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SEMINAR REPORT ON DIRET TO HOME (DTH)
ABSTRACT
DTH is a new technology and it has matured to its full
potential in other parts of the world. There are many
application has been found everyday for exploitation of
benefits of DTH
The word ËœDTHâ„¢ is synonymous with transmission of digital
video channel to home subscriberâ„¢s using a small dish
antenna. The DTH utilizes a technology which enables a home
to receive high speed internet broadband access data
communication, voice over internet protocol (IP) telephony
and much more using an open standard Digital Video
Broadcasting (DVB) technology. The video channels are
received with a suitable set top box. Capable of demodulating
Motion Picture Engineering Group (MPEG-2) standard videos. It
is for the return channel required for other services such as
voice over internet protocol and broadband access data
communications, that a return channel is also required for the
home terminal. The return channel via the satellite is called
RCS and is an open standard.
Hardware compatible with DVB-RCS technology are readily
available in the market in both Ku-band and C-band. DVB-RCS
is an international open standard for multimedia satellite
network where the return data rates in access of 2 Mbps are
possible using low cost user terminals. The forward ink is
usually at 40 Mbps.
Today, most satellite TV customers in developed television
markets get their programming through a direct broadcast
satellite (DBS) provider, such as DISH TV or DTH platform. The
provider selects programs and broadcasts them to subscribers as
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a set package. Basically, the providerâ„¢s goal is to bring dozens or
even hundreds of channels to the customerâ„¢s television in a form
that approximates the competition from Cable TV. Unlike earlier
programming, the providerâ„¢s broadcast is completely digital,
which means it has high picture and stereo sound quality. Early
satellite television was broadcast in C-band - radio in the 3.4-
gigahertz (GHz) to 7-GHz frequency range. Digital broadcast
satellite transmits programming in the Ku frequency range (10
GHz to 14 GHz). There are five major components involved in a
direct to home (DTH) satellite
A PATH TOWARDS DTH
On June 25, 1967, for two hours 26 nations of the world were
joined together by an invisible electromagnetic grid utilizing four
satellites. The London-based production, in glorious black and
white, was the first-ever use of satellites to simultaneously
interconnect remote corners of the world to a single program
event. The program, appropriately entitled "Our World," included
the Beatles debuting the song "All You Need Is Love" to an
audience estimated at more than 600 million.
During the course of the telecast, live feeds were interconnected
through a pair of early design Intelsats, an American experimental
satellite (ATS-1), and a Russian Molniya class bird. The New York
Times would write about the ground-breaking telecast, "Our World
was a compelling reaffirmation of the potential of the home
screen to unify the peoples of the world."
Less than three decades later, or approximately the period of one
generation of mankind, more than 30 million homes in the world
are equipped with their own satellite dishes. The early Intelsat,
ATS, and Molniya satellites were capable of relaying one (or at
most, two) simultaneous TV programs; each satellite of the
current generation easily can deliver as many as 200 program
channels to dish antennas less than one-thirtieth of the size
required for reception of the original "Our World" telecast.
Well before the turn of the century, virtually any location in Asia
or the Pacific will have direct access to hundreds of channels of
TV, high-speed Internet links, and thousands of radio program
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channels. It is not an exaggeration to suggest that satellites are
redesigning the very fabric of life by creating full-time universal
access to "our world."
All of this technology creates virtually unlimited opportunities for
new business enterprise and personal development. You are
holding in your hand a key that will unlock for you, your family,
and your business the "secrets" of the 21st century "Information
Revolution." There has never been a point in the history of the
world when so much opportunity has presented itself to mankind.
Use what you learn here wisely and your life will forever be
changed.
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The DTH
DTH stands for Direct-To-Home television. DTH is defined as the
reception of satellite programmes with a personal dish in an
individual home.
DTH does away with the need for the local cable operator and
puts the broadcaster directly in touch with the consumer. Only
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cable operators can receive satellite programmes and they then
distribute them to individual homes.
Working of DTH
A DTH network consists of a broadcasting centre, satellites,
encoders, multiplexers, modulators and DTH receivers.
A DTH service provider has to lease Ku-band transponders from
the satellite. The encoder converts the audio, video and data
signals into the digital format and the multiplexer mixes these
signals. At the user end, there will be a small dish antenna and
set-top boxes to decode and view numerous channels. On the
user's end, receiving dishes can be as small as 45 cm in
diameter.
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DTH is an encrypted transmission that travels to the consumer
directly through a satellite. DTH transmission is received
directly by the consumer at his end through the small dish
antenna. A settop box, unlike the regular cable connection,
decodes the encrypted transmission.
HOW DOES DTH REALLY DIFFER FROM CABLE TV
The way DTH reaches a consumer's home is different from the
way cable TV does. In DTH, TV channels would be transmitted
from the satellite to a small dish antenna mounted on the window
or rooftop of the subscriber's home. So the broadcaster directly
connects to the user. The middlemen like local cable operators
are not there in the picture.
DTH can also reach the remotest of areas since it does away with
the intermediate step of a cable operator and the wires (cables)
that come from the cable operator to your house. As we explained
above, in DTH signals directly come from the satellite to your DTH
dish.
Also, with DTH, a user can scan nearly 700 channels!
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Does one need to put two dish antennae and pay
double
Subscription per month if one has two TVs
For multiple connections in the same premises, one can use the
same connection. However, every television set will need to have
an individual STB.
Also, DTH is a national service and the STBs enable a viewer to
change service providers without changing the STB, even if one
moves from one city to another.
DTH RATHER THAN CABLE TV
DTH offers better quality picture than cable TV. This is because
cable TV in India is analog. Despite digital transmission and
reception, the cable transmission is still analog. DTH offers
stereophonic sound effects. It can also reach remote areas where
terrestrial transmission and cable TV have failed to penetrate.
Apart from enhanced picture quality, DTH has also allows for
interactive TV services such as movie-on-demand, Internet
access, video conferencing and e-mail. But the thing that DTH has
going for it is that the powerful broadcasting companies like Star,
Zee, etc are pushing for it.
In DTH, the payments will be made directly by the subscriber to
the satellite company offering the service.
A big problem that broadcasters face in India is the issue of
underreporting of subscribers by cable operators.
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Consider the cable operators pyramid. Right at the top is the
broadcaster. Next comes the Multi Service Cable Operator (MSOs)
like Siticable, InCable, etc. Below them are the Access Cable
Operators (ACOs) or your local cable guy who actually lays the
wires to your house.
The local cable operators or the ACOs then allegedly under-report
the number of subscribers they have bagged because they have
to pay the MSOs something like Rs 30-45 per household. Showing
a lesser number of households benefits ACOs.
With no way to actually cross check, the MSOs and the
broadcasters lose a lot. Broadcasters do not earn much in
subscription fees and are mostly dependent on advertisement
revenue to cover their costs, which is not sustainable and does
not
offer high growth in revenues for broadcasters.
The way out of this is to use a set-top box so that it will be clear
how many households are actually using cable or going for DTH
where broadcasters directly connect to consumers and can
actually grow revenues with a growth in the subscriber base.
Today, broadcasters believe that the market is ripe for DTH. The
prices of the dish and the set-top box have come down
significantly. Overall investments required in putting up a DTH
infrastructure has dropped and customers are also reaping the
benefits of more attractive tariffs.
The major thing that DTH operators are betting on is that the
service is coming at a time when the government is pushing for
CAS (conditional access system), which will make cable television
more expensive, narrowing the tariff gap between DTH and cable.
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WILL DTH BE CHEAPER THAN CABLE OR MORE
EXPENSIVE
DTH will be definitely more expensive than cable as it exists
today.
A set-top box is a must for DTH. Earlier, when CAS made set-top
box mandatory for households, the costs between DTH and cable
would not have been too wide.
But CAS on the backburner now -- which means no set-top box (a
must for DTH), the price gap between DTH and cable, will be
wide.
In Oct 2002, Siticable, which is owned by Zee, said that the cost
of the installation equipment, which includes the receiver dish
and the set-top box, would be priced at around Rs 3,900. Siticable
is looking to rope in 1 million subscribers in 15 months.
Other estimates say that digital cable set-top box may cost Rs
4,000, a DTH decoder dish is unlikely to cost less than Rs 7,000.
DTH's minimum subscription could be priced around Rs 500 per
month.
Some reports say that an entry level DTH STB will cost about Rs
7,000 (including taxes and installation cost at consumers end). A
more advanced STB with value added features like PVR (Personal
Video Recorder), PSTN connectivity, Gamming console, channel
management system, etc. may cost as much as Rs 15,000.
HISTORY OF DTH IN INDIA
DTH services were first proposed in India in 1996. But they did
not pass approval because there were concerns over national
security and a cultural invasion. In 1997, the government even
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imposed a ban when the Rupert Murdoch-owned Indian Sky
Broadcasting (ISkyB) was about to launch its DTH services in
India.
Finally in 2000, DTH was allowed. The new policy requires all
operators to set up earth stations in India within 12 months of
getting a license. DTH licenses in India will cost $2.14 million and
will be valid for 10 years. The companies offering DTH service will
have to have an Indian chief and foreign equity has been capped
at 49 per cent. There is no limit on the number of companies that
can apply for the DTH license.
MARKET COMPARISON OF D TH AND CABLE TV
The cable system is well entrenched in India and is showing quite
rapid growth. If DTH had come to India in 1996-97 (like Star had
originally attempted), then it could have made a significant
breakthrough.
Europe is an example of this. DTH developed there before cable
and now controls nearly 80 per cent of the total satellite
television subscriber base. But in US, cable rules because it came
before DTH.
DTH will definitely cut into the existing cable user base. It will
make the local cable operator less important and take business
away from him. It will give consumers greater choice.
But it is likely to be an up market premium product and most
middle class households will stick to cable.
THE BROADCAST TV PROBLEM
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Conceptually, satellite television is a lot like broadcast television.
It's a wireless system for delivering television programming
directly to a viewer's house. Both broadcast television and
satellite stations transmit programming via a radio signal.
Broadcast stations use a powerful antenna to transmit radio
waves to the surrounding area. Viewers can pick up the signal
with a much smaller antenna. The main limitation of broadcast
television is range. The radio signals used to broadcast television
shoot out from the broadcast antenna in a straight line. In order
to receive these signals, you have to be in the direct "line of
sight" of the antenna.
Small obstacles like trees or small buildings aren't a problem; but
a big obstacle, such as the Earth, will reflect these radio waves. If
the Earth were perfectly flat, you could pick up broadcast Photo
courtesy DirecTV television thousands of miles from the source.
But because the planet is curved, it eventually breaks the signal's
line of site. The other problem with broadcast television is that
the signal is often distorted even in the viewing area.
To get a perfectly clear signal like you find on cable, you have to
be pretty close to the broadcast antenna without too many
obstacles in the way.
THE SATELLITE TV SOLUTION
Satellite television solves the problems of range and distortion by
transmitting broadcast signals from satellites orbiting the Earth.
Since satellites are high in the sky, there are a lot more
customers in the line of site. Satellite television systems transmit
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and receive radio signals using specialized antennas called
satellite dishes.
Satellites are higher in the sky than TV antennas, so they
have a much larger "line of sight" range.
The television satellites are all in geosynchronous orbit,
meaning that they stay in one place in the sky relative to the
Earth. Each satellite is launched into space at about 7,000 mph
(11,000 kph), reaching approximately 22,200 miles (35,700 km)
above the Earth. At this speed and altitude, the satellite will
revolve around the planet once every 24 hours -- the same period
of time it takes the Earth to make one full rotation. In other
words, the satellite keeps pace with our moving planet exactly.
This way, you only have to direct the dish at the satellite once,
and from then on it picks up the signal without adjustment, at
least when everything works right.
At the core, this is all there is to satellite television. But as we'll
see in the next section, there are several important steps
between the original programming source and your television.
OVERVIEW OF DTH SYSTEMS
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Direct to Home are nothing but the Direct Broadcast Satellite
Television and Radio Systems. Geostationary satellites play an
important role for DTH systems. In general, DTH service is the one
in which a large number of channels are digitally compressed,
encrypted and beamed from very high power Geostationary
satellites. The programs can be directly received at homes. Also,
DTH transmission eliminates local cable operator completely,
since an individual user is directly connected to the service
providers.
An individual user has a small dish usually 45 to 60cm in diameter
and Low Noise Block Converter (LNBC) pointed towards satellite.
At home digital receiver i.e. Set top box is connected to TV which
receives digitally multiplexed channels from LNBC and gives RF
output for TV.
The satellite transmission is usually in Ku-Band. The digital
channels are first multiplexed and then QPSK modulated before
transmission. The small dish along with LNBC receives the signals
and LNBC converts these Ku band signals to Intermediate
Frequency based on the local IF which is typically 10.7GHz. Now,
the set top box receives the down-converted satellite signals and
performs the demodulation and de-multiplexing and finally D to A
conversion before making signal competent to TV.
The DTH receivers available in the Market are affordable and the
use of such systems is nowadays increasing dramatically in urban
as well as ruler areas.
DD Direct:
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DD Direct is launched by Doordarshan. It operates from NSS-6
Satellite and gives 33 free to air channels and 13 radio channels.
The transmission covers most of the India. The cost of the Dish,
LNBC and Set top box is around Rs. 2500/-. With this setup only
free to air channels are visible.
Dish TV:
Dish TV is launched by Essel Group. The Dish TV has different set
top box with Smart card facility to decode paid channels. The cost
of the unit is around Rs. 4000/-. So user can watch paid as well as
free channels and radio programs. The user has to pay monthly
rental for paid channels. The entire 'Zee Network' channels are
available on Dish TV. Dish TV Transmission is also from NSS-6
Satellite.
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NSS 6
NSS 6, Ku-band satellite with Ka band uplink capabilities, will
provide fully interactive access to high speed internet and other
multimedia communications. Additionally, it will provide direct to
home broadcasting services a well AS THE full compliment of
traditional enterprise telecommunications services across the
large coverage area stretching from the eastern Mediterranean
and southern Africa to Australia, Japan and Korea.
Access Cable Operators (ACOs) or your local cable guy who
actually lays the wires to your house. The local cable operators or
the ACOs then allegedly underreport the number of subscribers
they have bagged because they have to pay the MSOs something
like Rs 30-45 per household.
Showing a lesser number of households benefits ACOs. With no
way to actually cross check, the MSOs and the broadcasters lose
a lot. Broadcasters do not earn much in subscription fees and are
mostly dependent on advertisement revenue to cover their costs,
which is not sustainable and does not offer high growth in
revenues for broadcasters.
The way out of this is to use a set-top box so that it will be clear
how many households are actually using cable or going for DTH
where broadcasters directly connect to consumers and can
actually grow revenues with a growth in the subscriber base.
Specifications of NSS-6 Satellite:
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Location: 95 degree East
No. of Ku-Band Transponders (36MHz wide): 60
Saturated EIRP: 44-55 dBW.
Ku Band Uplink: 13.75 to 14.5 GHz
Ku Band Down links: 10.95 to 11.2 GHz
11.45 to 11.70 GHz
12.50 to 12.75 GHz
Modulation Type: QPSK
Symbol Rate: 27.5 Mb/s. Downlink for DD Direct: 12815,
12534,12898 GHz.
Look Angles for GMRT:
Azimuth= 130.51 degree from North.
Altitude= 57.26 degree
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THE CO M PONENTS
Programming sources are simply the channels that provide
programming for broadcast. The provider doesn't create original
programming itself; it pays other companies (HBO, for example,
or ESPN) for the right to broadcast their content via satellite. In
this way, the provider is kind of like a broker between you and the
actual programming sources. (Cable television companies work
on the same principle.)
The broadcast center is the central hub of the system. At the
broadcast center, the television provider receives signals from
various programming sources and beams a broadcast signal to
satellites in geostationary orbit.
The satellites receive the signals from the broadcast station and
rebroadcast them to the ground.
The viewer's dish picks up the signal from the satellite (or
multiple satellites in the same part of the sky) and passes it on to
the receiver in the viewer's house.
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The receiver processes the signal and passes it on to a standard
television.
SATELLITE
Geostationary satellites play an important role for DTH systems.
WHAT IS GEO STATIONARY SATELLITE
Geostationary satellites are positioned at an exact height
above the earth (about 36000 Km).
At this height they rotate around the earth at the same
speed as the earth
rotates around its axis, so in effect remaining stationary
above a point on
the earth (normally directly overhead the equator).
As they remain stationary they are ideal for use as
communications satellites and also for remote imaging as
they can repeatedly scan the same points on the earth
beneath them.
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Polar Orbiting satellites by comparison have a much lower
orbit, moving around the earth fairly rapidly, and scanning
different areas of the earth at relatively infrequent periods.
Motion of Geostationary Satellite around EARTH
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In above fig. , it is clear that geostationary satellite has
circular orbit.
In each orbit the time period remains same.
Orbital plane is same as equator.
Above 3 condition are necessary for a satellite to be a
geostationary
satellite.
Otherwise it will become geo synchronous satellite, which
appears oscillating to an observer on the earth at fix location
in sky.
Focusing of a particular position on earth
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Above fig. shows how geostationary satellite focuses a
part of earth.
We know that earth rotates on its axis.
Each such rotation has time period 24 hours.
Fig. explains how the same part of earth remains
always in the focus of geostationary satellite.
Geostationary satellite and earth both rotate in same
direction with same speed.
Derivation of radius of orbit
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Above is mathematical calculation of radius of
geostationary satellite.
¢ Fc- centrifugal force.
¢ Fg centripetal force.
¢ R radius of orbit.
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Receiver which point to a geostationary satellite
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In above fig. receiving antenna of DTH is shown located at
the top of a house.
Position of receivers at different location of earth
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Above fig. is there to explain that within the focus there
is a uplink station which transmits programs to satellite.
Satellite again broadcasts them to subscribers as a set
package. Basically, the providerâ„¢s goal is to bring
dozens or even hundreds of channels to the customerâ„¢s
television.
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A stationary satellite and orbit
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A LOOK OF GEOSTATIONARY SATELLITE IN ITS
ORBIT
KU BAND FREQUENCIES
Digital broadcast satellite transmits programming in the Ku
frequency range (10 GHz to 14 GHz).
Rain Fade Effects on Ku-band Transmissions
There is one major drawback to satellites down linking signals at
frequencies greater than 10 GHz: the signal wavelength is so
short that rain, snow, or even rain-filled clouds passing overhead
can reduce the intensity of the incoming signals (Figure 6-14). At
these higher frequencies, the lengths of the falling rain droplets
are close to a resonant submultiples of the signal's wavelength;
the droplets therefore are able to absorb and depolarize the
microwaves as they pass through the Earth's atmosphere.
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THE PROGRAMMING
Satellite TV providers get programming from two major sources:
national turnaround channels (such as HBO, ESPN and CNN)
and various local channels (the NBC, CBS, ABC, PBS and Fox
affiliates in a particular area). Most of the turnaround channels
also provide programming for cable television, and the local
channels typically broadcast their programming over the
airwaves.
Turnaround channels usually have a distribution center that
beams their programming to a geostationary satellite. The
broadcast center uses large satellite dishes to pick up these
analog and digital signals from several sources.
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Most local stations don't transmit their programming to satellites,
so the provider has to get it another way. If the provider includes
local programming in a particular area, it will have a small local
facility consisting of a few racks of communications equipment.
The equipment receives local signals directly from the
broadcaster through fiber-optic cable or an antenna and then
transmits them to the central broadcast center.
The broadcast center converts all of this programming into a
high-quality, uncompressed digital stream. At this point, the
stream contains a vast quantity of data -- about 270 megabits per
second (Mbps) for each channel. In order to transmit the signal
from there, the broadcast center has to compress it. Otherwise,
it would be too big for the satellite to handle. In the next section,
we'll find out how the signal is compressed.
COMPRESSION
The two major providers in the United States use the MPEG-2
compressed video format -- the same format used to store movies
on DVDs. With MPEG-2 compression, the provider can reduce the
270- Mbps stream to about 5 or 10 Mbps (depending on the type
of programming). This is the crucial step that has made DBS
service a success. With digital compression, a typical satellite can
transmit about 200 channels. Without digital compression, it can
transmit about 30 channels.
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At the broadcast center, the high-quality digital stream of video
goes through an MPEG-2 encoder, which converts the
programming to MPEG-2 video of the correct size and format for
the satellite receiver in your house.
The MPEG encoder analyzes each frame and decides how to
encode it. The encoder eliminates redundant or irrelevant data,
and extrapolates information from other frames to reduce the
overall size of the file. Each frame can be encoded in one of three
ways:
¢ As an intraframe - An intraframe contains the complete
image data for that frame. This method of encoding
provides the least compression.
¢ As a predicted frame - A predicted frame contains just
enough information to tell the satellite.
¢ Receiver how to display the frame based on the most
recently displayed intra frame or predicted frame. This
means that the frame contains only the data that relates to
how the picture has changed from the previous frame.
¢ As a bidirectional frame - To display a bidirectional frame,
the receiver must have the information from the
surrounding intraframe or predicted frames. Using data from
the closest surrounding frames, the receiver interpolates
the position and color of each pixel.
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This process occasionally produces "artifacts" -- little glitches in
the video image -- but for the most part, it creates a clear, vivid
picture.
The rate of compression depends on the nature of the
programming. If the encoder is converting a newscast, it can use
a lot more predicted frames because most of the scene stays the
same from one frame to the next. In other sorts of programming,
such as action movies and music videos, things change very
quickly from one frame to the next, so the encoder has to create
more intraframes. As a result, something like a newscast
generally compresses to a much smaller size than something like
an action movie.
ENCRYPTION AND TRANSMISSION
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After the video is compressed, the provider needs to encrypt it in
order to keep people from accessing it for free. Encryption
scrambles the digital data in such a way that it can only be
decrypted (converted back into usable data) if the receiver has
the correct decryption algorithm and security keys.
What is Encryption
Encryption is an electronic method of securing the video and
audio of any TV program so that satellite, cable, and broadcast TV
services can maintain control over the distribution of their signals.
To receive encrypted or "scrambled" TV services, cable and
SMATV system operators, hotel chains, private satellite networks,
and home dish owners must possess a compatible decoder that
can sense the presence of the encrypted TV signal and then
automatically decode the pictures and sound Premium program
services purchase the rights to movies from film production
companies with the understanding that every individual will pay
for the right to view them. Programmers also are very concerned
about hotels, bars, and other commercial establishments that
derive monetary benefit from signal piracy.
Within a particular region, program producers may license more
than one broadcast outlet for use of their programs. The program
producer may require that broadcasters encrypt their signals
whenever the broadcaster airs the producer's copyrighted
material. This strictly limits reception of the programming to the
market for which each broadcaster is licensed. In some areas of
the world, satellite broadcasters periodically must switch from a
free-to-air to an encrypted transmission mode whenever required
under their respective agreements with the program copyright
owners.
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Each IRD contains a unique numerical address number that is
installed at the factory. The satellite TV programmer's
authorization center sends a coded conditional access message
over the satellite that includes this unique address. This
authorization message can turn on an individual IRD so that it can
receive a particular service or group of services, or turn off an IRD
in the event that the subscriber fails to pay the required monthly
subscription fee.
Moreover, the authorization center can use this addressable
feature to selectively turn off and on large groups of decoders.
Group IRD control is used to selectively "black out" TV events,
such as a live championship boxing match, in certain countries for
which the programmer does not own the distribution rights.
Once the signal is compressed and encrypted, the broadcast
center beams it directly to one of its satellites. The satellite picks
up the signal with an onboard dish, amplifies the signal and uses
another dish to beam the signal back to Earth, where viewers can
pick it up.
Bit Rate
The amount of data information being transmitted in one second
of time is called the bit rate, expressed in bits per second (b/s). A
bit rate of one thousand bits per second is called a kilobit per
second (kb/s); one million bits per second a megabit per second
(Mb/s); and one billion bits per second a gigabit per second
(Gb/s).
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A bit rate of more than 200 Mb/s would be required to digitize a
broadcast quality video service without any signal impairment.
This would require the use of several satellite transponders to
relay just one uncompressed digital video signal. It therefore is
essential that some form of signal compression be used to
dramatically reduce the number of bits required for digital TV
transmissions.
THE MOVING PICTURES EXPERT GROUP
In 1988, the International Standards Organization (ISO) of the
International Telecommunication Union established the Moving
Pictures Experts Group (MPEG) to agree on an internationally
recognized standard for the compressed representation of video,
film, graphic, and text materials. The committee's goal was to
develop a relatively simple, inexpensive, and flexible standard
that put most of the complex functions at the transmitter rather
than the receiver. Representatives from more than 50
corporations and governmental organizations worldwide took part
in the MPEG committee's deliberations.
In 1991, the MPEG-1 standard was introduced to handle the
compressed digital representation of nonvideo sources of
multimedia at bit rates of 1.5 Mb/s or less. However, MPEG-1 can
be adapted for the transmission of video signals as long as the
video material is first converted from the original interlaced mode
to a progressively scanned format, which is subsequently
transmitted at half the normal field rate. MPEG-1 commonly is
encountered on IBM computers and other compatible platforms
with the ability to display files using the *.mpg extension. A few
TV programmers initially elected to use a modified form of MPEG-
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1 called MPEG-1.5 to transmit via satellite while the MPEG
committee developed a standard for source materials using
interlaced scanning. Although not an official standard, MPEG-1.5
was adopted for use for a wide variety of applications, including
the transmission of educational TV services and niche-program
channels.
The MPEG committee selected its final criteria for a new standard
in 1994 that resolves many of the problems with MPEG-1. The
MPEG-2 standard features higher resolution, scalability, and the
ability to process interlaced video source materials. MPEG-2 also
features a transport stream that allows multiple video, audio, and
data channels to be multiplexed at various bit rates into a single
unified bit stream. There are so many similarities between MPEG-
1 and MPEG-2, however, that MPEG-1 should be regarded as a
subset of the MPEG-2 specification.
MPEG-2 COMPRESSION TECHNIQUES
MPEG compression is accomplished through the use of four basic
techniques: preprocessing, temporal prediction, motion
compensation, and quantization coding. Preprocessing filters out
nonessential visual information from the video signal-information
that is difficult to encode, but not an important component of
human visual perception. Preprocessing typically uses a
combination of spatial and temporal nonlinear filtering.
Motion compensation takes advantage of the fact that video
sequences are most often highly correlated in time-each frame in
any given sequence is very similar to the preceding and following
frames. Compression focuses on coding the difference between
frames rather than the encoding of each frame in isolation.
Moreover, many of the changes that occur from frame to frame
can be approximated as translations involving small regions of
the video image. To accomplish this, an encoder scans
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subsections within each frame-called macro blocks-and identifies
which ones will not change position from one frame to the next.
THE BROADCAST CENTER
The broadcast center converts all of this programming into a
high-quality, uncompressed digital stream. At this point, the
stream contains a vast quantity of data †about 270 megabits
per second (Mbps) for each channel. In order to transmit the
signal from there, the broadcast center has to compress it.
Otherwise, it would be too big for the satellite to handle. The
providers use the MPEG-2 compressed video format †the same
format used to store movies on DVDs. With MPEG-2 compression,
the provider can reduce the 270-Mbps stream to about 3 or 10
Mbps (depending on the type of programming). This is the crucial
step that has made DTH service a success. With digital
compression, a typical satellite can transmit about 200 channels.
Without digital compression, it can transmit about 30 channels. At
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the broadcast center, the high-quality digital stream of video goes
through an MPEG-2 encoder, which converts the programming to
MPEG-2 video of the correct size and format for the satellite
receiver in your house.
COMMUNICATION CHANNEL AND BAND-WIDTH
In a communications system, the part that connects a data source
to a data sink is known as channel.
Bandwidth refers to the data transmission capacity of a
communications channel. The greater a channel's bandwidth, the
more information it can carry per unit of time.
The term technically refers to the range of frequencies that a
channel can carry. The higher the frequency, the higher the
bandwidth and thus the greater the capacity of a channel. This
capacity might more appropriately be referred to as throughput.
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For digital devices, the bandwidth is usually expressed in bits per
second (bps), kilobits per second (kbps) or megabits per second
(mbps). For analog devices, the bandwidth is expressed in cycles
per second, or Hertz (Hz).
The required bandwidth can vary greatly according to the type of
application. For example, the transmission of simple ASCII text
messages requires relatively little bandwidth, whereas the
transmission of high resolution video images requires a large
amount of bandwidth.
A major trend in networks at all levels (i.e., from LANs to the
Internet) has been increasing bandwidth. For example, the
development of optical fiber cable made possible a huge increase
in bandwidth as compared with copper wire cable, and the
bandwidth of optical fiber cable continued to increase both as a
result of improvements to the optical fiber itself and to the
transmitters and other devices used with it.
Nevertheless, bandwidth is often insufficient. This is due to such
factors as the continued increase in the numbers of users
(especially of the Internet), the growth in the demand for
applications which require more bandwidth and the high cost of
upgrading some portions of networks (particularly replacing
copper wire connections to individual homes and offices with
optical fiber). Thus, an important principle in the design of
network protocols continues to be the conservation of bandwidth.
For DTH system communication channel is air and Band-Width is :
Ku Band Uplink : 13.75 to 14.5 GHz
Ku Band Down links : 10.95 to 11.2 GHz
11.45 to 11.70 GHz
12.50 to 12.75 GHz
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RECEIVE TERMINALS
The receive terminals are basically for the reception of the signals
being beamed from the transmission station. The terminal
consists of the following:
¢ receive type solid offset antenna
¢ LNB feed system
¢ Interface cables &
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¢ Set top box
The terminal can be placed outside the window of a high rise
building, on the ground or a roof mount. The terminals can be
used as per the contents being beam by the transmission
stations.
SATELLITE RECEIVING ANTENNAS
Antenna size considerations:
The selection of the appropriate antenna size helps in keep in the
network up and healthy.
It is decided based on the following:
¢ Satellite EIRP at the particular location.
¢ Rain attenuation at the location.
¢ Adequate Eb/No for reception of excellent picture quality.
The satellite dish is a parabola of revolution, that is, a surface
having the shape of a parabola rotated about its axis of
symmetry. The resulting paraboloid shares one key property of
optical lenses: it is able to form an image of whatever object is
placed in front of it. The largest optical and radio telescopes
employ the parabolic reflector to gather and concentrate
electromagnetic radiation. Any antenna surface irregularities or
any departure from the precise parabolic shape will degrade the
image resolution. As is more often the case, however, lowresolution
performance is the result of the installer's failure to
grasp the importance of using good antenna assembly
techniques.
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The parabolic curve has the property of reflecting all incident rays
arriving along the antenna reflector's axis of symmetry to a
common focus located to the front and centre.
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The parabolic reflector receives externally generated noise along
with the desired signal. When the satellite dish tilts up towards
the "cold" sky, the antenna noise temperature is at its lowest
level. If the antenna must tilt downward to receive a low-elevation
satellite, however, the antenna's noise temperature will increase
dramatically because it is now able to intercept the "hot" noise
temperature of the Earth (Figure 5-3). The actual amount of noise
increase in this case is a function of antenna f/D ratio and
diameter. Minimum antenna elevation angles of 5 degrees, for Cband,
and 10 degrees, for Ku-band, above the site location's
horizon usually are recommended.
Antenna f/D Ratio
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The f/D ratio of the antenna is the ratio of focal length to dish
diameter, measured in the same units (Figure 5-10). A paraboloid
reflector that is 3 m in diameter and with a focal length of 1.26 m
therefore will exhibit an f/D of 0.42. The f/D ratio selected by the
antenna designer also determines the depth of the dish itself, that
is, the amount of contour or "wraparound" of the paraboloid
within its fixed diameter. A long-focus (high f/D) paraboloid
reflector will have a shallow contour, while a short-focus
paraboloid reflector resembles a deep bowl. The deepest
reflectors have a f/D ratio of 0.25. This places the focal point
directly in the plane of the antenna aperture.
An antenna design with a large value of f/D requires a feed horn
that has a narrower beam width, so that the edge illumination of
the antenna can be maintained. This typically is between 10 and
15 dB below the value produced at the center of the reflector.
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Conversely, a small value of f/D will require a feed horn with a
wider beam width.
Paraboloid antennas that are 3 m or less in diameter (at 4 GHz)
commonly use a 12-dB feed illumination taper, while larger
antennas will use a 15-dB taper. The antenna designer must
make a trade-off between antenna gain and noise temperature,
balancing the entry of random noise due to feed horn over
illumination or low antenna elevation angle with the noise
contribution of the antenna side lobes in the antenna radiation
pattern.
Although the long focal length employed by the shallow dish
design increases the illumination of the reflector surface, there
are distinct disadvantages to this design approach. Moreover,
antenna noise increases as antenna elevation increases. Shallow
dishes are more susceptible to intercepting Earth noise when
pointing at low elevation angles. Finally, the shallow dish is more
susceptible to picking up terrestrial interference from terrestrial
microwave stations.
The deep-dish design trades off gain in order to lower antenna
noise performance. The deep-dish design is an attractive
alternative for locations that potentially may experience
terrestrial interference (TI) problems or at installations that
require low antenna elevation angles. The deep-dish design
positions the feed horn relatively close to the rim of the reflector.
Therefore, the deep dish has a greater ability to shield the feed
horn from potential TI sources. However, the feed horn is so close
to the reflector that it cannot effectively illuminate the entire
surface.
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Antenna noise temperature is a function of the antenna f/D ratio and
diameter as well as the elevation angle of the dish as it points toward the
geostationary satellite's location in the sky
DISH MATERIALS AND CONSTRUCTION
The reflector's surface material must be constructed out of metal
in order to reflect the incoming microwave signals. Some antenna
reflectors appear to be manufactured out of plastic or fiberglass;
however, these dishes actually have an embedded metal mesh
material that reflects the incoming satellite signals to the front
and center of the dish.
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The solid one-piece metal antenna is most always the dish with
the best performance characteristics because there can be no
assembly errors and the reflector normally will maintain its
precise shape over the lifetime of the system. Solid petal
antennas constructed out of four or more metal panels are
generally the next best performance value, as potential assembly
errors are limited to variations along the seams between panels.
The installer can visually inspect these seams during assembly to
ensure that there are no variations in the surface curve from one
petal to the next. Installation errors almost never occur when this
type of antenna is assembled face-down on a flat, level surface.
Offset-fe e d Antennas
One oval dish design that is the antenna of choice for most digital
DTH satellite TV service providers is called the offset-fed antenna
(Figure 5-5). Here the manufacturer uses a smaller subsection of
the same paraboloid used to produce prime focus antennas (see
Figure 5-6), but with a major axis in the north/south direction, and
a smaller minor axis in the east/west direction.
The offset paraboloid eliminates aperture blockage, reduces
antenna noise temperature, and resists the accumulation of ice
and snow by placing the feed below the reflector and angling it
upwards. In this case, the reflector acts as if it were a portion of a
much larger paraboloid. But because only a portion of this
imaginary reflector exists, the feed is designed just to illuminate
that portion. The offset-fed antenna then performs just as it would
as a part of the larger dish, and directs its beam exactly the same
way.
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The offset-fed antenna design offers several distinct advantages
over its prime focus counterparts. There is no feedhorn blockage,
an important consideration when the antenna aperture is less
than one meter in diameter. Moreover, antenna designers can
reconfigure the required antenna aperture as a flatter, more
nearly vertical reflector, with the added advantage of pointing the
feed skywards, away from the hot-noise source of the Earth.
Because of these advantages, the offset-fed antenna can achieve
higher efficiency levels than prime focus antennas normally
attain, usually in the 70 percent range.
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The point is the dish's feed horn, which passes the signal onto the
receiving equipment. In an ideal setup, there aren't any major
obstacles between the satellite and the dish, so the dish receives
a clear signal.
In some systems, the dish needs to pick up signals from two or
more satellites at the same time. The satellites may be close
enough together that a regular dish with a single horn can pick up
signals from both. This compromises quality somewhat, because
the dish isn't aimed directly at one or more of the satellites. A
new dish design uses two or more horns to pick up different
satellite signals. As the beams from different satellites hit the
curved dish, they reflect at different angles so that one beam hits
one of the horns and another beam hits a different horn.
The central element in the feed horn is the low noise block
down converter, or LNB. The LNB amplifies the radio signal
bouncing off the dish and filters out the noise (radio signals not
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carrying programming). The LNB passes the amplified, filtered
signal to the satellite receiver inside the viewer's house.
Antenna Specifications:
Reflector Size
65cm
Type
Offset Feed
Frequency
10.7 to 12.75GHz
Antenna Gain
>36.75dBi @12.75GHz
3dB Beam width
<3.2 degree
10dB Beam width
<5.2 degree
Aperture efficiency
>70%
Surface Accuracy
<0.01"
Material
Steel! Aluminum
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Elevation Angel Range 15
to 50 degree
Cross Polar Discrimination on major axis.
>27dB
VSWR max
1.3
Noise Temperature <
35 Kelvin
Offset Angle
25 degree
F/D ratio
0.6
THE LOW NOISE BLOCK DOWN CONVERTER ( LNB)
The incoming satellite signal propagates down the waveguide of
the feed horn and exits into a rectangular chamber mounted at
the front of the low-noise block down converter (LNB), in which a
tiny resonant probe is located. This pickup probe, which has a
wavelength that resonates with the incoming microwave
frequencies, conducts the signal onto the first stage of electronic
amplification.
In addition to amplifying the incoming signal, the first stage of
electronic amplification also generates thermal noise internally.
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The internal noise contribution of the LNB is amplified along with
the incoming signal and passed on to succeeding amplifier
stages.
The LNB sets the noise floor for the satellite receiving system.
Today's high-performance LNB uses gallium arsenide
semiconductor and high electron mobility transistor (HEMT)
technologies to minimize the internal noise contribution of the
LNB.
The noise performance of any C-band LNB is quantified as a noise
temperature measured in Kelvinâ„¢s (K), while Ku-band LNB noise
performance is expressed as a noise figure measured in decibels
(Figure 4-10). Today's C-band LNB commonly achieves a noise
temperature of 40 K or less, while Ku-band noise figures of less
than 1 dB/K are commonly available. In either case, the lower the
noise performance rating of the LNB, the less noise introduced
into the LNB by its own circuitry.
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Figure ---- Noise temperature to noise figure conversion chart.
Universal LNB
A wide band product called a "universal" Ku-band LNB is available that can
switch electronically between the 10.7-11.7 and 11.7-12.75 GHz frequency spectra
to provide complete coverage of the entire Ku-band frequency range (Figure 4-15).
The receiver or IRD sends a switching voltage (13 or 17 volts DC) to the LNB that
automatically changes the LNB input frequency range to the desired frequency
spectrum (10.70-11.75 GHz or 11.7-12.75 GHz). Keep in mind, however, that any
universal LNB with an IF output frequency range of 950-2,050 MHz can only be
used effectively with a receiver or IRD that also has a comparable IF input
frequency range.
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Block diagram of a Ku-band Universal LNB.
LNB Gain
Another important LNB specification is the amount of
amplification or gain that each unit provides. This also is
measured in decibels. The consumer-grade LNB commonly
produces 50-65 dB of gain. The gain specification for any LNB is
important for ensuring that the signal arriving at the IF input of
the IRD is within a range of values in dBmV recommended by the
IRD manufacturer. For example, an LNB with a gain of 65 dB is
the logical choice for an installation where there is a long cable
run between the outdoor and indoor units. The installer uses an
LNB with 65 dB of gain to overcome signal attenuation through
the cable, while an LNB with 50 dB of gain is preferred for
installations with a short cable run between the outdoor and
indoor units.
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LNB Specifications:
I/P Range 1O.7GHz to 12.75GHz
Local Oscillator 9.75GHz or 10.6GHz
Noise Figure 0.5dB
Noise Temperature 35 Kelvin @ 290 degree
Gain 55dB 4
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SET TOP BOX
Simplified Block Diagram
The typical block diagram of the Set top box is as shown in
following figure,
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CONVENTI
ONAL
MIX. &
LOCAL
ADC QPSK
DEMO
D.
SYNCHRO
NIZER
VITERBI
DECODER
MPEG
TRANSPO
RT
DEMULTIP
LEXING
MPEG
DECOD
ER
DAC
VIDEO
CONTROL ENCODER
E
PROCESS
OR
CODITION
AL
ACCESS
SYSTEM
SMART
CARD
LNB
As per the diagram, the set top box accepts the entire down
converted band and separates out the individual transponder
frequency. Then signals are first converted to fixed IF and then
QPSK demodulated. The bandwidth of QPSK signals is 27.5 MHz as
the bit rate is 27.5 Mb/s. It is observed that 11 digital channels
are multiplexed in 27.5 MHz bandwidth. The power supply for
LNB, polarization selection signals as well as LO setting signals
are send by the set top box itself by using the same cable
between the LNB and set top box.
After the QPSK demodulation, the digital bit stream obtained
contains several multiplexed channels as well as error control
bits. The bit stream is processed to correct and detect errors, deinterleaved,
and decrypted. A digital demultiplexer then extracts
the bits for wanted channel, and sends them to MPEG decoder,
and finally generates analog Audio and Video signals with DIA
converters to drive TV set.
The paid channels are encrypted, and a smart card having the
correct key for decryption is required to view the paid channels.
The key is provided by the paying monthly rent by the user.
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DTH APPLICATIONS
a)To view pay & free-to-air TV channels of various DTH platform
on your home TV.
b) Doordarshan free-to-air services providing 40 TV channels with
no subscription fees is an attractive preposition to people in
urban and rural areas. These
channels comprises of DD channels and popular channels of news
, sports , information , entertainment etc.
c) One can scan the entire globe with a motorized dish using a CI
set top box with CAM modules and watch TV channels of several
DTH platforms visible to the
dish terminals.
d) A number has started IP broadcast with return channel on PSTN
line and this would be for education and other application.
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DTH-BENEFITS
Benefits of DTH extends to all sections of the society since DTH
has a reach in all areas whether it is remote or urban , it provides
equal benefits to everyone. Benefits of DTH are listed below:
¢ Cost effective communication, information and entertainment to
all .
¢ Small size terminals can provide up to 4000 TV channels and
2000 radio channels through a click of a button and thus brings
worldâ„¢s at least information, news, entertainment to your home .
¢ DTH services bypasses mediators and thus content provider
comes with customer directly.
¢ DTH services are transparent providing digital quality video,
audio, radio, and IP to all at equal prices and other benefits with
reliability.
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CONCLUSION
DTH projects in India are just a beginning and we are taking the
advantage of DTH revolution. Direct to home connects urban,
rural and remote areas of the country and provides desire
information communication, education and entertainment at the
click of a button.
1. Broadband noise will have negligible effect on GMRT
Observations, as the minimum separation distance is 90
meters with the assumption that there is no DTH system in 100
meter circle from any of the GMRT antennas. Care must be
taken for arm antennas.
2. Narrow band noise can cause RFI, in spectral line observations
below 400MHz, if located at about 2 km from a GMRT antenna.
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Further Work
1. It is useful to be able to control LNB without set top box so as
to understand the exact spectrum at LNB o/p. Effort is to be put
to make the circuit on page 18 (or some other approach) work.
2. Effect of Narrow band noise on GMRT must be studied in detail.
Towards this, a DTH Receiver needs to be installed on an
evaluation basis at the GMRT Guest Housel Recreation Room
and test observations in spectral line mode perfonned with
different "poorly made" coaxial cables to page link the LNB and STB.
Careful check for lines seen in nearby antennas like C3, C4,
and C9 etc in 235 and 325 MHz bands would help in getting a
clearer picture regarding the severity of the problem/s in a
controlled manner.
3. Finally, to restrict possible RFI, one can design a Hair Pin Filter
with provision of passing DC and 22 KHz tone which can be
added between the LNB and set top box. This will only allow
the required satellite signals and attenuate noise in the GMRT
band. Depending on the result of (2) above, we may have to
plan a strategy of adding such units BEFORE THE STB at
installations in nearby villages.
4. We may find that a simpler solution might be to buy a good
quality double shielded cable, assemble connectors
professionally and supply to the nearby villages, if the tests as
in (2) above does show interference to operation of GMRT.
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REFERENCES
1. EN 50221: Common Interface Specifications for conditional
access and other Digital video broadcasting decoder
applications.
2. IS 15377-2003: Digital Set Top Box for Direct to Home services
3. Effect of corDECT systems on GMRT (Internal Technical Report)
4. THE DIGITAL SATELLITE TV HANDBOOK
MARK E. LONG
5. mindstien.net
6. scribd.com
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7. googleearth.com
8 . howstuffwork.com
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