♦ Learn about : communication system , Transmitter , Communication channel , Receiver , Information , Channel Noise & Block diagram of Transmitter & Receiver ♦
A communication system is the set-up used in the transmission of information from one place to another . The present day communication systems are electrical , electronic or optical in nature.
A schematic model of an electrical communication system is shown in figure .
It consists of three major parts:
Transmitter , Communication channel and Receiver.
A transmitter transmits the information after modifying it to a form suitable for transmission. This modification is achieved by a process called Modulation.
The communication channel carries the modulated wave from the transmitter to the receiver. A receiver reconstructs the original message after propagation through the channel. This is achieved by a process, called ‘ Demodulation ’, which is the reverse of modulation.
(i) Information : Information itself is that which is conveyed. The amount of information contained in a message is measured in bits. The set or total number of messges, consists of individual messages-which may be distinguished from one another.
(ii)Transmitter : For sending information, the incoming signals are converted into electrical variations. A transmitter is required to process and possibly encode the incoming information so as to make it suitable for transmission and subsequent reception.
In a transmitter, the information modulates the carrier i.e. information is impressed on a high frequency carrier wave. The system may involve amplitude modulation, frequency modulation, pulse modulation or any combination of these.
(iii)Communication channel : the communication channel carries the modulated wave from the transmitter to the receiver. In case of telephony and telegraphy, communication channel is a transmission line. In optical communication, an optical fibre is the communication channel and in radio / TV broadcasting, communication channel is the free space itself.
(iv)Channel Noise : The term ‘channel’ usually refers to the frequency range allocated to a particular service or transmission.
Noise is unwanted energy, usually of random character, present in a transmission system, due to some known or unknown causes. Noise may distort the information at any point in a communications system. Obviously, noise will have its greatest effect, when the signal is weakest.
(v) Receiver : The most important function of receiver is demodulation and sometimes decoding as well. Both these processes are the reverse of the corresponding transmitter (modulator) processes.
Receivers range from a very simple crystal receiver with head phones, to a complex radar receiver. The output of a receiver may be fed to a loud speaker, video display unit, tele typewriter, TV picture tube, pen recorder or computer.
Note that the transmitter and receiver must be in agreement with the modulation and coding methods used.
The schematics of an arrangement for transmission and reception of a message signal are shown in figure :