What Is Modulation ? Why modulation is required? Types Of modulation

In this Article, we will talk about the modulation. And we will see what is modulation, why it is used in the communication, and what are the different types of modulation. 

Modulation

So, in a broad sense, communication is the exchange of information either wired or wirelessly between the two points which are far away from each other. So, this information could be voice signal, or email, or even a television signal. 

But here let's take the example of the voice signal. So, if we consider the speech signal or the human voice signal then it contains the frequencies up to 3 kHz. And if we consider the entire audible spectrum then it varies from 20 Hz to 20 kHz. So, this low frequency or the message signal is known as the baseband signal. And if we want to transmit this baseband signal to a far distance, then there are some challenges. 

So, we will talk about these challenges little later. But the thing is, the baseband signal cannot be transmitted directly. But with the help of the high-frequency periodic signal, it can be transmitted. So, this high-frequency periodic signal, which carries this baseband signal is known as the carrier signal.

But the question is how to impose this messagenor the baseband signal onto the carrier signal. Now, we know that any signal has three basic properties. That is amplitude, phase, and frequency. So, the modulation is the process, where one of these properties of the carrier signal like the amplitude, phase, or the frequency is changed according to the baseband signal. And based on which property is changed there are different types of modulation. But before that first of all, let's see, why this modulation is required. 

Why this modulation is required ?

1. The first reason to use this modulation is to reduce the antenna size. So, when the signal is transmitted wirelessly, then the size of the antenna is a very important parameter. So, this size of the antenna is proportional to the wavelength of the transmitted signal. And depending on the type of the antenna, it is the fraction of the transmitted wavelength. 

So, let's say, the size of the antenna is equal to λ/4. Now, the relationship between the frequency and the wavelength can be given by the following expression. So, for example, if the transmitted signal has a frequency of 10 kHz then its wavelength will be equal to c/f. That is equal to 3x 10^8 m/s divided by 10kHz. So, if we calculate the wavelength then twill come out as 30000 m. And if the side of the antenna is a quarter of the wavelength, then the length of the antenna will come around as 7500 m. And this size is actually impractical right!! On the other end, if the signal is transmitted at let's say 10 MHz, then the length of the antenna will reduce by the factor of 1000. That means in that case, the length of the antenna will be equal to 7.5 m. And in fact, this size of the antenna is manageable right !! So, that is one of the reasons for the modulation.

2. Then the second reason is to reduce interference. so, just imagine, in a single room if three persons are talking to the same person then that person won't get anything right !!Similarly, let's say, if 5 different voice signals which are around in the same frequency range are transmitted without the modulation through a single channel, then there will be interference between them, and at the receiver, we won't get the transmitted signal. But instead of that, if each signal is modulated at the different carrier frequency, and all carrier frequencies are relatively far away from each other, then there won't be any interference and all the signals can be demodulated at the receiver and the message signal of each transmitter can be retrieved. So, using the modulation, the interference can be reduced. And at the same time, the multiplexing of the different signals is also possible. And the example which I showed you earlier is the example of the frequency division multiplexing. Where the different message signals are modulated at the different carrier frequencies and because of that, they can be transmitted simultaneously. so, these are some of the important aspects, why the modulation is used in the communication. So, now let's see the different types of modulation.

Types of modulation

 So, broadly there are two types of modulation. 

• Analog modulation
• Digital modulation.

If the message signal is analog, then it is known as the analog modulation and similarly, 

If the message signal is in digital form or in terms of 1s and 0s then it is known as the digital modulation. 

first of all, let's talk about the analog modulation. 

Analog modulation

So, based on the type of the carrier signal, this analog modulation can be further classified into two categories. That is 

• Continuous-wave modulation
• Pulse modulation. 

If the carrier signal is a continuous wave signal like a sinewave signal then it is known as the continuous wave modulation. 

If the carrier signal is a pulse signal, then it is known as the pulse modulation.

Continuous-wave modulation

And first of all, let's see the different types of continuous-wave modulation.

So, mainly there are three types of modulation. That is 

• Amplitude modulation, 
• Frequency modulation
• Phase modulation.

So, in the case of the amplitude modulation, the amplitude of the carrier wave changes according to the message signal. So, if this is the message signal and the high-frequency signal is the carrier signal then the amplitude-modulated wave.

Similarly, in the case of the frequency modulation, thethe frequency of the carrier wave changes according to the message signal. So, with the increase in the amplitude of the message signal, the frequency of the modulated signal will also increase. Similarly, with the reduction in the amplitude, the frequency of the modulated signal will also reduce. 

And similarly, in case of the phase modulation, the phase of the modulated signal changes according to the message signal.

Pulse modulation. 

 let's see the different types of pulse modulation techniques. So, mainly there are four different types of modulation techniques. That is 

• Pulse-amplitude modulation
• The pulse width modulation
• The pulse position modulation
• The pulse code modulation. 

So, let's briefly see all these techniques one by one. So, in all these modulation techniques, the message signal is analog in nature while the carrier signal is a pulse train of the finite frequency. 

So in case of the pulse-amplitude modulation, the width or the duration of the pulse will remain the same, but the height or the amplitude of the pulse signal will change according to the message signal.

Similarly, in case of the pulse width modulation, the height or the amplitude of the pulse will remain the same, but the width of the pulse will change according to the message signal. So, here the message signal amplitude is sample dat the rising edge of each pulse and the width of the modulated signal is proportional to the sampled amplitude. And in fact, in the pulse width modulation,

Similarly, in case of the pulse position modulation, the width and the amplitude of the pulse will remain fixed but the position of the modulated signal will change according to the input signal. So, at each rising edge, the message signal is sampled, and based on the amplitude of the input signal, the position of the modulated signal will change. That means as the amplitude of the input signal increases, then this modulated pulse will further go away from the reference position. And in fact, it can be generated with the help of the PWM signal. That means by generating the pulse at every falling edge of the PWM signal, this PPM signal can be generated. Alright, so now let's move to the next modulation scheme.

So, in the pulse code modulation, the message signal is sampled at the finite interval, and the sampled value is quantized using the quantization process. And then using the analog to digital converter, the message signal is encoded. For example, if the sampled value is encoded in 5 bits, then for every sampled value, we will get a pulse code of 5 bits. So, this is a very brief overview of the pulse code modulation.

let's briefly talk about some of the digital modulation techniques.

Digital modulation

So, in a very broad sense, mainly there are three different techniques. That is 

• Amplitude shift keying
• Phase-shift keying
• Frequency-shift keying.

So, in all these digital modulation schemes, the message signal is in digital form. 

So, this amplitude shift keying is one form of digital amplitude modulation where the amplitude of the modulated signal changes according to the digital bitstream. In this scheme, binary signal 1 is represented by the fixed amplitude of the carrier wave. While the binary 0 is represented by 0.

Similarly, in the case of the frequency-shift keying, the frequency of the modulated signal changes according to the digital bitstream. So, here the binary 1 is represented by one frequency. While binary 0 is represented by a different frequency. 

And likewise, in the case of the phase-shift keying, the phase of the modulated signal is changed according to the digital bitstream. So, in a binary phase-shift keying, binary1 and 0 are represented by the different phases. And to be precise, they are 180 degree apart from each other. So, when there is a transition from 1 to 0in the message signal, or from 0 to 1 in the message signal, then there is a 180-degreephase shift in the modulated signal. So, that is a brief overview of the phase shift keying. So, in summary, we have seen that what is modulation, why it is used and we have briefly seen the different types of modulation techniques. So, in future videos, we will see some of these modulation schemes in detail. So, if you have any questions or suggestions, do let me know here in the comments section below.