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INTRODUCTION
Television has become a wide ranging and powerful medium of mass communication in this ever shrinking world. It had now become possible to have an international programs with global coverage by linking national TV systems through satellites. Communication satellites in geo stationary orbits have opened new vistas for global television broadcasting. Direct to Home (DTH) broadcasting satellites for television have been launched by several countries including India. These satellites emit high power radiations enough to be received directly through low cost medium to small size dish antennas on conventional TV receivers augmented by a front end converter. The INSAT 2 & 4 series satellites launched by India are widely being used for TV programme and for other communications.
SATELLITE CHANNELS
The frequencies for satellite communication as regulated by the International Telecommunication Union (ITU) are broadly as shown:
Band of frequencies
2.2 FEED AND LNBC:
After reflection from the PDA the signal strength is maximum at the focus of PDA, so a feed is placed at focus points with the help of mounting plate. For the reception of S-band signal we use helical feed while for C-band signal we use polarized circular feed. For amplification and down conversion of the received signal the LNBC (low noise block down converter) is connected at the output of the feed. LNBC down converts the signal in the IF ranges of 950 MHz to 1150 MHz for S-band and 950 MHz to 1750 MHz for C-band signal. The local oscillator frequency of S-band LNBC is 3650 MHz and that of C-band signal is 5150 MHz.22V DC supply is fed to the LNBC through RF coaxial cable connecting the satellite TV receivers. The down converted IF signal from LNBC is fed to satellite receiver for further processing.
DIGITAL SATELLITE TV RECEIVERS:
A digital satellite TV receiver is convenient product that allows us to view a variety of programme provided through satellite. The satellite receiver accepts signal in the range of 950 MHz to 2150 MHz from LNBC with level of -25 dbm to -65 dbm. These are also called integrated receiver decoders (IRDs).These are suitable for single carrier per channel (SCPC) and multiple channel per carrier (MCPC) applications. These are capable of decoding the components of digitized video signal encoded in the format 4:2:0 /4:2:2 which corresponds to the video signal components such as the luminance signal(y) and the chrominance signal(R-Y) and (B-Y).The purpose of these receivers are to process the IF signal received from the LNBC, demodulate it and provide high quality analog video and studio signals. These receivers also provide automatic support for PAL/NTSC system.
VIDEO SIGNAL:
The video signal output from a video camera and other video equipments is of composite nature. It consists of the following components:
1. Luminance (brightness) information
2. Color information (Chroma signal)
3. H and V blanking pulses
4. H and V sync pulses
5. Color burst signal (about 10 cycles of color subcarrier signal).