14.1 INTRODUCTION
There was always a persistent quest to increase the coverage area of broadcast- ing. Before the advent of the satellite broadcasting, the terrestrial broadcasting, which is basically localized, was mainly providing audio and video services. The terrestrial broadcasting has a major disadvantage of being localized and requires a large number of transmitters to cover a big country like India. It is a gigantic task and expensive af- fair to run and maintain the large number of transmitters. Satellite broadcasting, came into existence in mid-sixties, was thought to provide the one-third global coverage simply by up-link and down-link set-ups. In the beginning of the satellite broad- casting, up-linking stations (or Earth Stations) and satellite receiving centers could had only been afforded by the Governments organizations. The main physical con- straint was the enormous size of the transmitting and receiving parabolic dish anten- nas (PDA). In the late eighties the satellite broadcasting technology had undergone a fair improvements resulting in the birth of cable TV. Cable TV operators set up their cable networks to provide the services to individual homes in local areas. It rap- idly grew in an unregulated manner and posed a threat to terrestrial broadcasting. People are now mainly depending on cable TV operators. Since cable TV services are unregulated and unreliable in countries like India now, the satellite broadcasting tech- nology has ripened to a level where an individual can think of having direct access to the satellite services, giving the opportunity to viewers to get rid of cable TV. Di- rect-to-Home satellite broadcasting (DTH) or Direct Satellite Broadcasting (DBS) is the distribution of television signals from high powered geo- stationary satellites to a small dish antenna and satellite receivers in homes across the country. The cost of DTH receiving equipment’s is now gradually declining and can be afforded by com- mon man. Since DTH services are fully digital, it can offer value added services, vid- eo-on-demand, Internet, e- mail and lot more in addition to entertainment. DTH re- ception requires a small dish antenna (Dia60 cm), easily be mounted on the roof top, feed along with Low Noise Block Converter (LNBC), Set-up Box (Integrated Re- ceiver Decoder, IRD) with CAS (Conditional Access System). A bouquet of 40 to 50 video programs can simultaneously be received in DTH mode.
14.2 UPLINK CHAIN
DTH broadcasting is basically satellite broadcasting in Ku-Band (14/12 GHz). The main advantage of Ku-Band satellite broadcasting is that it requires physically man- ageable smaller size of dish antenna compared to that of C-Band satellite broadcast- ing. C-Band broadcasting requires about 3.6 m dia PDA (41dB gain at 4 GHz) while Ku-Band requires 0.6 m dia PDA (35dB gain at 12 GHz). The shortfall of this 6 dB is compensated using Forward Error Correction (FEC), which can offer 8 to 9 dB cod- ing gain in the digital broadcasting. Requirement of transmitter power (about 25 to 50Watts) is less than that of analog C-band broadcasting.
The major drawback of Ku-Band transmission is that the RF signals typically suf- fer 8 to 9dB rain attenuation under heavy rainfall while rain attenuation is very low at C-Band. Fading due to rain can hamper the connectivity of satellite and therefore rain margin has to be kept for reliable connectivity. Rain margin is provided by operating transmitter at higher powers and by using larger size of the dish antenna (7.2m PDA).
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Fig 14.1 DTH to UPLINK Setup
Fig.1 shows schematic of uplink chain proposed to broadcast bouquet of 30 video programs in Doordarshan, Prasar Bharati, India. 30 video programs may either be down-linked from satellites or taken from other sources like video tape recorders, video cameras etc. in digital format. These sources are fed to Router whose outputs are divided in three groups A, B and C. Each group contains 10 video sources multiplexed in a Multiplexer.These three multiplexed streams are digitally (QPSK modulation) modulated individually at 70 MHz Intermediate Frequency (IF). Each group is further doubly up-converted, first conversion at L-Band (950-1450 MHz) and second conver- sion at Ku-Band (12-14 GHz)
14.3 DOWN-LINK CHAIN
Down-Link or receiving chain of DTH signal is depicted in Fig.2. There are mainly three sizes of receiving antenna, 0.6m, 0.9m, and 1.2m. Any of the sizes can easily be mounted on rooftop of a building or house. RF waves (12.534GHz, 12.647GHz, 12.729 GHz) from satellite are picked up by a feed converting it into electrical sig- nal. The electrical signal is amplified and further down converted to L-Band (950-1450) signal. Feed and LNBC are now combined in single unit called LNBF. The L-Band signal goes to indoor unit, consisting a set-top box and television through coaxial cable. The set-top box or Integrated Receiver Decoder (IRD) down con- verts the L-Band first IF signal to 70 MHz second IF signal, perform digital demod- ulation, de-multiplexing, decoding and finally gives audio/video output to TV for viewing
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CONCLUSION
Doordarshan is the oldest and the biggest Broadcasting media in India.
In my training session I learned a lot. Not only in technical field but also in social field too. I got a great experience of working in a Public Sector Company.
I learned about the recent trends in Broadcasting Media and also the market strategies to maximize the profit using limited resources.
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REFERENCES
1. Monochrome and Colour television by R R Gulati 2. www.ddlucknow.com
3. www.ddinews.gov.in 4. www.scribd.com