Modulation is a technique which is used to enhance the capability of an information signal to be transmitted over a longer distance, safely and securely. In simple words, modulation is addition of a simple message signal to a high frequency carrier signal which is capable of being transmitted over a longer distance thus enabling the message signal to be transmitted.Modulation is achieved by varying one or more properties (amplitude, frequency or phase) of a periodic waveform called carrier signal according to the massage signal.
For example, in the diagram below, we have a low frequency message signal which is required to be transmitted. But in order to transmit it we would need the message to be modulated over a high frequency carrier signal. And on the receiver side we would require a demodulator which will eliminate the carrier wave and extract the message signal.
Now the question is how modulation enables a signal to be transmitted over a longer distance. Let me give you an example. Let’s suppose we are to transmit a signal of a very low frequency of 1Hz (wavelength, λ = 300,000,000 m). Now we know that antenna height required to transmit a signal is λ/4, so in order to transmit a signal of 1Hz we would require an antenna which is 75000 Km high. But if we are to transmit a signal of let’s say 100MHz (λ =3m) the antenna height required will be 0.75 meter. That could well be the reason to have low frequency message signal to be modulated over a very high frequency carrier signal.
In addition to this modulation enables simultaneous transmission of multiple signals. For example we can have number of radio stations transmitting the same voice bands over a number of frequency band carriers and still we can tune our radio set to one particular frequency to get the particular audio being transmitted.And yes, to increase the Signal to Noise ratio (SNR), we would need the modulation. SNR is actually the ration of useful information to the irrelevant information received.
SNR = PSignal / PNoiseSo in order to increase the signal to noise ration we need to increase the Power of signal to be transmitted and which is something achieved by Modulation.