Communication is a process of transmission and reception of various information’s / messages and for its success it is necessary that the sender and receiver understand a common language which may be lucid or complex. Historically a longer distance communication started with the discovery of telegraphy. But the 1st telegraphy was introduced in 1938 at USA and Europe communication link started in 1850. An Italian scientist Marconi 1stly established the radio transmission in 1901. In India JC Bose made a significant contribution in this regard. The first coastal telephone service was started in 1915, while co-axial cables with multi channels were laid down in 1941. The Telstar satellite was launched in 1962 and through it the satellite communication era began. The first communication satellite (geo-stationary satellite) ’Early Bird’ was launched in 1965 which was useful for both North American and European continents. In 1970 optical fiber communication started in USA and Trans-Atlantic optical fibers were laid down in 1988 and through it a revolution took place in the entire field of the communication.
A communication system is a well furnished setup used in the transmission-reception of information/ message from one place to another. At present broadly communication system has its two types-electrical communication system and optical communication system. Every communication system has three essential elements Which can be stated as bellow-
(a) Transmitter : It processes and encodes the information/ message and makes it suitable for the transmission. But the information/ message signal for communication can be analog or digital.
(b) Communication Channel : The medium through which information/ message’ propagates from transmitter to receiver is called communication channel. The link is an example of communication channel in which many channels may coexist together.
(c) Receiver : The receiver of the communication system receives and decodes the signal of information/ message.
Important terms associated to Communication System
1 . Signal : Information / message which is converted into the electrical form and which is suitable for the transmission is called a signal and it can be analog or digital.
(a) Analog Signal : The signal in which current or voltage changes its magnitude (amplitude) continuously with time is called an analog signal. The examples of analog signals are speech, concert, music etc.
(b) Digital Signal : The signal in which current or voltage have only two discrete values then it is called a digital signal. The examples of digital signals are letters printed of a novel, listing of any data on the computer, all the operations of digital electronics etc.
But an analog signal can be converted directly and suitably into a digital signal and vice-versa. The analog signals are found usually in the form of pulses.
2. Transducer : Any device (system) which converts any physical signal into the electoral signal is called transducer. The simple example of transducer is a micro phone which converts speech signals into electrical signals.
3. Noise : The disturbance/ distortion in the transmission and processing of information/ message signals in a communication is called noise. A better communication system has negligible noise.
4. Attenuation : The loss of strength of a signal during its propagation through the communication channel is called attenuation. In every communication system attenuation more or less exists.
5. Amplification : The process of increasing the strength of the transmitted signal using some suitable electronic circuit like transistor is called amplification and employed device is called amplifier.
6. Range : It is the largest distance between the transmitter and receiver where the signals are received in the proper strength without any noise attenuation.
7. Bandwidth : It is usually the range of frequencies over which the communication system operates.
8. Repeater : It is a combination of transmitter, amplifier and receiver which is frequently used in the communication system specially where the extended range of communication is required.
Transmission medium/Communication Channel
This is a link through which information message signal transmits from the source to destination without any noise or distortion. Transmission medium/ communication channel is of two types-
(a) Guided transmission Medium : It is a communication medium/ channel which is used in point-to-point communication between a single transmitter and a receiver. Optical fibres are the major example of guided transmission medium which is used in the line communication. Thus parallel wire lines, twisted pair, co-axial cable etc are the examples of guided transmission medium.
(b) Unguided Transmission Medium : It is a communication medium in which point-to-point contact between the transmitter and receiver is not available. Free space is an example of unguided transmission medium. Thus it is used in space communication and satellite communication. That’s why in free space radio waves transmit easily whose frequency range in 10^5 Hz-I0^9 Hz which is subdivided further as the following table-
The original low frequency of information message signal can not be transmitted to a longer distance. 80 this low frequency signal requires superposition with a high frequency carrier wave and the process involved is called modulation. Here the low frequency signal is called modulating signal / wave and the resulting transmitted wave is called modulated wave.
Types of Modulation There are three types of modulation :
(a) Amplitude Modulation (AM) : In amplitude modulation, the amplitude of high frequency carrier wave is varied in accordance to the instantaneous amplitude of the modulating signal. Such type of modulation is generally used in the commercial transmission of audible signals / waves.
(b) Frequency Modulation (FM) : In frequency modulation, the frequency of high frequency carrier wave is varied in accordance to the instantaneous frequency of the modulating signal. Such type of modulation is generally used in the transmission of television.
(c) Pulse (Phase) Modulation (PM) :
Demodulation is the inverse process of modulation which is performed in a receiver to recover the original modulating signals.
Thus demodulator performs the following functions
1. To select the desired signal and to reject the unwanted signals.
2. To amplify and to demodulate the desired signal.
3. To display the original modulating signal in a descent manner.