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What is modulation and demodulation? Different types of modulation methods ?

Modulation and Demodulation


Modulation

Audio or video waves received through transducers, known as signal webs, are difficult to transmit over long distances. High frequency carrier waves are used to transmit these signal waves over long distances as radio waves. The amplitude, frequency or phase of this carrier wave is changed according to the instantaneous value of the signal wave. Such changes are called modulation.

In other words, mixing the signal with the carrier wave is called modulation. Modulation is mainly of two types, namely Amplitude Modulation (AM) and Frequency Modulation (FM).


Use of Modulation: How far a real signal can be transmitted by a transmitter depends on two factors, namely --

(i) Transmitting Frequency

(ii) Transmitter Power


Modulation is used to send the signal farther. Different types of modulation methods:

(1) Amplitude Modulation (AM)

(ii) Frequency Modulation (FM)

(iii) Pulse Modulation (PM)

(iv) Frequency Shift Keying (FSK) 

(v) Quadrature Amplitude Modulation (QAM)) 

(iii) Pulse Code Modulation (PCM)


Amplitude Modulation: When the amplitude of the high frequency carrier wave varies according to the intensity of the signal, it is called amplitude modulation. In amplitude modulation, the amplitude of the carrier wave changes according to the intensity of the signal, but the frequency of the modulated wave remains the same, i.e., equal to the carrier frequency. The diagram below shows the principle of amplitude modulation. It can be seen that both +ve and -ve of the carrier wave are changed according to the half cycle signal. For example, when the signal is +ve amplified, the carrier amplitude also increases. On the other hand the amplitude of the carrier wave decreases during the –ve half cycle of the signal. The electronic circuit with the help of which amplitude modulation is done is called modulator.


Following are the points to note about amplitude modulation --

(a) The amplitude of the carrier wave varies with the intensity of the signal.

(b) Amplitude variation of carrier wave occurs at signal frequency fs.

(c) The frequency of the modulated wave of the amplitude wave remains the same i.e. equal to the carrier frequency.


Frequency Modulation: When the frequency of the carrier wave changes according to the intensity of the signal, it is called frequency modulation. In frequency modulation the carrier wave changes only according to the frequency of the signal. But the amplitude of the modulated wave remains the same. The frequency of the carrier wave depends on the instantaneous amplitude of the varying signal.

Differences and Characteristics of Amplitude Modulation and Frequency Modulation:

Amplitude modulation:


  • The bandwidth of amplitude modulation is twice the highest frequency of the signal wave. bandwidth, | BW = 2 x f lower to s(max) where f lower to s(max) is the maximum frequency of the modulating frequency. 
  • This type of modulation is easily affected by noise as the amplitude of the modulated wave varies. So noise cannot be easily avoided, because noise also changes the signal amplitude. As a result noise remains in the output. 
  • Fidelity in this method is not better than modulation.


Frequency Modulation:


    • The formula BW = 2nx f lower to m is used for bandwidth in case of frequency modulation. Here is the highest order of significant side bands.
    • In frequency modulation the amplitude remains unchanged and therefore is not easily affected by noise. Apart from that, noise can be easily avoided in this method. Amplitude changes at the receiver can be suppressed by limiters. As a result, the effect of noise is not found.
    • Fidelity in frequency modulation is better than frequency modulation.


Demodulation: The method by which a modulated signal is separated from the high-frequency signal and the low-frequency signal. It is called demodulation. Non-linear devices such as diodes, transistors etc. are used for demodulation. By processing the Amplitude Modulated Wave, Diode and RC Circuit are used to recover the signal wave. With the help of this circuit the envelope of the wave is determined. It is also called Envelope Modulation.




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