Advantages and Disadvantages of Amplitude Modulation

Introduction to Amplitude Modulation

Amplitude refers to the peak value of any property. For a sigmoid wave, it is the maximum height of the wave. 

Modulation is the process of varying the properties of a carrier wave under the signal. Modulation can be of three types.

• Amplitude modulation occurs when the signal varies the amplitude of a carrier wave
• Frequency modulation occurs when the frequency of a carrier wave is varied as per the signal
• Phase modulation occurs when variation in the instantaneous phase of a carrier wave follows the signal pattern

What is Amplitude Modulation?

Amplitude modulation involves two kinds of waves.

• A carrier wave is a high-frequency wave
• A signal wave is a low-frequency wave that possesses the information

After the modulation process, a modulated wave emerges, having the frequency of the carrier wave. The amplitude of this modulated wave changes based upon the superposition of amplitude of the signal wave with that of the carrier wave.

Need for Amplitude Modulation

Transmission of the low-frequency signals presents many difficulties. 

• As the wavelength is inversely proportional to the frequency. Low-frequency signals have high wavelengths
• Due to these high wavelengths, the height of the antenna required is also very large
• Power radiated by this antenna is also very less and therefore, signals vanish after some distance
• Low-frequency signals such as sound waves usually have a wider frequency range
• So, there is also the chance of mixing different signals 

Modulation, especially amplitude modulation, came into the picture to deal with these issues.

Modulation Index

It refers to the ratio of the signal wave’s amplitude with the carrier wave’s amplitude. It represents the depth of modulation undergone by a modulated wave. The percentage of modulation is calculated by multiplying the modulation index by 100. 

• If the modulation index is 1, then the percentage of modulation will be 100%, which implies perfect modulation
• If the modulation index is less than 1, then the percentage of modulation will be less than 100%
• This will lead to under-modulation
• It implies that the modulation process has not been efficient
• If the modulation index is more than 1, then the percentage of modulation will be more than 100% which will result in over-modulation
• It implies that sidebands are extending beyond the allowed bandwidth
• Thus, they are interfering with other users and distorting the signal on the reception
• Broadcast stations take preventive measures to stop over-modulation

Advantages of Amplitude Modulation

• The cost of amplitude modulation is low
• Amplitude modulated signals are reflected by the earth from the ionosphere layer
• Thus, they travel a long distance and cover a wider area
• Required carrier frequency is low
• It requires a simple transmitter and receiver
• Thus transmission and reception of an audio signal are less cumbersome
• Demodulation of amplitude-modulated signals is less complex due to the use of a circuit having few components

Disadvantages of Amplitude Modulation

• Amplitude modulated signals are highly susceptible to peripheral noises
• Receivers of these signals find it difficult to distinguish between signals and noises
• Thus, the quality of the audio signal is of poor standard
• If the amplitude modulated signal is weak, the receiver requires a complex arrangement
• Amplitude modulation is less efficient in terms of bandwidth usage
• The bandwidth of the modulated signal is twice the frequency of the signal wave
• Amplitude modulation is also less efficient in terms of power usage
• Only the power in the sidebands is used

Applications of Amplitude Modulation

• Amplitude modulation is used to transmit the signal over a long distance
• It is also used for radio transmission
• It is also applied in two-way communication for air traffic control

Variations of Amplitude Modulation

• Single Sideband

It is used for high-frequency communications. In this modulation type, the carrier component and one sideband component are removed. Thus, this variation is more efficient in frequency and power usage.

• Quadrature Amplitude Modulation

It is used for transmitting digital signals. In this modulation type, two carriers are used, which are 90° out of phase. The information contained in both carriers is combined.

Conclusion

Overall, amplitude modulation is a simple process compared to frequency and phase modulation. Advanced techniques in communication systems are increasingly replacing it. But the principles underlying amplitude modulation are still relevant for a theoretical understanding of the transmission of signals and the practical improvement of advanced techniques. It is also a high-scoring topic for intermediate students and aspirants preparing for competitive examinations if adequately understood.