_televisionhttp://en.wikipedia.org/wiki/High-definition_televisionhttp://en.wikipedia.org/wiki/Cable_televisionhttp://en.wikipedia.org/wiki/Terrestrial_televisionhttp://en.wikipedia.org/wiki/Personal_computerhttp://en.wikipedia.org/wiki/Personal_computerhttp://en.wikipedia.org/wiki/Personal_computerhttp://en.wikipedia.org/wiki/Set-top_boxhttp://en.wikipedia.org/wiki/Set-top_boxhttp://en.wikipedia.org/wiki/Set-top_boxhttp://en.wikipedia.org/wiki/Set-top_boxhttp://en.wikipedia.org/wiki/Set-top_boxhttp://en.wikipedia.org/wiki/Satellite_dishhttp://en.wikipedia.org/wiki/Satellite_dishhttp://en.wikipedia.org/wiki/Satellite_dishhttp://en.wikipedia.org/wiki/Communications_satellitehttp://en.wikipedia.org/wiki/Communications_satellitehttp://en.wikipedia.org/wiki/Communications_satellitehttp://en.wikipedia.org/wiki/Television

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SATELLITE T.V. RECEPTION

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TECHNOLOGY

Satellites used for television signals are generally in either naturally highlyelliptical (with inclination of +/-63.4 degrees and orbital period of about

12 hours, also known as Molniya orbit) or geostationary orbit 37,000 km

(22,300 miles) above the earthsequator.

Satellite television, like other communications relayed by satellite, starts with a

transmitting antenna located at an uplink facility. Uplink satellite dishes are

very large, as much as 9 to 12 meters (30 to 40 feet) in diameter. The

increased diameter results in more accurate aiming and increased signal

strength at the satellite. The uplink dish is pointed toward a specific satellite

and the uplinked signals are transmitted within a specific frequency range, so

as to be received by one of the transponders tuned to that frequency range

aboard that satellite. The transponder 'retransmits' the signals back to Earth

but at a different frequency band (a process known as translation, used toavoid interference with the uplink signal), typically in the C-band(48 GHz) or

Ku-band(1218 GHz) or both. The leg of the signal path from the satellite to

the receiving Earth station is called the downlink

http://en.wikipedia.org/wiki/Inclinationhttp://en.wikipedia.org/wiki/Molniya_orbithttp://en.wikipedia.org/wiki/Equatorhttp://en.wikipedia.org/wiki/Uplinkhttp://en.wikipedia.org/wiki/Transponderhttp://en.wikipedia.org/wiki/C_bandhttp://en.wikipedia.org/wiki/Ku_bandhttp://en.wikipedia.org/wiki/Ku_bandhttp://en.wikipedia.org/wiki/Ku_bandhttp://en.wikipedia.org/wiki/Ku_bandhttp://en.wikipedia.org/wiki/Ku_bandhttp://en.wikipedia.org/wiki/Ku_bandhttp://en.wikipedia.org/wiki/Ku_bandhttp://en.wikipedia.org/wiki/C_bandhttp://en.wikipedia.org/wiki/C_bandhttp://en.wikipedia.org/wiki/C_bandhttp://en.wikipedia.org/wiki/Transponderhttp://en.wikipedia.org/wiki/Uplinkhttp://en.wikipedia.org/wiki/Equatorhttp://en.wikipedia.org/wiki/Molniya_orbithttp://en.wikipedia.org/wiki/Molniya_orbithttp://en.wikipedia.org/wiki/Molniya_orbithttp://en.wikipedia.org/wiki/Inclination

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TRANSMISSION OF AUDIO AND VIDEO

SIGNALS

The image captured is combined with other electronic

content (text and graphics) plus audio. The combined image is amplified and transmitted via

AM (amplitude modulation) and FM (frequencymodulation) carrier waves to either a satellite feed orfrom direct transmission to a television receiver.

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THE RECEIVER DECODES THE

SIGNAL

The electronic signalis decoded by thereceiver; splitting the

FM wave to theaudio section andthe AM wave to thevideo section of the

television.

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THEBLACK AND WHITE TV SIGNAL

In a black-and-white TV,the screen is coated withwhite phosphor and theelectron beam "paints" animage onto the screen by

moving the electron beamacross the phosphor a lineat a time.

To "paint" the entire screen, electronic circuits inside the TV

use the magnetic coils to move the electron beam in a "rasterscan" pattern across and down the screen. The beam paints one

line across the screen from left to right. It then quickly flies

back to the left side, moves down slightly and paints another

horizontal line, and so on down the screen, like this:

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The blue lines represent lines that the

electron beam is "painting" on the screen

from left to right, while the red dashed

lines represent the beam flying back to the

left. When the beam reaches the right side

of the bottom line, it has to move back to

the upper left corner of the screen, as

represented by the green line in the figure.

When the beam is "painting," it is on, andwhen it is flying back, it is off so that it does

not leave a trail on the screen.

As the beam paints each line from left to

right, the intensity of the beam is changed

to create different shades of black, gray

and white across the screen. Because thelines are spaced very closely together, your

brain integrates them into a single image. A

TV screen normally has about 480 lines

visible from top to bottom.

The Black and

WhiteTV Signal

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EARLY TELEVISION IMAGERY

Early Television did had

lower scan rates and

reduced image quality.

As image capture devices

improved after the National

Television System

Committee (NTSC)

established standards by

which broadcasters and

manufacturers alike

adhered.

Interlaced transmissions

became an early standard.

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INTERLACED

IMAGES

Interlaced images allow

for easier transmission

of moving images athigher resolution.

Half pictures in 1/60tha

second..30fps.

Trade offs include some

image jitter:

Jagged edges from motion

occur because the object is

in a different location every

1/60 of a second. The even

lines show the object inone position while the odd

lines show the image in a

different position.

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INTERLACED IMAGES

Motion artifacts and horizontal "line twitter" are the most notoriousNTSC artifacts.

The closer you sit to your video display device and the larger thevideo display device appears, the easier it will be to see NTSC artifactsin images.

Some newer television sets employ powerful image processing thatcan make NTSC artifacts very difficult to find. HDTV (high-definitiondigital television) includes standards for higher-resolutionprogressive scanning, which eliminates the video image artifacts wehave endured for over 50 years.

Unfortunately, many HDTV products have chosen the higher

resolution 1080i format (1080 lines interlaced) to use to converteverything regardless of how it was broadcast or recorded. This isunfortunate because interlace artifacts remain quite visible even inthe 1080i format.

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THE CATHODE RAY TUBE

The cathode-ray tube (CRT), the oldest version of the television, consists of a vacuum tube

with a narrow end and a wide end. The narrow end contains an ion gun, which shoots out a

series of charged particles of electricity. A series of electromagnets guide the particles tospecific points on the wide end of the tube, the screen that viewers look at. Phosphors,

substances that light up when a charged electrical particle hits them, coat the screen's inner

surface. The ion gun essentially sprays the image at the screen, much like a paint gun

sprays paint onto a surface.

Different kinds of phosphors produce different colors, but for color television, only red, blue,

and green are needed. Using these colors in various combinations and intensities can create

all the colors the human eye can see. As energy travels from the ion gun to the phosphors, it

is filtered to strike the exact point on the screen needed to produce a specific hue. In

combination, all of these colored pixels create a color image.

Cathode-ray tubes are quite heavy due to the large amount of glass they contain, and

relatively inefficient, especially when used in large-screen televisions. For this reason, newtechnologies were developed to make lighter sets with crisper images. In addition, the

development of high definition (HD) digital broadcast signals made bigger screens more

popular since the images were of higher quality. Plasma and LCD televisions were created in

response.

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COLOR TV

SCREEN

A color TV screen differs

from a black-and-white

screen in three ways: Three electron beams (Red,

Green, Blue)that move

simultaneously across the screen.

The screen is coated with red,

green and blue phosphors

arranged in dots or stripes.

On the inside of the tube, very

close to the phosphor coating,

there is a thin metal screen called

a shadow mask. This mask is

perforated with very small holes

that are aligned with the

phosphor dots (or stripes) on the

screen.

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COLOR TV SCREEN

To create a white dot,red, green and bluebeams are fired

simultaneously -- thethree colors mixtogether to createwhite. The absence ofsignal is black.

All other colors on aTV screen arecombinations of red,green and blue

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HOW COLORS ARE CREATED

IN TV

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COMPOSITE VIDEO

SIGNAL

A signal that contains all threeof these components --intensity information,horizontal-retrace signals, and

vertical-retrace signals -- iscalled a composite video signal.

One line of a typical compositevideo signal looks somethinglike this:

The horizontal-retrace signals are 5-microsecond (abbreviated as "us" in the

figure) pulses at zero volts. Electronics inside the TV can detect these pulses and use

them to trigger the beam's horizontal retrace. The actual signal for the line is a

varying wave between 0.5 volts and 2.0 volts, with 0.5 volts representing black and 2

volts representing white. This signal drives the intensity circuit for the electronbeam. In a black-and-white TV, this signal can consume about 3.5 megahertz (MHz)

of bandwidth, while in a color set the limit is about 3.0 MHz.

A vertical-retrace pulse is similar to a horizontal-retrace pulse but is 400 to 500

microseconds long. The vertical-retrace pulse is serrated with horizontal-retrace

pulses in order to keep the horizontal-retrace circuit in the TV synchronized.

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Following these eight cycles, a phase shiftin the chrominance signal indicates the color to display. The amplitude of

the signal determines the saturation. The following table shows you the relationship between color and phase:

Color PhaseBurst 0 degrees

Yellow 15 degreesRed 75 degrees

Magenta 135 degreesBlue 195 degrees

Cyan 255 degreesGreen 315 degrees

A black-and-whiteTV filters out andignores thechrominance signal.A color TV picks itout of the signal anddecodes it, alongwith the normalintensity signal, to

determine how tomodulate the threecolor beams.

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THE PLASMA SCREEN

A plasma screen television consists of a number of tiny cells filled with neon

and xenongases. Each cell is linked to an electrode, which, when fired, excites the

gases contained in the cell. The gases emit charge particles, much like the ion gun,that interact with phosphors coating the glass inside each cell. The phosphors light

up, creating the image seen on the television screen. The large number of cells in a

plasma screen makes for a great number of pixels, rendering a clearer and brighter

image.

Compared to other technologies, plasma TVs produce some of deepest blacks,which means that the contrast ratio is very high. They also have very high refresh

rates, so images with a lot of motion don't blur as they can on other televisions. If

the image remains static, however, it can burn into the screen, creating a

permanent discoloration; this is more common in older plasma TVs, and can also

occur with CRT screens. Plasma screens can be set to be very bright, which

requires a lot of electricity. They also tend to be thicker than LCD televisions,

although much thinner than CRTs.

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THE LCD SCREEN

LCD televisions also use cells to create images. Rather than exciting gases as plasma

TVs do, however, the cells contain a set of red, blue, and green filters covered by a layer

of liquid crystals sandwiched between two pieces of glass. Depending on the display type,

each cell is linked to either electrodes or thin film transistors (TFT), which trigger the

necessary cells to create the image. A backlight most often cold-cathode fluorescent

lamp lights up the screen so the image can be seen.

While LCDs are very light and thin, they are subject to "dead" pixels, where one or more

cells on the screen do not change. Viewing LCD screens from an angle can also lower thepicture quality. They have slower response times than plasma or CRT televisions as well,

so images can "ghost" or blur in movement.

More recent versions of the LCD television use light-emitting diodes (LEDs) as the light

source rather than cold-cathode fluorescent lamps. LED televisions require less electricity

than regular LCD screens, and take up even less space. Also, LEDs generally emit a

brighter white light, making these screens especially vivid.

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BIBLIOGRAPHY

http://en.wikipedia.org/wiki/Television

Britannica Encyclopedia

http://www.tvhistory.tv/index.html

www.howstuffworks.com

http://en.wikipedia.org/wiki/Televisionhttp://www.tvhistory.tv/index.htmlhttp://www.tvhistory.tv/index.htmlhttp://en.wikipedia.org/wiki/Television

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