If you are in cable industry or working for cable industry then there is basic need to understand the modulation techniques used in the cable tv headend. This is the basic concept used in the CATV Headend. In cable tv industry we use the terms DVB stands for Digital Video Broadcasting.
DVB – C : Digital Video Broadcasting For Cable
DVB – S : Digital video broadcasting For Satellite
In Digital Video Broadcasting system we transmit the digital signals. These signals are in the form of 0 or 1. These two digits are transmitted as a information. If we say 1 Mega Bit transfer of information then it means there is 1 million bits are transferred as a information at the receiving end. Now to transmit these information at long distance we use the digital modulation.
What Is Modulation
Modulation is a technique used for to send the modulating signal by superimposing it on high frequency carrier wave. Thus there are two terms came into modulation that is modulating signal and carrier wave. Modulation avoid the interference between two signal and also avoid the errors at receiving end of signal also enable the signal to transmit long distance.
In traditional analog headend we were using either Amplitude Modulation or Frequency Modulation.
Types Of Modulation
Amplitude modulation is type of modulation in which only amplitude of carrier wave changes. While frequency remains same.
Thus in amplitude modulation we superimpose our modulating signal to amplitude varying carrier wave.
Frequency modulation is type of modulation in which only Frequency of carrier wave changes. While Amplitude remains same. So we superimpose our modulating signal to high frequency carrier wave.
Phase Modulation is type of modulation in which phase of the carrier wave is changes while amplitude remains same. This is also known as Phase Shift Keying.
In a sin wave if there is starting point of both singals are same then both the signals are in same phase. If starting point of two signals are different like one signal is starting from 0 and other signal is starting in delay of 90 degree then these signals are called the out of phase by 90 degree.
When the amplitude remains same but the frequency of the carrier wave changes then this type of modulation is called the Frequency Shift Keying.
In other category where amplitude changes then we call it Amplitude Shift Keying or AMP.
A combination of FSK and AMP is used in the Cable TV. For satellite communication we use the QPSK this is more advanced and known as Quadrature Phase Shift Keying.
Bit Error Rate Or B.E.R.
Bit Error Rate defined as the errors available in the signal at the receiving end so it defines the accracy of the signal at the receiving end. A lower bit error rate denotes the better quality of the signal.
When we transmit 10,000 Bits if there is only 1 bit error rate then its normal while by applying error correction techniques we can reduce this bit error rate. After we apply the error correction we probabilly gets the 1 bit error in 100,000 million Bits of information.
Modulation In Digital Headend – QAM
In digital headend we use the QAM modulation it is known as Quadrature Amplitude Modulation. The QAM modulation implies the PSK and AMP or Phase Shift Keying and Amplitude Shift keying. There are many types of QAM Modulation we use in the digital headend like
Basic requirement for digital headend in terms of modulation is that
There should not be interference between channels because we transfer here large number of channels through one cable which satisfy this QAM modulation technique.
There should be good carrier to noise ratio which aslo satisfy this QAM modulation.
This is why we use the QAM modulation in digital CATV headend.
This was all about the digital modulation in digital headend.