Modulation

Modulation is a technique which is used to enhance the capability of an information signal to be transmitted over a longer distance, safely and securely. In simple words, modulation is addition of a simple message signal to a high frequency carrier signal which is capable of being transmitted over a longer distance thus enabling the message signal to be transmitted.

Modulation is achieved by varying one or more properties (amplitude, frequency or phase) of a periodic waveform called carrier signal according to the massage signal.

For example, in the diagram below, we have a low frequency message signal which is required to be transmitted. But in order to transmit it we would need the message to be modulated over a high frequency carrier signal. And on the receiver side we would require a demodulator which will eliminate the carrier wave and extract the message signal.

Now the question is how modulation enables a signal to be transmitted over a longer distance. Let me give you an example. Let’s suppose we are to transmit a signal of a very low frequency of 1Hz (wavelength, λ = 300,000,000 m). Now we know that antenna height required to transmit a signal is λ/4, so in order to transmit a signal of 1Hz we would require an antenna which is 75000 Km high. But if we are to transmit a signal of let’s say 100MHz (λ =3m) the antenna height required will be 0.75 meter. That could well be the reason to have low frequency message signal to be modulated over a very high frequency carrier signal.

In addition to this modulation enables simultaneous transmission of multiple signals. For example we can have number of radio stations transmitting the same voice bands over a number of frequency band carriers and still we can tune our radio set to one particular frequency to get the particular audio being transmitted.

And yes, to increase the Signal to Noise ratio (SNR), we would need the modulation. SNR is actually the ration of useful information to the irrelevant information received.

                                                SNR = PSignal / PNoise

So in order to increase the signal to noise ration we need to increase the Power of signal to be transmitted and which is something achieved by Modulation.

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Introduction to Communication Systems

A system used to send information from one place to another is called a Communication System. A communication system can be as simple as talking to a friend or talking over a string telephone and can be as complex as a guided anti-missile system.
On the basis of information to be transmitted communication systems can be classified as:

Physical Communication System: This is a communication system the information is transmitted in physical form from one place to another.

Optical Communication System: In Optical communication the information is transmitted in the form of light over the communication channel (which is optical fiber) from one place to another.

Electronic Communication System: Here in this case the information is transformed to an electrical signal and sent over from one place to another.

In figure 1.1, a string telephone is illustrated as a basic communication system where a normal voice in its physical form is being transmitted over a string. With simplicity comes the limitation. Although this is a very easy to build easy to use communication but the question is how effective is this if one has to transmit the voice over a distance of a kilometer. This might not be possible using a string telephone because of two reasons. First one being the lesser power being transmitted through the string and then the increased interference from the medium over a longer distance. That can be overcome by using an Electronic Communication System.

 

While in a string telephone a physical signal is transmitted, in an electronic communication system an electronic signal is transmitted. An electronic signal can be lesser prone to interference as compared to the physical signal and can be transmitted on a higher power level and of course over a longer distance. A land line telephone is an example of Electronic Communication System.

Now we are in a position to identify the basic elements of a communication system. As illustrated in the figure below a communication system consists of Communication Message, Source, Transmitter, Channel, Receiver and Destination. Noise is an unwanted addition to the communication, which we can hardly eliminate from the communication system.

 

Source of Message: Message or Input information is something which we want to send over. It can be a plain text, voice or a video and the source is the origination of the actual message to be transmitted by the communication system. Sometimes the Source of information is coupled with an input transducer. Transducer is a device which converts one form of energy to another. Here the input transducer converts the information to be transmitted to its electrical equivalent message signal. The input information is converted to electrical signals because they travel with the speed of light and the information can be transmitted in a faster way.

Example: Microphone is an input transducer which converts audio input to an electrical signal.

Transmitter: Transmitter is a device that makes input electrical information suitable for efficient transmission over a given channel. The original signal is baseband signal and the transmitter modulates or changes the characteristics like amplitude, frequency or phase of the high frequency carrier signal according to input electrical information and send it over the communication channel.

Channel: The information generated at source side may need to travel hundreds or thousands of miles via channel to reach destination. Communication channel is the media by which information is sent. The channel could be a wired line such as copper wire, optical fiber or it could be a wireless medium (atmosphere).

Noise and Distortion: Noise or distortion is the unwanted interference on transmitted signal.  It is the phenomenon which changes the shape of the communication signal that may mislead the destination about the content of the message transmitted. As the signal travels through channel, there is loss in the strength or amplitude of the signal which is known as attenuation. Noise is unwanted interference on transmitted signal. There are two types of noise, Internal and External Noise.

è Internal Noise: It is the distortion of signal due to thermal motion of electrons or random emission, diffusion, recombination of carriers.

è External Noise: It is the interference caused by outer world which includes the other communication signals, lightening, electrical switching, automobile ignition etc.

Destination/Receiver: Destination is the point we are trying to communicate the information to. Receiver is the device that receives information from channel and extracts the actual message form the received signal. It also amplifies the signal and removes noise or distortion from the attenuated received signal. Sometimes the receiver is coupled with a transducer at the destination side which converts electrical output to a form of message required by user, for example speech, image video text etc. The loud speaker is an example of output transducer where electrical input is converted to an audio output.

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