A message signal usually spreads over a range of frequencies, called the signal base width. That is why message signals are also called base band signals, representing the band of frequencies of the original signal. Suppose we wish to to transmit an electrical signal in the audio frequency (AF) range (20 Hz to 20 kHz) over a long distance. we cannot do it directly thus there is necessity of modulation.

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Necessity of modulation?

• Size of antenna or aerial

An antenna or aerial is needed, both for transmission and reception. Each antenna should have a size comparable to the wavelength of the signal, (at least ), so that time variation of the signal is properly sensed by the antenna.

For an audio frequency signal of frequency f = 15 kHz, the wavelength, .

The length of the antenna = \dfrac{\lambda}{4} = \dfrac{20000}{4} = 5000 [/latex] meter. To set up an antenna of vertical height 5000 meter is practically impossible to construct and operate.

If transmission frequency were raised to 1 MHz, then .

The length of antenna would be , i.e., 

As high powers are needed for good transmission, therefore, for given antenna length, wavelength  should be small or frequency f should be high.

Thus, this factor also points out to the need of using high frequency transmission.

• Mixing up of signals from different transmitters.

When my people are talking at the same time, we just cannot make out who is talking what? Similarly, when many transmitters are transmitting baseband information signals simultaneously, they get mixed up and there is on way to distinguish between them. The possible solution is, communication at high frequencies and allotting a band of frequencies to each transmitter so that there is no mixing. This is what is being done for different radio and T.V. broadcast stations.

All the three reasons explained above suggested that there is a need for transmission at high frequencies. This is achieved by a process, called modulation.

What is Modulation?

Basic to the field of communications is the concept of modulation. Modulation is the process of putting information onto a high-frequency carrier for transmission. In essence, then, the transmission takes place at the high frequency (the carrier) which has been modified to “carry” the lower-frequency information. The low-frequency information is often called the intelligence signal or, simply, the intelligence. It follows that once this information is received the intelligence must be removed from the high-frequency carrier – a process known as demodulation. At this point you may be thinking, why bother to go through this modulation/demodulation process? Why not just transmit the information directly? The problem is that the frequency of the human voice ranges from about 20 to 300 Hz. If everyone transmitted those frequencies directly as radio waves, interference would cause them all to be ineffective. Another limitation of equal importance is the virtual impossibility of transmitting such low frequencies since the required antennas for efficient propagation would be miles in length. Thus, there is necessity of modulation.

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The solution is modulation, which allow propagation of the low-frequency intelligence with a high-frequency carrier. The high-frequency carriers are chosen such that only one transmitter in an area operates at the same frequency to minimize interference, and that frequency is high enough so that efficient antenna sizes are manageable. There are three basic methods of putting low-frequency information onto a higher frequency.

So we can define modulation as ‘Modulation is the process by which some characteristics (amplitude, frequency or phase angle) of a voltage (usually sinusoidal voltage), carrier voltage is varied in accordance with the instantaneous value of some other voltage called the modulating voltage.’

Let the carrier voltage be sinusoidal and its instantaneous value v_c at time t be given by,

 …….(1)

Where.

V_c is the amplitude of the carrier voltage,

 is the angular frequency of carrier in radians/second?

 is the phase angle

Then, 

Where, fc is the frequency of the carrier in Hz.

Types of Modulation

During the process of modulation, any one of the three parameters viz Vc;  and  of the carrier wave is varied in accordance with information or message signal, which is the modulation base band audio frequency signal m(t). This would result in three types of modulation.

1. Amplitude Modulation: In amplitude modulation, the amplitude of the carrier is varied according to the instantaneous value of the modulating voltage.
2. Frequency Modulation: In frequency modulation, frequency of the carrier is varied according to the instantaneous value of the modulating voltage.
3. Phase Modulating: In phase modulating, phase angle of the carrier is varied according to the instantaneous value of the modulating voltage.

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Source: electronicspani