About Digital Headend Transmodulators
Transmodulators are used to digitized the signal. This is usend to convert the QPSK signals to QAM modulator signals. Transmodulators can be implemented with our Traditional Analog Headend System.Transmodulators can be tuned for particular frequency as we need.
Thus we need to use individual transmodulator for each frequency. Trans modulators are not that much costly device.So we can use transmodulators as our channels requirement. It is preferred that we need to use frequency starting from 300MHz that is UHF band frequency because in UHF band each frequency difference is 8MHz in PAL-G system.
Thus starting with 300Mhz to 550MHz total 69 analog channels are available.So each transmodulator provides about 8 channels for 64 QAM modulation then we gets total 700 channel in this frequency range by using these transmodulator.
Transmodulators working and Transmodulators Diagram
Each transmodulator have QPSK signal as input and RF signal as output. We use the combiner to combine the output of all the transmodulators.Then after combiner's output can be mixed with our traditional analog headend system that is as per our requirement.
Block Diagram Of Digital Headend Transmodulators Is Given Below,
As shown above in the figure there are thre QPSK signal that we are getting from three different Dish Antenna. Transmodulators are having two different section (1) QPSK Demodulator (2) QAM Modulator The L band tuner tune the frequency and QPSK demodulator demodulate the channels. QAM Modulator down shift the frequency between 48 MHz to 860MHz.
Then the output of each transmodulator is combined by combiner and we mix rest of all analog channels in the mixer. One thing to keep in mind is that if you are using transmodulator number 1 of 306MHz or S21 then we need to switch off the modulator of 298MHz in analog headend system to avoid the interference. Second thing is that the difference between analog signal level and digital signal level should be 10DB.In other words at the time of mixing the digital signal level(306Mhz) should be 10dB less than the analog signal level(290MHz).
Types Of Digital Headend Transmodulators
There are basically two types of digital headend transmodulators, (1) Non Intelligent (2) Intelligent As shown above in the figure there are Non-intelligent transmodulators. Because these transmodulator supports only FTA channels. On the other hand Intelligent transmodulators supports PAY channels means there is slot for Smart Card.
Drawbacks Of Digital Headend Transmodulators
In low cost transmodulators the Encryption cant be employed. We cant pass the PSI SI Tables. So we are not able to give EPG Number or LCN number and EPG information will not be displayed on STB. The series of Channels would be same as broadcasting side.We can not change it. This is all about the Digital Headend Transmodulators.
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