Video Engineering-DirectBroadcast Satellites / DTH

7/31/2019 Video Engineering-DirectBroadcast Satellites / DTH

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DTH/DBS

By vikas kaduskar

Date: 29th sep 2010


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DIRECT-TO-HOME (DTH) DIRECT BROADCAST

SATELLITE (DBS)

(Direct Broadcast Satellite) A one-way TV

broadcast service from a communications

satellite to a small round or oval dish antenna

no larger than 20" in diameter. Using a highly

compressed digital signal in the 11-15GHz Ku-

band, DBS offers every household in the country

a service similar to cable TV. Prior to DBS, costlyequipment and very large dishes were required,

and tuning stations was complicated because

content was available on multiple satellites.


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Direct broadcast satellite (DBS) is a term used

to refer to satellite television broadcasts

intended for home reception, also referred toas direct-to-home signals. It covers both

analog and digital television and radio

reception, and is often extended to otherservices provided by modern digital television

systems, including video-on-demand and

interactive features. A "DBS service" usuallyrefers to either a commercial service, or a

group of free channels available from one

orbital position targeting one country.


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Terminology confusion In certain regions of the world, especially in North

America, DBS is used to refer to providers of subscription

satellite packages, and has become applied to the entireequipment chain involved. With modern satellite providersin the United States using high power Ku-bandtransmissions using circular polarization, which result insmall dishes, and digital compression (hence bringing in an

alternative term, Digital Satellite System, itself likelyconnected to the proprietary encoding system used byDirecTV, Digital Satellite Service), DBS is often misused torefer to these. DBS systems are often driven by paytelevision providers, which drives further confusion.

Additionally, in some areas it is used to refer to specificsegments of the Ku-band, normally 12.2 to 12.7 GHz, asthis bandwidth is often referred to as DBS or one of itssynonyms. In comparison, European "Ku band" DBSsystems can drop as low as 10.7 GHz, which is in fact in the

X band.


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ree serv ces Germany is likely the leader in free-to-air DBS, with

approximately 40 analogue and 100 digital channels

broadcast from the SES Astra 1 position at 19.2E. These arenot marketed as a DBS service, but are received inapproximately 12 million homes, as well as in any homeusing the German commercial DBS system, Premiere.

The United Kingdom has approximately 90 free-to-airdigital channels, for which a promotional and marketingplan is being devised by the BBC and ITV, to be sold as"Freesat". It is intended to provide a multi-channel servicefor areas which cannot receive Freeview, and eventuallyreplace their network of UHF repeaters in these areas

India's national broadcaster, Doordarshan, promotes afree-to-air DBS package as "DD Direct Plus", which isprovided as in-fill for the country's terrestrial transmission

network


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The Broadcast TV Problem 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 broadcasttelevision and satellite stations transmit programming via aradio signal (see How Radio Works for information aboutradio broadcasting).

Broadcast stations use a powerful antenna to transmit radiowaves to the surrounding area. Viewers can pick up the signalwith a much smaller antenna. The main limitation ofbroadcast television is range. The radio signals used tobroadcast television shoot out from the broadcast antenna ina straight line. In order to receive these signals, you have tobe in the direct "line of sight" of the antenna. Small obstacleslike trees or small buildings aren't a problem; but a bigobstacle, such as the Earth, will reflect these radio waves.


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If the Earth were perfectly flat, you could pick up

broadcast 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, youhave to be pretty close to the broadcast antenna

without too many obstacles in the way.


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Photo courtesy DirecTV


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Satellites are higher in the sky than

TV antennas, so they have a much

larger "line of sight" range.


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The television satellites are all in geosynchronousorbit, meaning that they stay in one place in the skyrelative to the Earth. Each satellite is launched into

space at about 7,000 mph (11,000 kph), reachingapproximately 22,200 miles (35,700 km) above theEarth. At this speed and altitude, the satellite willrevolve around the planet once every 24 hours --

the same period of time it takes the Earth to makeone full rotation. In other words, the satellite keepspace 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 withoutadjustment, at least when everything works right.(See How Satellites Work for more information on

satellite orbits.)


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Unlike earlier programming, the provider's

broadcast is completely digital, which means it

has much better picture and sound quality (seeHow Digital Television Works for details). Early

satellite television was broadcast in C-band

radio -- radio in the 3.4-gigahertz (GHz) to 7-GHz frequency range. Digital broadcast satellite

transmits programming in the Ku frequency

range (12 GHz to 14 GHz ).


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e omponents There are five major components involved in a direct to home (DTH)

satellite system: the programming source, the broadcast center, thesatellite, the satellite dish and the receiver.

Programming sources are simply the channels that provide programmingfor broadcast. The provider doesn't create original programming itself; itpays other companies (HBO, for example, or ESPN) for the right tobroadcast their content via satellite. In this way, the provider is kind of likea 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 programmingsources and beams a broadcast signal to satellites in geostationary orbit.

The satellites receive the signals from the broadcast station andrebroadcast them to the ground.

The viewer's dish picks up the signal from the satellite (or multiplesatellites in the same part of the sky) and passes it on to the receiver inthe viewer's house.

The receiver processes the signal and passes it on to a standard television.


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A satellite dish is just a special kind of antennadesigned to focus on a specific broadcast source.The standard dish consists of a parabolic (bowl-

shaped) surface and a central feed horn. To transmita signal, a controller sends it through the horn, andthe dish focuses the signal into a relatively narrowbeam.

The dish on the receiving end can't transmitinformation; it can only receive it. The receiving dishworks in the exact opposite way of the transmitter.When a beam hits the curved dish, the parabolashape reflects the radio signal inward onto aparticular point, just like a concave mirror focuseslight onto a particular point.


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The central element in the feed horn is the low

noise blockdown converter, or LNB. The LNB

amplifies the radio signal bouncing off the dishand filters out the noise (radio signals not

carrying programming). The LNB passes the

amplified, filtered signal to the satellite receiver

inside the viewer's house.


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CONDITIONAL ACCESS SYSTEM

A conditional access system (CAS) is a system bywhich electronic transmission of digital media,especially satellite television signals through cable,is limited to subscribed clients. This is called

conditional access. The signal is encrypted and isunavailable for unauthorised reception. A set-topbox containing a conditional access module isrequired in the customer premises to receive and

decrypt the signal. CAS is now becoming a significant concern for

major companies in the DVB-H market becausebroadcasters are looking to control the digital

signals that they will be broadcasting.

C diti l ("CA") i h i f


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Conditional access ("CA") is the protection ofcontent by requiring certain criteria to be metbefore granting access to this content. The term

is commonly used in relation to digital televisionsystems, most notably satellite television.

Under the DVB, conditional access systemstandards are defined in the specificationdocuments for DVB-CA (Conditional Access),DVB-CSA (the Common Scrambling Algorithm)and DVB-CI (the Common Interface). These

standards define a method by which a digitaltelevision stream can be obfuscated, with accessprovided only to those with valid decryptionsmart cards.

This is achieved by a combination of scrambling and


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This is achieved by a combination of scrambling andencryption. Encryption is the process of protecting thesecret keys that are transmitted with a scrambledstream inorder to enable the descrambler to work at the receiving

end. The scrambler key, called thecontrol word

, must, ofcourse, be sent to the receiver, and is encrypted as anentitlement control message (ECM). The CA subsystem inthe receiver will decrypt the control word only whenauthorised to do so; that authority is sent to the receiver in

the form of an entitlement management message (EMM).The control word is typically changed at intervals of 10seconds. The ECM is changed at perhaps monthly intervalsto discourage unauthorised viewing; this being apparentlynot sufficient, TPS has lowered this interval down to about

12 minutes. The decryption cards are read, and sometimes updated with

specific access rights, through a Conditional Access Module(CAM), a PCMCIA format card reader, or a built-in cardreader meeting DVB-CI standards, such as that in the Sky

Digibox.


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WHAT IS CAS?

CAS stands for conditional access system,

which is a digital mode of transmitting TV

channels trough a set-top box (STB). The

transmission signals are encrypted andviewers need to buy a set-top box to receive

and decrypt the signal. The STB is required to

watch only pay channels, not free-to-airchannels, like Doordarshan.

What is a set top bo ? Ho m ch


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What is a set-top box? How much

does it cost? The set-top box is the device that enables a subscriber view pay

channels. This instrument decodes signals from the cable operator forviewing a pay channel. It can also monitor the number and duration ofchannels viewed by the subscriber.

Analog STBs cost between Rs 3,000-3,500, while digital STBs cost 5,500to 7,000.

The Telecom Regulatory Authority of India (which also regulates India's

cable television regime) has asked cable TV service providers in CASareas to offer digital set-top boxes on a monthly rental scheme of Rs 30and a refundable security deposit of Rs 999.

Subscribers will also have another option of not paying any securitydeposit but the monthly rental will be higher at Rs 45 per STB.

For analogue boxes, the rent will be Rs 23 per month per STB. Multi-system operators, like Hathway, now plan to give out STBs to theirsubscribers at a nominal rental of Rs 1 per day. If you change youraddress, you may have to go in for a new MSO who covers your newlocation.


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Advantages of CAS For viewers: Under, CAS viewers can watch only what they would like

to watch, than what the cable operator has on offer. Subscribers save

money they now spend on unnecessary channels. They will get bettertransmission (because of the use of optic fibre instead of metal cables).The cable operators will no longer have any control over the pricing ofchannels.

For broadcasters: It benefits broadcasters as they always had tograpple with the issue of cable operators not declaring the actual

number of subscribers, and hence suffering losses. With CAS, they canfind out the exact number of subscribers with a cable operator.

For cable TV operators: They need to pay a part of the subscriptionfees to the broadcasters only for the actual number of end users whoopt for the channel. This allows operators to price their channelsaccording to their popularity.

For advertisers: It gives a far more accurate indicator of programmepopularity with only the actual subscribers of each channel beingaccounted for.

Documents

Video Engineering-DirectBroadcast Satellites / DTH