Modulation
Modulation is an important step of communication system. Modulation is defined as the process whereby some characteristic (line amplitude, frequency, phase of a high frequency signal wave (carrier wave) is varied in accordance with instantaneous value intensity of low frequency signal wave (modulating wave.)
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Need for modulation :
i) To separate signal from different transmitters :
Audio frequencies are within the range of 20 Hz to 20 kHz. Without modulation all signals at same frequencies from different transmitters would be mixed up. There by giving impossible situation to tune to any one of them. In order to separate the various signals, radio stations must broadcast at different frequencies.
Each radio station must be given its own frequency band. This is achieved by frequency translation as a result of modulation process.
ii) Size of the antenna :
For efficient transmission the transmitting antennas should have length at least equal to a quarter of the wavelength of the signal to be transmitted. For an electromagnetic wave of frequency 15 kHz, the wavelength λ is 20 km and one-quarter of this will be equal to 5 km. Obviously, a vertical antenna of this size is impractible. On the other hand, for a frequency of 1 MHz, this height is reduced to 75 m.
Also, the power radiated by an antenna of length l is proportional to (l/λ). This shows that for the same antenna length, power radiated is large for shorter wavelength. Thus, our signal which is of low frequency must be translated to the high frequency spectrum of the electromagnetic wave. This is achieved by the process of modulation.
iii)To Reduce Antenna Height.(practicability of antennas height).
iv)Transmit The Information To long Distance Without interference.
v) Reduce band width.
Other answers have described modulation as it applies to communications applications, where the information to be conveyed is superimposed onto a carrier. My answer will apply the concept of modulation to power electronics applications, particularly in DC-AC inverters.
Refer to Figure 1. This shows a typical 3-phase inverter bridge. Six electronic switches are arranged in three-phase bridge configuration, and are supplied by a DC voltage source. These switches are high-power electronic devices. The devices are shown as transistors, but can be SCRs, GTOs, Darlington transistors, MOSFETs or IGBTs. The most suitable devices will be governed by conversion power, voltage level and switching frequencies requirements. The load is shown as an induction motor, but may be the primary of a three-phase transformer or any other A.C. load.

Figure 1: Three-phase Inverter Switches and Waveforms
If each of the three phases are switched in this manner, but with the switching of each phase 120 electrical degrees delayed from the previous phase, the three waveforms of the centre points (VAO, VBO, VCO) are as shown in the figure. If the difference in voltage between any two phases is measured, (VAB in the figure), the result is a 6-step or quasi-square wave. VBC and VCA are the same except are phase shifted 120 degrees from each other.
Thus the output of the three-phase inverter bridge is a three-phase waveform. The frequency of the waveform is set by the switching frequency. The RMS voltage is controlled by modulating the waveform. In practice, the waveform is pulse width modulated with chops.. These chops are of varying size. The purpose is twofold: to make the inverter output currents more closely resemble a sine wave, and to enable adjustment of the RMS output voltage.

Figure 2: Sinusoidal Modulation of Inverter Output Waveform
A method of modulation often used is sinusoidal modulation. . Refer to Figure 2. A triangle waveform is compared with a sine waveform. When the sine wave exceeds the triangle wave, the top switch of the inverter phase is turned on. When the triangle wave exceeds the sine wave, the bottom switch is turned on. The resultant current waveform to the load is nearly sinusoidal with very little harmonic distortion. If the load is an induction motor, no further filtering is required.
If the inverter is fixed frequency, such as in a UPS, then further filtering is needed. The harmonic distortion content is a function of the modulation frequency of the carrier wave. At a high modulation frequency, the first significant harmonic will be at high frequency and low magnitude, which means that harmonics are relatively easy to filter out with a low-pass LC filter. Typical modulation frequency with IGBT power switches is 4kHz.
In the process of modulation, the baseband signal is translated i.e., shifted from low frequency to high frequency . This frequency shift is proportional to the frequency of carrier.
Advantages of Modulation
1. Reduction in the height of antenna
2. Avoids mixing of signals
3. Increases the range of communication
4. Multiplexing is possible
5. Improves quality of reception
We will discuss each of these advantages in detail below .
1. Reduction in the height of antenna
For the transmission of radio signals, the antenna height must be multiple of λ/4 ,where λ is the wavelength .
λ = c /f
where c : is the velocity of light
f: is the frequency of the signal to be transmitted
The minimum antenna height required to transmit a baseband signal of f = 10 kHz is calculated as follows :
The antenna of this height is practically impossible to install .
Now, let us consider a modulated signal at f = 1 MHz . The minimum antenna height is given by,
This antenna can be easily installed practically . Thus, modulation reduces the height of the antenna .
2. Avoids mixing of signals
If the baseband sound signals are transmitted without using the modulation by more than one transmitter, then all the signals will be in the same frequency range i.e. 0 to 20 kHz . Therefore, all the signals get mixed together and a receiver can not separate them from each other .
Hence, if each baseband sound signal is used to modulate a different carrier then they will occupy different slots in the frequency domain (different channels). Thus, modulation avoids mixing of signals .
3. Increase the Range of Communication
The frequency of baseband signal is low, and the low frequency signals can not travel long distance when they are transmitted . They get heavily attenuated .
The attenuation reduces with increase in frequency of the transmitted signal, and they travel longer distance .
The modulation process increases the frequency of the signal to be transmitted . Therefore, it increases the range of communication.
4. Multiplexing is possible
Multiplexing is a process in which two or more signals can be transmitted over the same communication channel simultaneously .
This is possible only with modulation.
The multiplexing allows the same channel to be used by many signals . Hence, many TV channels can use the same frequency range, without getting mixed with each other or different frequency signals can be transmitted at the same time .
5. Improves Quality of Reception
With frequency modulation (FM) and the digital communication techniques such as PCM, the effect of noise is reduced to a great extent . This improves quality of reception .
We need to modulate signal for following reason basically :
1: Transmission of signal to longer distance : For transmitting a signal to the longer distance you need to have maximum amplitude and frequency which is impossible to acquire from original signal.
2: Multiplexing the signal: Modulating signals on different frequencies give us the option to mix and send multiple signals through a single channel and separate out easily.
3: Reduction in Antenna height: Well, Antenna theory will be helpful for understanding this. Simply I remembered that height H of antenna is proportional to the wavelength (Lamda) of the signal.
4: Noise immunity: Modulated signals are high frequency high amplitude signals and hence noise is not so easily affecting the information as every modulation technique using different phenomena to store data.
5: Noise availability: Modulated signals consist frequencies and amplitudes decided by the designer and so according to the channel noise and interference we can decide parameters and avoid noise.
6: Ease of transmission and reception: As we know baseband i.e. original signals can't be transmitted directly as they are not supported by most of channels and also fade away after particular distance, so we need modulation.
This one's easy. I'll give you two instances why modulation is needed.
INSTANCE 1:
Consider you're on a bank of one side of a river and I'll give you a paper and tell you to throw it to the other side of the bank! Is it possible to throw the paper which doesn't have suitable mass to other side of the river?
The answer is No. So in order to make the paper reach the other side of the river I'll use a stone with the paper and throw it, now it'll reach the other side.
In communication the message signal will be of low frequency which is like paper (light weight) in order to make it heavy, a carrier signal(stone) is used to send data from transmitting station to the receiving station.
INSTANCE 2:
Consider you have food and you have to take it to school for lunch, will you carry the food with your hand? No right? You'll use a carrier box and you'll eat the food not the carrier box.
In communication food(message signal) cannot be transmitted as it is, it needs a carrier box(carrier signal)to which is used to enclose the message signal from the transmitter end and carry the modulated signal through the medium and at the receiver the carrier box(carrier signal) is removed by demodulation it removes the envelope and obtains the food(message signal).
This is why modulation and demodulation is required in communication systems.
Dictionary definition: changing any one of the parameters of carrier signal such as amplitude, frequency or phase of the signal with respect to the input voltage of the message signal at that particular instance of time and keeping the other parameters constant is called as modulation.
Few other reasons why modulation is required is as follows.
1) TO REDUCE THE HEIGHT OF THE ANTENNA
2) TO MULTIPLEX MORE NUMBER OF SIGNALS
3) TO REDUCE THE NOISE & DISTORTIONS
4) TO NARROW BANDING THE SIGNAL
5) TO REDUCE EQUIPMENT COMPLEXITY
Very simply because of physics. Let's say you want to communicate from one person to another using just a really really long wire (cable communications). The way that you model a long wire is using a infinite number of capacitors and inductors, as shown below:

For simplicity, you can say that both connections have a shared ground, so all the bottom inductors don't participate, and all the capacitors connect to ground on the bottom. If we say all the capacitances are the same and all inductances are the same (again an approximation that the wire is homogeneous), and you sum these infinitely, you will get a DC resistance. What this means it that this lossless long wire (meaning it has no resistors in it), will transmit nothing at DC, because all of the voltage will drop across the resistor.
Thus the only way to transfer anything across a long wire is through AC (alternating current). Thus you need high frequency modulation in order to communicate.
In the modulation process, two signals are used namely the modulating signal and the carrier .
The modulating signal is nothing but the baseband signal or information signal while the carrier is a high frequency sinusoidal signal .
In the modulation process, some parameter of the carrier wave (such as amplitude, frequency or phase ) is varied in accordance with the modulating signal . This modulated signal is then transmitted by the transmitter .
The receiver demodulates the received modulated signal and gets the original information signal back .
Thus, demodulation is exactly opposite to modulation .
In the process of modulation the carrier wave actually acts as carrier which carries the information signal from the transmitter to receiver .
Need of Modulation
You may ask, when the baseband signal can be transmitted directly why to use the modulation ?
The answer is that the baseband transmission has many limitations which can be overcome using modulation . It is explained below .
In the process of modulation, the baseband signal is translated i.e., shifted from low frequency to high frequency . This frequency shift is proportional to the frequency of carrier.
Advantages of Modulation
1. Reduction in the height of antenna
2. Avoids mixing of signals
3. Increases the range of communication
4. Multiplexing is possible
5. Improves quality of reception

Modulation is a process in which the feature (amplitude, phase or frequency ) of the carrier signal is changed in accordance with instantaneous value of modulating signal.
Modulation is needed for many reasons. Some of them are given below :
• To increase the bandwidth of the signals
• To reduce the antenna size.
• To reduce the noise and distortions.
• To multiplex the signals
• To reduce the interference at the time of transmission of the information.
In a carrier communication system, the baseband signal of a low-frequency spectrum is translated to a high frequency spectrum. This is achieved through modulation. The aim of this topic is to explore the reasons for using modulation. Modulation is defined as a process by virtue of which, some characteristic of a high frequency sinusoidal wave is varied in accordance with the instantaneous amplitude of the baseband signal.
Two signals are involved in the modulation process. The baseband signal and the carrier signal. The baseband signal is to be transmitted to the receiver. The frequency of this signal is generally low. In the modulation process, this baseband signal is called the
modulating signal. The waveform of this signal is unpredictable. For example, the waveform of a speech signal is random in nature and cannot be predicted. In this case, the speech signal is the modulating signal.
The other signal involved with the modulation is a high frequency sinusoidal wave. This signal is called the carrier signal or carrier.
The frequency of the carrier signal is always much higher than that of the baseband signal. After modulation, the baseband signal of low frequency is transferred to the high frequency carrier, which carries the information in the form of some variations. After the completion of the modulation process, some characteristic of the carrier is varied such that the resultant variations carry the information.
The carrier signal is represented by the equation:
A=E sin(ωt + ϕ)——(1)
equation-(1) is an indicator that this equation represents the carrier signal. The components of this equation arc as follows:
• A: Instantaneous amplitude of the carrier
• E: Amplitude of the carrier
• ϕ: Initial phase of the carrier signal
• Angular frequency of the carrier, such that = 2πfc, Where fc is the frequency carrier, also called the central frequency
Equation (1) has three parameters namely, amplitudes (E), frequency (ω), and phase (f). In principle, these parameters have constant values for a particular sinusoidal wave. According to the definition of Modulation, some characteristic of the carrier signal is varied in accordance with the modulating signal. After modulation any one of the three parameters of the carrier signal, namely, frequency, or phase, is varied keeping the remaining two constant.
The baseband signal is then carried by these variations. The type of the modulation is decided by the parameter chosen to vary.
For example, if amplitude of the carrier is chosen to vary in accordance with the instantaneous amplitude of the baseband signal, keeping frequency and phase constant, the resulting modulation called amplitude modulation. Frequency modulation and phase modulation are also obtained in a similar way.
Low-frequency baseband sign it is thus translated to a high frequency carrier such that the information is coded in the variations in one of the parameters of the carrier. At the receiver side, these variations are detected through the demodulation process to recover the original baseband signal.
The following can be summarized with reference to modulation.
• The baseband signal is known as the modulating signal.
• The baseband signal is a low-frequency signal.
• The carrier signal is always a high frequency sinusoidal wave.
• After modulation, the carrier is said to be modulated by the modulating.
• The output of the modulator is called the modulated signal.
During the modulation process, the modulating, signal varies the frequency, y, amplitude, or phase of the carrier in accordance with its instantaneous amplitude.
The process of modulation in a communication system increases its cost and complexity. This may be considered as a disadvantage. However, modulation is extensively used in most communication systems. There is a definite need for using modulation. There can be problems if modulation is not used. Scrutinizing these problems can explain why modulation is required.
The baseband signal will be transmitted as it is. If modulation is not employed however, the system designer could confront the fallowing problems:
1) TO REDUCE THE ANTENNA HEIGHT
2) TO MULTIPLEX THE MORE NUMBER OF SIGNALS
3) TO REDUCE THE NOISE & DISTORTIONS
4) TO NARROW BANDING THE SIGNAL
5) TO REDUCE EQUIPMENT COMPLEXITY
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Source: https://www.quora.com/Why-do-we-need-modulation