how tv works.ppt
TRANSCRIPT
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HOW TELEVISION WORKS?A Simplified
Viewpoint
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INTRODUCTIONA 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.
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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 highly
elliptical (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 VIDEOSIGNALS
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 THESIGNALThe electronic signal
is 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 andWhiteTV 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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INTERLACEDIMAGES Interlaced images allowfor 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 notorious
NTSC 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 TVSCREEN A color TV screen differsfrom 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 CREATEDIN TV
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COMPOSITE VIDEOSIGNAL 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
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