What is Modulation

Modulation is a technique of superimposing a low frequency information signal over a high frequency carrier signal for its transmission. This is done by modulating one of the parameters of the high frequency carrier wave in accordance with the instantaneous amplitude of the information signal, also known as the modulating signal. The carrier wave can be modulated in terms of amplitude, frequency, phase etc. For example: – In case of amplitude modulation, the amplitude of the carrier wave is varied in accordance with the amplitude of the modulating wave. It’s maximum at the positive peak of the modulating wave, equal to the original carrier amplitude when the modulating wave amplitude is zero and is minimum at the negative peak of the modulating wave. The frequency of the carrier in this modulation scheme remains constant throughout.

Modulation can be either analog or digital depending upon the nature of the information signal. Digital modulation techniques are known as shift keying techniques and these can be binary or M-ary depending upon the number of bits represented by a modulated symbol. A few examples of digital modulation techniques are:-Amplitude Shift Keying (ASK), Gaussian Minimum Shift Keying (GMSK), Phase Shift Keying (PSK) etc. On the receiver side, this high frequency modulated wave is demodulated to receive the original modulating or information signal.

Modulation is essential in any practical communication system because:-

The original low frequency information signal is not capable of long distance transmission due to its rapid attenuation by the communication media especially in case of the atmosphere. So it is superimposed on a high frequency carrier wave to achieve a modulated wave of a much higher energy that can be transmitted to long distances. The height of antenna for the transmission of a signal is given by the formula lambda/4, where lambda is the wavelength of the signal. If the original information signal is to be transmitted, then the antenna height required will be in kilometers. This is not practically possible and so to reduce the antenna height required, modulation is necessary. If all the information signals from different source stations are transmitted in to the atmosphere without any modulation, then they will interfere with each other and ultimately destroy all the information. Modulation allows these source stations to use mutually exclusive frequency bands for their transmissions which prevent such a scenario.

Attempting to transmit the information signal over longer distances without the use of modulation is very much like trying to shout out an audio message with your mouth over the whole perimeter of the coverage area which is both impractical and impossible. Therefore, modulation is of uttermost importance is any communication system.

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