satellite TV-Direct Broadcast Satellite System,DBS TV
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This page covers satellite TV system basics. It describes direct broadcast satellite system(DBS). It explains DBS TV receiver block diagram with packet format.
The most common method for TV signal distribution to cover large areas on the earth is using satellite. The satellite is placed at the height of about 36000 Km. The satellite is used as relay station. The TV signal which need to be distributed is first up-converted to higher microwave frequency and passed to the satellite using directional antennas. The satellite receives these signals and translates to another RF carrier frequencies and transmit them back on the earth. Cable TV companies pick these signals and distributes them to subscribers.
The concept where in cable TV companies have been bypassed and subscribers directly receive the signals from the satellite is referred as direct broadcast satellite (DBS) TV. The latest DBS systems employ low noise amplifiers in the receiver and will usually have very small dish antenna (about 18 in diameter). DBS TV systems provide high quality video and audio due to digital techniques.
Satellite TV signal Transmitter and Receiver
The TV signal is uplinked to the satellite using C band or Ku band. There are two parts in this satellite frequency allocations viz. uplink frequency and downlink frequency. C band uses 5925 to 6425 MHz as transmit frequencies and 3700 to 4200 MHz as receive frequencies. Ku band uses 14 to 14.5 GHz as transmit frequencies and 12 to 12.5 GHz as receive frequencies.
The video signal frequency modulates the microwave carrier as mentioned on one of the 24 channels. The audio along with video frequency modulates sub-carrier in the 5 to 8 MHz range. 6.2 and 6.8 MHz are the most commonly used carriers. stereo sound is used to modulate 5.58 and 5.76 MHz. The video occupies spectrum from 0 to 5 MHz.
The uplink signal(5925-6425 MHz) as mentioned in received by the satellite and down converted and ampplified using transponder module on board. The downconverted lower frequency signal(3700-4200 MHz) is transmitted back to the earth.
The satellite TV receiver provides the signal to the consumer TV set. It mainly houses parabolic dish antenna, low noise amplifier and down converter, IF circuitry. The IF circuits along with demodulators segregate both voice and video and provide interfacing with the TV set. In addition satellite receiver will have control circuits to change the position of the satellite dish antenna.
Antenna is the most critical part in the communication link here. The signal of the satellite on earth very in strength. Due to this the areas on earth where received signal is strong will require smaller less gain antennas. The areas where received signal is weak requires high gain larger size antennas.
Satellite TV receiver usually will be of single conversion type or dual conversion type. single conversion type will have 70 or 140 MHz IF. Dual conversion type will have 1st IF of about 770MHz(mostly in 600-1500MHz range) and second IF of about 70/140 MHz. The dual conversion helps in image frequency rejection.
DBS TV Receiver
The Direct Broadcast Satellite(DBS) system is the latest advancement in satellite TV industry. It is all digital system and uses data compression techniques. It helps in achieving high quality picture as well as sound.
The DBS satellite systems use antenna of size 18 inch in diameter. There are special digital broadcast satellites in the orbits which provides DBS TV service. Few of them are DirecTV, USSB and PrimeStar.
DBS satellites usually operates in Ku band. 14 to 14.5 GHz is the uplink frequency and 10.95 to 12.75GHz is the downlink frequency used. DBS satellites will have both right hand as well as left hand circular polarizations supported using helical antennas. The channel capacity can be doubled using both the polarizations of the transmitted signal.
The digital signal consisting of video, audio and control information is packed as per the DBS TV packet format mentioned below in fig1. Each packet is of size 147 bytes. This is divided into 2 bytes of SCID and flags, 1 byte used as packet type and continuity counter, 127 bytes of data and 17 bytes used as redundant for FEC.
SCID is a 12 bit number which identifies video program being transported by the packet. The additional 4 bits here defines encryption is being applied or not.
Figure depicts block diagram of the DBS receiver system. Let us understand the entire reception process.
1. The horn antenna in the DBS receiver picks up the Ku band signal and translates the same of 500 MHz bandwidth to L band frequency in the range from 950 to 1450MHz. Control circuitry will help select RHCP or LHCP signals. This RF signal is sent to the receiver
2. The signal now is passed through second mixer which helps in channel selection with the tuning of LO or synthesizer.
3. Second IF is then demodulated to recover the originally transmitted digital signal.
4. This is passed through the FEC(Forward Error Correction) module. The FEC helps in detecting and correcting the errors.
5. The FEC corrected data is stored in the RAM from where both video and audio portions are separated.
6. These separated signals are passed to their respective compression units. DBS TV system uses digital compression standard referred as MPEG2. MPEG stands for Motion Picture Experts Group.
7. These video and audio signals are converted to analog form using D/A converters.
8. These analog converted signals are given to RF demodulator which is the interface to the TV set.
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