Question

(a) Give three reasons why modulation of a message signal is necessary for long distance transmission.
(b) Show graphically an audio signal, a carrier wave and an amplitude modulated wave.

Answer 1

Hint: In communication, we need to send signals from one place to another over large distances. But a simple transverse wave signal can get a lot of distortions due to the obstacles in the path of the communication due to which need some ways to ensure efficient signal transmission from one place to another.

Complete step by step answer:
(a). In communication, we use electromagnetic waves which are transverse in nature. They are used to transmit a signal from a sending antenna to a receiving antenna.
The message signal cannot be sent directly in its original form from one place to another because that way the signal dies out very quickly and long distance communication is not possible.
Modulation allows a number of benefits for communication.
It allows us to transmit a signal over a range of frequencies called bandwidth of the channel.
The modulated signals do not interfere with each other.
Lower the frequency of a signal, larger is the antenna required to transmit that signal. But through modulation, we can send signals by modulating it to a higher frequency and be able to use a smaller antenna.
We have different ways in which we can modulate our message signal and then transmit it safely without excessive distortion from one place to another. These methods are amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM).

(b). Following diagram represents an audio signal, a carrier wave and an amplitude modulated wave.

Note: A signal must not be modulated beyond a certain limit. This limit is defined by the modulation index which, for amplitude modulation, is equal to ratio of amplitude of the modulated wave to the amplitude of the carrier wave. Its value should not exceed one which can lead to over modulation.