What is DTH TV, DTH Satellite TV Wiki, DTH TV basics
We will try to explain the very basics of Satellite DTH TV and thus will serve as your DTH Satellite TV Wiki or DTH TV Wiki ("Wiki" is from the Hawaiian word WikiWiki, meaning "quick" or "tell me quickly").
Satellite DTH TV works from signals received directly on the dish antenna placed outdoors facing the direction of the satellite. So for DTH satellite TV to work, there is no need for any cables from outside the compound of the house or flat building. The only cable required is from the dish antenna to the Set Top Box (STB) Decoder near the TV and then from the STB or Set Top Box, a connecting cable to the TV.
Difference between cable TV and DTH TV
The difference between Cable TV and DTH TV is that in cable TV long cables connect your TV from the Cable TV operators place. There is a lot of signal loss due to the long lengths the cable through which the TV signals has to pass through. The cable operator gets his signals from Satellites and then converts it into lower frequency signals and then passes those lower frequency signals through cables to your house or flat.
Satellite TV on the other hand sends the signals from the satellites high above the earth through the air at very high frequencies, which is received by the Dish Antenna and the LNB converts those very high frequencies to lower frequencies, amplifies the signals and sends it through a short cable to the Set Top Box. In DTH there is no signal loss and you get strong TV signals in the Set Top Box and so get very good picture and sound quality on the TV set. Cable TV on the other hand has a lot of signal loss and disturbance and hence DTH TV is much better than Cable TV.
What is Satellite Footprint
Satellites are launched by rockets and placed in a Geosynchronous orbit around the earth at about 36,000 kilometer above the earth's equator. Geosynchronous orbit means that the satellite is traveling at the same speed at which the earth is rotating and so the satellite is always in the same position above the earth. The small image here shows the satellite beam hitting the earth between North and South America. The video animation below shows how the signal footprint remains in the same position over a small area over the earth, even though the earth is constantly moving or rotating. The Video shows very clearly how a Geosynchronous satellite with its footprint works.
How satellite sends signals to earth
The DTH satellite sends a beam of signals to the earth which is like shining a giant torch-light towards the earth. In the video animation here you can see an imaginary beam hitting the earth, in an area over the Isthmus of Panama (which connects North and South America) and parts of Cuba and Florida.
Similar to the torch lighting up an area of the earth, the signals from the satellite are focused over a small area of the earth. The area where the signals hit the earth is known as the 'footprint' of the satellite. Any Dish Antenna placed within the footprint of the satellite can pick up signals and decode it to show us the TV pictures. In the center of the Satellite footprint, the signal is the strongest and you need only a very small dish antenna to pick up the DTH signals. The further you move away from the centre of the footprint, the signal gets weaker, and you will need bigger diameter dish antennas to receive the signals. The DTH footprint map of satellites show the signal strength, the size of dish required is inversely proportional to the signal strength. That is for stronger signal you need smaller dishes to receive DTH TV. The minimum size of dish for DTH in India is 50 cm or half meter diameter.
DTH dish size to receive DTH TV
The values shown in the map above is the signal strength of the DishTV DTH. The higher the strength of the signal, the smaller the DTH dish size to receive those signals. The measure of a DTH satellite signal strength in an area of the earth is measured by a unit called EIRP (equivalent isotropically radiated power). The higher the EIRP values in an area the stronger the satellite signals are, and in that area only a smaller dish antenna will be required to receive the signals. The image above shows the EIRP signal strength of the NSS6 satellite which transmits the DTH signals of India’s first and largest DTH service ‘DishTV’.
|Dish Size (Cms.)||50||50-60||55-65||60-75||65-85||75-95||85-105||95-120|
From the table above it is seen that the dish antenna size required for DishTV in the middle of India is only 50 centimetres or half a meter. But as you go further away from the central parts of India the signal strength decreases and you need a larger size dish antenna to receive the DishTV signals. For example the Dish TV signal strength over Pakistan is 44 dBW and to receive this weaker signal you need a larger dish, from 1 meter to 1.2 meters.
In the DishTV signal strength map above the Middle East areas is shown to have no signal strength. But the Dish TV signals can be received in the Middle East areas of UAE, Oman, Qatar, Kuwait, Saudi Arabia, etc. by using a larger dish antenna of sizes up to 150 cms or 1.5 meters. With a 120 cms or 1.2 meter dish size for Dish TV DTH in Dubai it is possible to get good reception, but if you can get a 1.5 meter dish there will be better reception in the Middle East countries. Similarly to receive Airtel DTH in UAE, Oman, Qatar, Kuwait, Saudi Arabia, etc. the Airtel DTH dish antenna size will be 120 cms or 1.2 meters and more.
How DTH satellite TV works
The first commercial home DTH service started in Europe with the 'Astra-1' satellite launched in December 1982. Thus began the world revolution of Satellite TV. People in India could watch a cricket match being played in England instantaneously as each ball is being bowled. How this is achieved is explained in a very simple way below.
The picture above on the left shows how the TV signals are sent up to a satellite by the broadcaster in UK and the subscriber in India receives the reflected signals from the satellite through his small dish antenna sitting on his house roof or balcony. Sending signals to the satellite is known as 'Uploading' and we the DTH customers in India are 'Downloading' the content. For TV signals to travel round the world require a chain of satellites. An earth station receives signal from one satellite and downloads those signals only to retransmit immediately by uplinking to another satellite. So a TV signal to reach India from England will involve more than one satellite.
The picture on the top right shows how the DTH satellite dish in our homes are reflecting and concentrating the signals on to the LNB (Low Noise Block downconverter) which is placed at the focal point of the parabolic satellite DTH dish. The transmission from the satellite high up (36,000Km) in Geostationary orbit is sent by radio electromagnetic waves or Microwaves at a high frequency (10 to 13 GHz). This is because the very high frequency radio waves can travel great distances without too much loss of power. But these high frequency waves cannot pass through walls or other obstacles, so our DTH dish must be placed in line of sight of the Satellite. This is why DTH dishes are always placed on top of roofs and the dish must point exactly to the direction of the satellite. Once the Satellite signals are collected by the LNB, it reduces the frequency of the TV transmission. This is because high frequency waves lose its signal strength when it passes through the coaxial cable connecting the Dish LNB to the STB or satellite receiver. The higher the frequency through a coaxial cables the greater the loss of signal strength. So the LNB reduces the frequency from 10+ GHz to less than 2 GHz.
How DTH TV picture is formed
A TV video picture basically consists of 25 'frames' per second, each frame is a single still image. When these still images are shown very fast one after another at the rate of 25 frames per second, it creates in our minds a smooth moving picture.
In a very simplified way, we can explain how each frame or picture is formed on a TV by the use of the picture on the right. The TV picture is actually painted on the screen by an electron beam. The electron beam "paints" the image on the screen by moving from left to right starting from the top left hand corner (1). When it reaches the other end of the screen the beam offs and quickly moves back to the left side (2) and at the same time going down a little bit. It repeats this process till it finally reaches the bottom right hand corner. Now the beam has painted one "frame" which is a complete picture, like a photo, on the TV screen. The electron beam offs and then moves back up and repeat the process (3). All of this is done in fractions of a second, since the beam has to paint about 25 full pictures on the screen per second. As the beam paints each line from left to right, the intensity and characteristics of the beam is changed to create different shades of colours across each of the pixels of the screen.
There are 3 standard resolutions for the number of horizontal lines. In the old CRT TV it was 480 lines. The HD Ready and the ordinary DVD players have 720 lines. HDTV and Blu-Ray DVD players have 1080 lines.
TV transmission companies, in order to reduce the Bandwidth (the amount of data to be transmitted per second) required to transmit TV pictures, devised a method by which each frame of a picture is split into 2. The odd lines starting from the top, that is line 1, line 3, line 5, etc are made into one frame. Similarly the even lines 2, 4, 6, 8, etc are made into the second frame.
This means that the TV transmitting station only has to transmit only half the picture information or data at a time, even though, instead of 25 frames per second they now have to transmit 50 frames per second. This method of transmitting is called 'Interlaced Scanning'. The TV now receives 50 frames per second and it combines 2 frames into one and shows it on the TV. The 'i' after the horizontal line numbers represents that it is an interlaced signal.
Progressive Scans on the other hand transmits the complete picture at the same time. The 'p' after the horizontal line resolution stands for progressive. Of the two scanning methods, the progressive is the better one, because it avoids problems like 'judder' or 'jitter' or 'flickering' in the TV full picture when the 2 interlaced pictures are combined. So in effect the best quality picture from today's TV technology is available from a 1080p device. Right now, in India 1080p is only available from Blu-ray DVD.
TV video signals cannot be transmitted in its original picture format. This will require too much bandwidth or too much data to be sent through the transmission. So digital compression technologies are used to compress the video signals as explained below.
A TV picture has 50 frames per second being shown on the TV screen. Let us take the example of a man lifting his hand. In the first of the frames (the first of fifty pictures per second) shows his hand down. In each of the following pictures his hand moves up a little, till on the 50th frame his hand is up. When these 50 frames are shown so fast at 50 frames per second, our eyes do not see the single pictures, but we see it as a smooth motion of his hands going up.
Compression of the 50 frames in the above example can be explained very simply as follows. In the example only the man's hand moves, the rest of his body is not moving. So the picture frame is divided into 2 portions, one portion has the man's whole body which is not moving and another part which only has his moving hand. So the basic digital compression technique is to transmit the non-moving part of his whole body frame once per second and the moving parts of the frame at the rate of 50 frames per second. This greatly reduces the amount of data to be transmitted. The compressed TV signals are recombined by the satellite decoder STB. The single non-moving part is added to each of the 50 frames per second of the moving hand and complete frames at the rate of 50 frames per second are sent to the TV by the satellite decoder STB. The most popular compression technologies are called MPEG2 and MPEG4. We discuss these 2 technologies in more detail in our dedicated page MPEG2 vs MPEG4
The above described method does not greatly reduce the quality of the picture. But the amount of data can also be reduced by sacrificing the quality of the pictures by reducing its resolution. This method is resorted to by DTH operators when they want to add more channels within limited bandwidth.