 Okay, so wherever we last class, we have started this chapter, communication system, right? So we have a small, you know, a small topic left on this chapter, communication system that is modulation, okay? So this will take around maybe 30 to 40 minutes, okay? Write down guys, modulation and its necessity. We know that modulation is something which transmitter does, right? You have message signal, you have a message signal, okay? What transmitter does? It processes it, processing by the transmitter and then this message is transmitted in the medium, then this message goes in the medium, okay? You cannot just transmit the message signal, otherwise the message will just get died down, okay? So there has to be some processing that is done by the transmitter, okay? So this particular topic actually talks about what is this processing all about? This processing is the modulation, okay? Now the need for the modulation, there are three reasons for the modulation. There are three reasons why processing of the message signal is required, okay? The first is the size of the antenna, first one is size of antenna, okay? Now size of the antenna is very important when you design anything. You don't want size of the antenna to be, you know, hundreds of meters. So if you take typical message signal, right, like the frequency with which we speak, okay? It will be just few kilohertz, okay? Let us say we are speaking with 20 kilohertz frequency and that is what the frequency of the message signal will be, okay? So this becomes the message signal, fine? So this is an electromagnetic signal which travels with the speed of light, 3 into 10 raise to power, 8 meter per second, this is the speed of this message signal, fine? So it is seen that whenever you design a receiver antenna, there is a transmitter antenna which transmitted, which transmits the signal and one receiver antenna that receives the signal, fine? This transmits and this receives, okay? The size of the receiver, size of the receiver is of the order of lambda by 4, okay? Now can you quickly find out if you have this message signal, which is 20 kilohertz message signal, what is the wavelength of this message signal? For an EM wave, we have speed of light is equal to lambda into frequency. This relation you should remember. From this relation, quickly get the value of lambda. So lambda will come out to be, let's say if you substitute the values, speed of light is 3 into 10 raise to power 8, okay? This will be lambda into frequency that is 20 kilohertz, 20 into 10 raise to power 3, fine? So wavelength will come out to be 3 by 2 into 10 raise to power 4 meters, fine? So this is a very large wavelength. It runs into kilometers, okay? Like 15 kilometers. So you cannot have an antenna whose size is lambda by 4, right? So it becomes impractical. So that is why you need to process the signal, fine? You cannot just transmit it. So therefore, there needs to be some processing to be done, which will increase this frequency. So if you increase the frequency, if frequency increases, then automatically lambda will decrease because lambda into frequency, this is speed of light, okay? So that is the reason why I want to process this signal and somehow increase the value of frequency so that the value of lambda goes down and then you will require lesser size antenna, fine? So this is one of the reason why you need the processing or modulation, okay? Second one is effective power radiated by the antenna, right down? Effective power radiated by the antenna. The next reason for processing to be done is power radiated by antenna, okay? So power radiated by the antenna is proportional to 1 by wavelength square, are you getting it? So since power is proportional to 1 by wavelength square, so if wavelength is very large, if wavelength is very large, the power radiated by the antenna goes down very fast, okay? So what do you want? You want wavelength to be very less, okay? So if you want to transmit your message signal, okay? Which has very large wavelength, like few kilometers wavelength, okay? Then the power radiated by the antenna will come out to be very, very less, okay? So you don't want very less power to be transmitted. You want your signals which are getting transmitted to be strong enough, right? So that is why you need to do the processing, okay? So this is the second reason for processing, third reason, right down? Another reason for you to do the processing of message signal is mixing up of signals, right? Ultimately, these signals, they are EM waves, okay? So like for example, any EM wave, you can write down as, you know, a wave equation like this. So this is an EM wave. Now if you just throw without doing any processing all the signals in air, then there could be some interference, okay? So there will be whatever we have learned, constructive interference, destructive interference, anything can happen, all right? So because of that, I mean that's not a favorable situation, right? You don't want your signals to get mixed up and do some interference. So that is the reason why again you need to do some sort of processing so that these signals do not get mixed up, okay? So these are the three reasons which are there for you to do the processing of the message signal before throwing it in the medium, okay? These are the, these are just reasons why you need to do the processing. Now let us talk about the kind of processing, okay? Write down types of processing or types of modulation. Processing is modulation, okay? Okay, so basically our core purpose is what? Our core purpose is to increase the frequency, okay? Our core purpose to do modulation is to increase the frequency without losing the message signal. Getting it? You should be able to extract message at the receiver end, okay? So the way you do the processing is like this. You have a carrier wave which is called CT, okay? Carrier wave is a high frequency wave. This is a high frequency wave, write down. So it's a very high frequency wave and it will just rapidly change its, I'm not able to draw the, you know, very nice thing. So it is a very high frequency wave like this, okay? This is carrier wave. This is called carrier wave and then you have your message. This is the message wave which will have higher frequency or lower frequency. It will have much lower frequency, fine? So the message signal will be like this, okay? So this is a high frequency wave. This is called carrier and this is the message. So basically you want this message to be carried away by this high frequency wave. This is called modulation, okay? Now the way this message is carried by this high frequency wave, the way it get carried will give the name of the modulation, how this modulation is done, okay? Like for example, if you have to travel, let us say 100 kilometers or suppose you have to go to Kerala, okay? If you have to go to Kerala, you can go by train, you can go by bus, you can go by flight, okay? So it depends how you reach there, fine? So the mode of transport will determine, you know, will give the name, you travel by air or whatever it is, fine? So basically the way high frequency wave carry this low frequency wave will give the name to the type of modulation here we are dealing with, okay? So there are three common way of doing modulation. One is write down amplitude modulation which is called AM, amplitude modulation, fine? So what we do here is that we mix carrier wave and message wave in such a manner that the amplitude will vary with a frequency of message, okay? All of you please draw this, this is the envelope. First draw this, let then I will explain, okay? You can see that the amplitude of this wave, this amplitude of this modulated wave is changing and the frequency of the change of this modulation amplitude of this modulated wave has a frequency that is equal to the message signal, okay? But if you just look at the wave, if you just look at the wave, the wave is actually this which is rapidly changing, okay? So this is a very high frequency wave but then if you look at the way the amplitude is getting changing, the amplitude is following the frequency of the message, okay? So this is called amplitude modulation, fine? And then we have frequency modulation, so this is AM. So I think you might have heard of AM radio or FM radio, right? FM is frequency modulation. So what you receive in FM radio station is frequency modulated wave, okay? So in amplitude modulation, the frequency of change in model, frequency of change in amplitude is the frequency of message but in frequency modulation, the frequency itself changes, okay? So there is a frequency of change in frequency and that is equal to the frequency of the message, okay? The way you can visualize the frequency modulated wave is like this, all of you draw this. So you can see that the frequency is high in this zone, this is high frequency, okay? And this is lower frequency, getting it? So the frequency of this entire signal is changing. It fluctuates in a regular intervals of time and that fluctuation is the frequency of the message. So message is carried on the way the frequency is getting fluctuated, okay? So here this is amplitude modulation, this is frequency modulation. There is one more way of modulating and that is called the phase modulation. So this is phase modulation, PM, okay? Now the way the phase gets modulated, the diagram looks similar to amplitude modulation but I can explain better using the, you know, wave equation. For example, the wave equation is AM sin of omega t plus phi. I am not writing a kx here because I am keeping x to be fixed. I am observing everything at x equal to 0, okay? Now here you have three things in this wave. You have amplitude, you have frequency and you have phase, okay? So when the amplitude changes with the frequency of message, it becomes amplitude modulation. When the frequency itself changes with the frequency of the message, it is called frequency modulation, okay? And then when the phase changes with the frequency of the message, it is called phase modulation. Are you getting it? So you have any doubts? If you have any doubt, please type in the message, yes or no? You have doubts or not? Okay, okay. So I assume that Snigda is speaking on all of your behalf, okay, good, fine, all right. So this is what, I mean this is a very, very crude way of explaining modulation, okay? Modulation is a very involved topic. You will probably spend a month or so when you will do your engineering in electronics and communication just on modulation, okay? So I have just done a basic introduction.