direct to home tvs

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CHAPTER-1

INTRODUCTION

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 cable operators can receive satellite programs and then they distribute

them to individual homes.

Figure (i) DTH satellite TV

When satellite television first hit the market in the early 1990s, home dishes were

expensive metal units that took up a huge chunk of yard space. In these early years,

only the most die-hard TV fans would go through all the hassle and expense of putting

in their own dish. Satellite TV was a lot harder to get than broadcast and cable TV.

Today, we see compact satellite dishes perched on rooftops all over the United States.

Drive through rural areas beyond the reach of the cable companies, and we'll find

dishes on just about every house. The major satellite TV companies are luring in more

consumers every day with movies, sporting events and news from around the world

and the promise of movie-quality picture and sound.

Satellite TV offers many solutions to broadcast and cable TV problems. Though

satellite TV technology is still evolving, it has already become a popular choice for

many TV viewers.
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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

multiplexers 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 end, receiving

dishes can be as small as 45 cm in diameter.

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 set-top box, unlike the regular cable connection, decodes the

encrypted transmission.

1.1 DTH OVER CABLE TV

1. Cable Tv is analog and DTH employs digital transmission. Therefore, DTH offersbetter quality picture than cable Tv. It is less susceptible to transmission noise . DTH

offers all the advantages of digital over analog signal.


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2. DTH can reach remote areas where terrestrial transmission and cable TV havefailed to penetrate. It does away with the intermediate step of a cable operator and

wires that come from the cable operator that come to our house.

3. A big problem that broadcasters face in India is issue of underreporting ofsubscribers by cable operators.

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.

1.2 DTH OVER BROADCAST TV

Conceptually, satellite TV is a lot like broadcast TV. 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
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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.

PROBLEMS WITH BROADCAST TV

1. Range: The main limitation of broadcast TV is range. The radio signals used tobroadcast 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 TV thousands of miles from the source. But because the planet is

curved, it eventually breaks the signal's line of sight.

2. Distorted signal: The other problem with broadcast TV is that the signal isoften 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.

DTH SOLUTION

1. Satellite TV solves the problems of range and distortion by transmitting broadcastsignals from satellites orbiting the Earth. Since satellites are high in the sky, there are a

lot more customers in the line of sight. Satellite TV systems transmit and receive radio

signals using specialized antennas called satellite dishes.

2. The TV satellites are all in geosynchronous orbit, meaning that they stay in oneplace 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. This

way, we 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.

CHAPTER-2

DTH SATELLITE SYSTEM

2.1 COMPONENTS OF DTH SATELLITE
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Early satellite TV viewers were explorers of sorts. They used their expensive dishes to

discover unique programming that wasn't necessarily intended for mass audiences.

The dish and receiving equipment gave viewers the tools to pick up foreign stations,

live feeds between different broadcast stations, NASA activities and a lot of other stuff

transmitted using satellites.

Early satellite TV viewers who used C-band radio for their broadcasts were able to

catch wild feeds of syndicated programs, sporting events and news. These broadcasts

were free, but viewers had to hunt them down -- they didn't get previewed or listed

like regular broadcast programming. These signals still exist, and Satellite Orbit

magazine publishes a list of today's wild feeds

Some satellite owners still seek out this sort of programming on their own, but today,

most satellite TV customers get their programming through a direct broadcast

satellite (DBS) provider, such as DirecTV or DISH Network. The provider selects

programs and broadcasts them to subscribers as a set package. Basically, the provider's

goal is to bring dozens or even hundreds of channels to your TV in a form that

approximates the competition, cable TV.

Unlike earlier programming, the provider's broadcast is completely digital, which

means it has much better picture and sound quality .Early satellite television was

broadcast in C-band radio -- radio in the 3.7-gigahertz GHz to 6.4-GHz frequency

range. Digital broadcast satellite transmits programming in the Ku frequency

range (11.7 GHz to 14.5 GHz ).

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

broadcasting (DBS) satellite system: the programming source, the broadcast centre,

the satellite, the satellite dish and the receiver.
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Figure (ii) DTH components

Programming sources are simply the channels that provide programming forbroadcast. 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 TV companies work on the same principle.)

The broadcast centre is the central hub of the system. At the broadcast centre, theTV provider receives signals from various programming sources and beams a

broadcast signal to satellites in geosynchronous orbit.

The satellites receive the signals from the broadcast station and rebroadcast them toEarth.

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

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

2.2SATELLITE TV PROGRAMMING

Satellite TV providers get programming from two major sources:

1.National turnaround channels (such as HBO, ESPN and CNN) and2. Local channels (the ABC, CBS, Fox, NBC and PBS affiliates in a particular area).


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Most of the turnaround channels also provide programming for cable TV, and the local

channels typically broadcast their programming over the airwaves.

Turnaround channels usually have a distribution center that beams their programming

to a geosynchronous satellite. The broadcast center uses large satellite dishes to pick

up these analog and digital signals from several sources.

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.

2.3COMPRESSION TECHNIQUES

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
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success. With digital compression, a typical satellite can transmit about 200 channels.

Without digital compression, it can transmit about 30 channels.

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 two ways:

2.3.1 INTRAFRAME COMPRESSION TECHNIQUES

Intraframe compression is compression applied to still images, such as photographs

and diagrams, and exploits the redundancy within the image, known as spatial

redundancy. Intraframe compression techniques can be applied to individual frames

of a video sequence. Various intraframe techniques are given below

1. Sub-samplingSub-sampling is the most basic of all image compression techniques and it reduces the

amount of data by throwing some of it away. Sub-sampling reduces the number of bits

required to describe an image, but the quality of the sub-sampled image is lower than

the quality of the original. Sub-sampling of images usually takes place in one of two

ways.

In the first, the original image is copied but only a fraction of the pixels from the

original are used, as illustrated below.

Alternatively, sub-sampling can be implemented by calculating the average pixel

value for each group of several pixels, and then substituting this average in the

appropriate place in the approximated image. The latter technique is more complex,

but generally produces better quality images.


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In this example the pixels in every second row and every second column are ignored.

To compensate for this, the size of the remaining pixels is doubled. The same

procedure is illustrated below.

For example, an image might be sub-sampled by 2 in both the x and y directions, thus

every second line and every second column of the image is completely ignored. When

the sub-sampled image is displayed at the same size as the original image the size of

the pixels is doubled. This is known aspixel doubling.

When coding colour images, it is common to sub-sample the colour component of the

image by 2 in both directions, while leaving the luminance component intact. This is

useful because human vision is much less sensitive to chrominance than it is to

luminance, and sub-sampling in this way reduces the number of bits required to

specify the chrominance component by three quarters.

Sub-sampling is necessarily lossy, but relies on the ability of human perception to fill

in the gaps. The receiver itself can also attempt to fill in the gaps and try to restore the

pixels that have been removed during sub-sampling. By comparing adjacent pixels of

the sub-sampled image, the value of the missing in-between pixels can beapproximated. This process is known as interpolation. Interpolation can be used to


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make a sub-sampled image appear to have higher resolution than it actually has and is

usually more successful than pixel doubling. It can, however, result in edges becoming

blurred.

2. Coarse Quantization

Coarse quantization is similar to sub-sampling in that information is discarded, but the

compression is accomplished by reducing the numbers of bits used to describe each

pixel, rather than reducing the number of pixels. Each pixel is reassigned an

alternative value and the number of alternate values is less than that in the original

image. In a monochrome image, for example, the number of shades of grey that pixels

can have is reduced. Quantization where the number of ranges is small is known

as coarse quantization.

These three images have different numbers of colors. The first has thousands of

different colored pixels. The middle has 64 different colors and the rightmost uses

only 8 different colors.

Photographic quality images typically require pixels of 24 bits, but can be reduced to

16 bits with acceptable loss. Images with 8 bits, however, are noticeably inferior to

those with 16 bits per pixel. Coarse quantization of images, often called bit depth

reduction, is a very common way of reducing the storage requirements of images.

3. Vector quantization


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Vector quantization is a more complex form of quantization that first divides the input

data stream into blocks. A pre-defined table contains a set of patterns for blocks and

each block is coded using the pattern from the table that is most similar. If the number

of quantization levels (i.e. blocks in the table) is very small, as in Figure 2.5, then the

compression will be lossy. Because images often contain many repeated sections

vector quantization can be quite successful for image compression.

In this example of vector quantization a sequence of symbols is divided into blocks of

four symbols and the blocks are compared to those in a table. Each block is assigned

the symbol in the table entry it most resembles. These symbols form the compressed

form of the sequence. On decompression an approximation of the original sequence is

generated.

4. Transform Coding

Transform coding is an image conversion process that transforms an image from the

spatial domain to the frequency domain. The most popular transform used in image

coding is the Discrete Cosine Transform (DCT). Because transformation of large

images can be prohibitively complex it is usual to decompose a large image into

smaller square blocks and code each block separately.


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Instead of representing the data as an array of 64 values arranged in an 8x8 grid, the

DCT represents it as a varying signal that can be approximated by a collection of 64

cosine functions with appropriate amplitudes. The DCT represents a block as a matrix

of coefficients. Although this process does not in itself result in compression, the

coefficients, when read in an appropriate order, tend to be good candidates for

compression using run length encoding or predictive coding.

The most useful property of DCT coded blocks is that the coefficients can be coarsely

quantized without seriously affecting the quality of the image that results from an

inverse DCT of the quantized coefficients. It is in this manner that the DCT is most

frequently used as an image compression technique.

2.3.2 INTERFRAME COMPRESSION TECHNIQUES

Interframe compression is compression applied to a sequence of video frames, rather

than a single image. In general, relatively little changes from one video frame to the

next. Interframe compression exploits the similarities between successive frames,

known as temporal redundancy, to reduce the volume of data required to describe the

sequence.

There are several interframe compression techniques, of various degrees of

complexity, most of which attempt to more efficiently describe the sequence by

reusing parts of frames the receiver already has, in order to construct new frames.

1. Sub-samplingSub-sampling can also be applied to video as an interframe compression technique, by

transmitting only some of the frames. Sub-sampled digital video might, for example,

contain only every second frame. Either the viewers brain or the decoder would be

required to interpolate the missing frames at the receiving end.


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2. Difference CodingDifference coding, or conditional replenishment, is a very simple interframe

compression process during which each frame of a sequence is compared with its

predecessor and only pixels that have changed are updated. In this way only a fraction

of the number of pixel values is transmitted. The images below are successive frames

from the table tennis sequence and illustrates how, frequently, very little changes from

one frame to the next.

Figure (iii) Consecutive frames from the Table Tennis sequence.

If the coding is required to be lossless then every changed pixel must be updated.

There is an overhead associated with indicating which pixels are to be updated, and if

the number of pixels to be updated is large, then this overhead can adversely affect

compression.Two modifications can alleviate this problem somewhat, but at the cost of introducing

some loss. Firstly, the intensity of many pixels will change only slightly and when

coding is allowed to be lossy, only pixels that change significantly need be updated.

Thus, not every changed pixel will be updated. Secondly, difference coding need not

operate only at the pixel level, but at the block level.

3. Block Based Difference Coding

If the frames are divided into non-overlapping blocks and each block is compared

with its counterpart in the previous frame, then only blocks that change significantly

need be updated. If, for example, only those blocks of the Table Tennis frame that

contain the ball, lower arm and bat were updated, the resulting image might be an

acceptable substitute for the original.

Updating whole blocks of pixels at once reduces the overhead required to specify

where updates take place. The 160x120 pixels in the Table Tennis frame can be split


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into 300 8x8 pixel blocks. Significantly fewer bits are required to address one of 300

blocks than one of 19200 individual pixels. If pixels are updated in blocks, however,

some pixels will be updated unnecessarily, especially if large blocks are used. Also, in

parts of the image where updated blocks border parts of the image that have not been

updated, discontinuities might be visible and this problem is worse when larger blocks

are used. Clearly the choice of block size must be an informed one so as to achieve the

best balance between image quality and compression.

Block Based Difference Coding can be further improved upon by compensating for

the motion between frames. Difference Coding, no matter how sophisticated, is almost

useless where there is a lot of motion. Only objects that remain stationary within the

image can be effectively coded. If there is a lot of motion or indeed if the camera itself

is moving, then very few pixels will remain unchanged. Even a very slow pan of a still

scene will have too many changes to allow difference coding to be effective, even

though much of the image content remains from frame to frame. To solve this problem

it is necessary to compensate in some way for object motion.

4. Based Motion Compensation

Block based motion compensation, like other interframe compression techniques,

produces an approximation of a frame by reusing data contained in the frame's

predecessor. This is completed in three stages.


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Figure(iv) Past Frame - Current frame to be coded

First, the frame to be approximated, the current frame, is divided into uniform non-

overlapping blocks, as illustrated below (left).

Then each block in the current frame is compared to areas of similar size from the

preceding orpastframe in order to find an area that is similar. A block from the

current frame for which a similar area is sought is known as a target block.

The location of the similar ormatching blockin the past frame might be different from

the location of the target block in the current frame. The relative difference in

locations is known as the motion vector. If the target block and matching block are

found at the same location in their respective frames then the motion vector that

describes their difference is known as azero vector. The illustration below shows the

motion vectors that describe where blocks in the current frame (below left) can be

found in past frame (above left).

Current frame to be coded divided into blocks. Motion vectors indicating where

changed blocks in the current frame have come from. Unchanged blocks are marked

by dots. .

Finally, when coding each block of the predicted frame, the motion vector detailing

the position (in the past frame) of the target blocks match is encoded in place of the

target block itself. Because fewer bits are required to code a motion vector than to

code actual blocks, compression is achieved.

During decompression, the decoder uses the motion vectors to find the matching

blocks in the past frame (which it has already received) and copies the matching


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blocks from the past frame into the appropriate positions in the approximation of the

current frame, thus reconstructing the image. In the example used above, a perfect

replica of the coded image can be reconstructed after decompression. In general this is

not possible with block based motion compensation and thus the technique is lossy.

The effectiveness of compression techniques that use block based motion

compensation depends on the extent to which the following assumptions hold.

Objects move in a plane that is parallel to the camera plane. Thus the effects ofzoom and object rotation are not considered, although tracking in the plane parallel

to object motion is.

Illumination is spatially and temporally uniform. That is, the level of lighting isconstant throughout the image and does not change over time.

Occlusion of one object by another, and uncovered background are not considered.Bidirectional motion compensation uses matching blocks from both a past frame and

a future frame to code the current frame. A future frame is a frame that is displayed

after the current frame. Considering the chess board example, suppose that a player is

fortunate enough to have a once lost queen replace a pawn on the board. If the queen

does not appear on the board before the current move then no block containing the

queen can be copied from the previous state of play and used to describe the current

state. After the next move, however, the queen might be on the board. If in addition to

the state of play immediately before the current move, the state of play immediately

following is also available to the receiver, then the current image of the chess board

can be reproduced by taking blocks from both the past and future frames.

Bidirectional compression is much more successful than compression that uses only a

single past frame, because information that is not to be found in the past frame might

be found in the future frame. This allows more blocks to be replaced by motion

vectors. Bi-directional motion compensation, however, requires that frames be

encoded and transmitted in a different order from which they will be displayed.

Motion Compensated Video Compression Overview


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Figure(v) Flow of information through the motion compensation process.

As described in the section on Interframe Compression techniques, block based

motion compensation uses blocks from a past frame to construct a replica of the

current frame. The past frame is a frame that has already been transmitted to the

receiver. For each block in the current frame a matching block is found in the past

frame and if suitable, its motion vector is substituted for the block during transmission.

Depending on the search threshold some blocks will be transmitted in their entirety

rather than substituted by motion vectors. The problem of finding the most suitable

block in the past frame is known as the block matching problem.

Block based motion compensated video compression takes place in a number of

distinct stages. The flow chart above illustrates how the output from the earlier

processes form the input to later processes. Consequently choices made at early stages

can have an impact of the effectiveness of later stages. To fully understand the issues

involved with this type of video compression it is necessary to examine each of the

stages in detail. These stages can be described as:

Frame Segmentation Search Threshold Block Matching Motion Vector Correction
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Vector Coding Prediction Error Coding

1. Frame SegmentationThe current frame of video to be compressed is divided into equal sized non-

overlapping rectangular blocks. Ideally the frame dimensions are multiples of the

block size and square blocks are most common. Chan et al. used rectangular blocks of

16 x 8 pixels, claiming that blocks of this shape exploit the fact that motion within

image sequences is more often in the horizontal direction than the vertical.

Block size affects the performance of compression techniques. The larger the block

size, the fewer the number of blocks, and hence fewer motion vectors need to be

transmitted.

However, borders of moving objects do not normally coincide with the borders of

blocks and so larger blocks require more correction data to be transmitted .Small

blocks result in a greater number of motion vectors, but each matching block is more

likely to closely match its target and so less correction data is required. Lallaret et al.

found that if the block size is too small then the compression system will be very

sensitive to noise. Thus block size represents a trade off between minimising the

number of motion vectors and maximising the quality of the matching blocks. The

relationship between block size, image quality, and compression ratio has been the

subject of much research and is well understood.

For architectural reasons block sizes of integer powers of 2 are preferred and so block

sizes of 8 and 16 pixels predominate. Both the MPEG and H.261 video compression

standards use blocks of 16x16 pixels.

2. Search ThresholdIf the difference between the target block and the candidate block at the same position

in the past frame is below some threshold then it is assumed that no motion has taken

place and a zero vector is returned. Thus the expense of a search is avoided. Most

video codecs employ a threshold in order to determine if the computational effort of a

search is warranted.


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3. Block MatchingBlock matching is the most time consuming part of the encoding process. During

block matching each target block of the current frame is compared with a past frame in

order to find a matching block. When the current frame is reconstructed by the

receiver this matching block is used as a substitute for the block from the current

frame.

Block matching takes place only on the luminance component of frames. The colour

components of the blocks are included when coding the frame but they are not usually

used when evaluating the appropriateness of potential substitutes orcandidate blocks.

Figure (vi) Corresponding blocks from a current and past frame, and the search

area in the past frame.

The search can be carried out on all of the past frame, but is usually restricted to a

smallersearch area centred around the position of the target block in the current frame

(see above figure). This practice places an upper limit, known as the maximum

displacement, on how far objects can move between frames, if they are to be coded

effectively. The maximum displacement is specified as the maximum number of pixels

in the horizontal and vertical directions that a candidate block can be from the position

of the target block in the original frame.

The quality of the match can often be improved by interpolating pixels in the search

area, effectively increasing the resolution within the search area by allowing

hypothetical candidate blocks with fractional displacements.


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The search area need not be square. Because motion is more likely in the horizontal

direction than vertical rectangular search areas are popular. The CLM460x MPEG

video encoder for example, uses displacements of -106 to +99.5 pixels in the

horizontal direction, and -58 to +51.5 pixels in the vertical. The half pixel accuracy is

the result of the matching including interpolated pixels. The cheaper CLM4500, on the

other hand, uses 48 pixels in the horizontal direction, and 24 in the vertical, again

with half pixel accuracy.

If the block size is b and the maximum displacements in the horizontal and vertical

directions are dx and dy respectively, then the search area will be of size

(2dx + b)(2dy + b). Excluding sub-pixel accuracy it will contain (2dx + 1)(2dy+1)

distinct, but overlapping, candidate blocks. Clearly the larger the allowable

displacement the greater the probability of finding a good match. The number of

candidate blocks in the search area, however, increases quadratically as the

displacement increases, which can result in a large number of candidate blocks being

compared to the target block. Considering every candidate block in the search area as

a potential match is known as an Exhaustive Search,Brute Force Search,

orFullSearch

Matching Criteria

In order for the compressed frame to look like the original, the substitute block must

be as similar as possible to the one it replaces. Thus a matching criterion, ordistortion

function, is used to quantify the similarity between the target block and candidate

blocks. If, due to a large search area, many candidate blocks are considered, then the

matching criteria will be evaluated many times. Thus the choice of the matching

criteria has an impact on the success of the compression. If the matching criterion is

slow, for example, then the block matching will be slow. If the matching criterion

results in bad matches then the quality of the compression will be adversely affected.

Fortunately a number of matching criteria are suitable for use in video compression.

Sub-Optimal Block Matching Algorithms

The exhaustive search is computationally very intensive and requires the distortion

function (matching criteria) to be evaluated many times for each target block to be

matched. Considerable research has gone into developing block matching algorithms


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that find suitable matches for target blocks but require fewer evaluations. Such

algorithms test only some of the candidate blocks from the search area and choose a

match from this subset of blocks. Hence they are known as sub-optimal algorithms.

Because they do not examine all of the candidate blocks, the choice of matching block

might not be as good as that chosen by an exhaustive search. The quality-cost trade-off

is usually worthwhile however.

4. Motion vector correctionOnce the best substitute, ormatching block, has been found for the target block, a

motion vector is calculated. The motion vector describes the location of the matching

block from the past frame with reference to the position of the target block in the

currentframe.

Motion vectors, irrespective of how they are determined, might not correspond to the

actual motion in the scene. This may be due to noise, weaknesses in the matching

algorithm, or local minima. The property that is exploited in spatially dependent

algorithms, can be utilised after the vectors have been calculated in an attempt to

correct them. Smoothing techniques can be applied to the motion vectors that can

detect erratic vectors and suggest alternatives. The alternative motion vectors can be

used in place of those suggested by the BMA. Usually the candidate blocks to which

they refer are first evaluated as potential matches and the corrected motion vector used

only if the block is suitable

Smoothing motion vectors, however, can add considerable complexity to a video

compression algorithm and should only be used where the benefits outweigh these

costs. If frames are going to be interpolated by the receiver then motion vector

correction is likely to be worthwhile. Smoothing can also reduce the amount of data

required to transmit the motion vector information, because this information is

subsequently compressed and smooth vectors can be compressed more efficiently.

Vector smoothing causes problems of its own. Smoothing can cause small objects to

be coded badly because their motion vectors might be considered erroneous when they

are in fact correct. Smoothing such motion vectors can adversely affect the quality of

the compressed image. Stiller found that averaging schemes blurred sharp

discontinuities in image sequences. Because these discontinuities might be the result


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of object boundaries they should be preserved .To correct these problems Kim & Park

developed a sophisticated correction process that yielded better results than other

methods .Lallauret & Barba devised a technique that reconsidered every motion vector

that did not concur with its immediate neighbours to the left and above .

5. Vector coding

Once determined, motion vectors must be assigned bit sequences to represent them.

Because so much of the compressed data will consist of motion vectors, the efficiency

with which they are coded has a great impact on the compression ratio. In fact up to

40% of the bits transmitted by a codec might be taken up with motion vector data.

Fortunately, the high correlation between motion vectors and their non-uniform

distribution makes them suitable for further compression. This compression must be

lossless.

Efficient coding of motion vectors is a subject of research in its own right and many

authors have offered suggestions on which techniques work best. Any one of the

lossless general purpose compression algorithms are suitable for coding vectors.

Akansu et al investigated the relative efficiency of Arithmetic, Adaptive Huffman, and

Lewpel-Ziv Coding .They found that the arithmetic and Huffman techniques

performed best and that adaptive techniques using short term statistics performed

better than those using long term statistics. The ISO/IEC video compression standard

known as MPEG specifies variable length codes to be used for motion vectors. The

zero vector, for example, has a short code, because it is the most frequently occuring.

Lallauret & Barba tested two methods of coding motion vectors. The first was a

predictive method where a prediction of the motion vector was calculated based on its

predecessors in the same row and column. The prediction errors were then Huffman

coded. The second technique grouped the motion vectors into blocks. If all the vectors

in a block were the same, then only one was transmitted. Blocks that did not contain a

homogenous set of vectors were labelled and the vectors described as per as their first

method.


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6. Prediction Error codingAlthough the battery of techniques described thus far can code video very

successfully, they rarely generate perfect replicas of the original frames. Thus the

difference between a predicted frame and the original uncompressed frame might be

coded. Generally this is applied on a block by block basis and only where portions of

the coded frame are significantly different from the original. Transform coding is most

frequently used to achieve this and completely lossless coding is rarely a goal.

2.4 SATELLITE TV SIGNAL

Satellite signals have a pretty long path to follow before they appear on your TV

screen in the form of your favourite TV show. Because satellite signals contain such

high-quality digital data, it would be impossible to transmit them

without compression. Compression simply means that unnecessary or repetitive

information is removed from the signal before it is transmitted. The signal is

reconstructed after transmission.

2.4.1 STANDARDS OF COMPRESSION

Satellite TV uses a special type of video file compression standardized by the Moving

Picture Experts Group (MPEG). With MPEG compression, the provider is able to

transmit significantly more channels. There are currently five of these MPEG

standards, each serving a different purpose. DirecTV and DISH Network, the two

major satellite TV providers in the United States, once used MPEG-2, which is still

used to store movies on DVDs and for digital cable television (DTV). With MPEG-2,

the TV provider can reduce the 270-Mbps stream to about 5 or 10 Mbps (depending

on the type of programming).

Now, DirecTV and DISH Network use MPEG-4 compression. Because MPEG-4 was

originally designed for streaming video in small-screen media like computers, it can

encode more efficiently and provide a greater bandwidth than MPEG-2. MPEG-2

remains the official standard for digital TV compression, but it is better equipped to

analyze static images, like those you see on a talk show or newscast, than moving,

dynamic images. MPEG-4 can produce a better picture of dynamic images through use

of spatial (space) and temporal (time) compression. This is why satellite TV using

MPEG-4 compression provides high definition of quickly-moving objects that

constantly change place and direction on the screen, like in a basketball game.
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2.4.2 MPEG STANDARDS

All MPEG standards exist to promote system interoperability among your computer,

television and handheld video and audio devices. They are:

MPEG-1: the original standard for encoding and decoding streaming video andaudio files.

MPEG-2: the standard for digital television, this compresses files for transmissionof high-quality video.

MPEG-4: the standard for compressing high-definition video into smaller-scalefiles that stream to computers, cell phones and PDAs (personal digital assistants).

MPEG-21: also referred to as the Multimedia Framework. The standard thatinterprets what digital content to provide to which individual user so that media

plays flawlessly under any language, machine or user conditions.

2.4.3 ENCRYPTION AND TRANSMISSIONAfter the video is compressed, the providerencrypts it 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.

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.
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CHAPTER-3

DTH SATELLITE RECEIVER

3.1 SATELLITE DISH

When the signal reaches the viewer's house, it is captured by the satellite dish. A

satellite dish is just a special kind of antenna designed to focus on a specific broadcast

source. The standard dish consists of aparabolic (bowl-shaped) surface and a

central feed horn. To transmit a signal, a controller sends it through the horn, and the

dish focuses the signal into a relatively narrow beam.

Figure (vii) The curved dish reflects energy from the feed horn, generating a

narrow beam

The dish on the receiving end can't transmit information; it can only receive it. The

receiving dish works in the exact opposite way of the transmitter. When a beam hits

the curved dish, the parabola shape reflects the radio signal inward onto a particular

point, just like a concave mirror focuses light onto a particular point.
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Figure (viii) The curved dish focuses incoming radio waves onto the feed horn.

In this case, the point is the dish's feed horn, which passes the signal on to 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 blockdown converter, or LNB.

The LNB amplifies the radio signal bouncing off the dish and 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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3.2 SATELLITE RECEIVER

Figure (ix) Satellite Receiver

The end component in the entire satellite TV system is the receiver. The receiver has

four essential jobs:

It de-scrambles the encrypted signal. In order to unlock the signal, the receiverneeds the proper decoder chip for that programming package. The provider can

communicate with the chip, via the satellite signal, to make necessary adjustments

to its decoding programs. The provider may occasionally send signals that disruptillegal de-scramblers as an electronic counter measure (ECM) against illegal users.

It takes the digital MPEG-2 or MPEG-4 signal and converts it into an analog formatthat a standard television can recognize. In the United States, receivers convert the

digital signal to the analog National Television Systems Committee (NTSC)

format. Some dish and receiver setups can also output an HDTV signal.

It extracts the individual channels from the larger satellite signal. When you changethe channel on the receiver, it sends just the signal for that channel to your TV.Since the receiver spits out only one channel at a time, you can't tape one program

and watch another. You also can't watch two different programs on two TVs

hooked up to the same receiver. In order to do these things, which are standard on

conventional cable, you need to buy an additional receiver.

It keeps track of pay-per-view programs and periodically phones a computer at theprovider's headquarters to communicate billing information.
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Receivers have a number of other features as well. They pick up a programming

schedule signal from the provider and present this information in an onscreen

programming guide. Many receivers have parental lock-out options, and some have

built-in digital video recorders (DVRs), which let you pause live television or record it

on a hard drive.

These receiver features are just added bonuses to the technology of satellite TV. With

its movie-quality picture and sound, satellite TV is becoming a popular investment for

consumers. Digital cable, which also has improved picture quality and extended

channel selection, has proven to be the fiercest competitor to satellite providers. The

TV war is raging strong between satellite and digital cable technologies as well as

between the providers who offer these services. Once considered luxuries in most

households, satellite and digital cable are becoming quite common as providers bundle

TV with Internet and phone services to offer competitive deals and win over

customers.

4. CONCLUSION
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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 GMRTObservations, 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, iflocated at about 2 km from a GMRT antenna.


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5. FUTURE SCOPE

1. To view pay & free-to-air TV channels of various DTH platform on our home TV.

2. Doordarshan free-to-air services providing 40 TV channels withno 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.

3. One can scan the entire globe with a motorized dish using a CIset top box with CAM modules and watch TV channels of several

DTH platforms visible to the dish terminals.

4. A number has started IP broadcast with return channel on PSTN line and this wouldbe for education and other applications.

6. REFERENCES


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[1]. EN 50221: Common Interface Specifications for conditionalaccess and other Digital video broadcasting decoder

applications.

[2]. IS 15377-2003: Digital Set Top Box for Direct to Home services.[3]. http://www.newmediarepublic.com/dvideo/compression/adv06.html

[4]. http://www.newmediarepublic.com/dvideo/compression/adv07.html[5]. The digital satellite TV handbook by Mark E. Long[6]. www.howstuffworks.com/satelliteTV[7]. Satellite Direct-to-Home - IEEE[8]. Development of satellite TV distribution and broadcasting-IEEE-

Electronics & Communication Engineering Journal-August 1992.
http://www.newmediarepublic.com/dvideo/compression/adv06.htmlhttp://www.newmediarepublic.com/dvideo/compression/adv06.htmlhttp://www.newmediarepublic.com/dvideo/compression/adv07.htmlhttp://www.howstuffworks.com/satelliteTVhttp://ieeexplore.ieee.org/iel5/5/33232/01566626.pdf?arnumber=1566626http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=2219http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=2219http://ieeexplore.ieee.org/iel5/5/33232/01566626.pdf?arnumber=1566626http://www.howstuffworks.com/satelliteTVhttp://www.newmediarepublic.com/dvideo/compression/adv07.htmlhttp://www.newmediarepublic.com/dvideo/compression/adv06.html

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