Introduction to Communication Systems
Electronic Communication
Communication is the process of exchanging information. The basic components of a communication system are: Source, transmitter, channel, receiver and destination (user of information). Below figure depicts the general form of an electronic communication system.
Block diagram of Electronic Communication System
Description of a communication system is given as under:
- The sender (Information Source) who creates the message to be transmitted
- A medium that carries the message
- The receiver (User of Information) who receives the message
INFORMATION SOURCE
A source is a device that generates messages or data to be transmitted. It may be a simple microphone, computer keyboard. The messages generated by source can be in analog or digital form. An example of analog and digital information sources is given below.
- Analog Information Sources: Microphone, camera, TV signal
- Digital Information Sources: Teletype, output of computer – consists of discrete symbols or letters.
TRNSMITTER
First job of transmitter is to convert input messages produced by source (ex: voice signal) into electrical form suitable for transmission. For voice messages, a transducer called microphone is used to translate sound into an electronic audio signal. For TV, a camera converts the light information into video signal. Transducer is the general term given to a device that converts one form of energy into another form. Note that speaker is also a transducer device, which is used to convert electrical signal back to voice signal.
Also note that modulation is the main function of the transmitter. In modulation, message signal is mathematically mixed or superimposed with high frequency carrier signal. Other functions of transmitter include, restriction of range of audio frequencies, amplification etc. Note that all these processing are done just to ease the transmission of signal from transmitter to receiver.
Advantages of modulation are: long distance transmission, high speed of transmission. Modulation also allows the reliable (practically achievable) sizes of antennas at transmitter and receiver.
CHANNEL
In a communication system, the transmitter is located at one place and the receiver is located at some other place. Channel is the physical medium that connects them electrically. Channel is the medium through which message signal is transmitted. Depending upon the type of communication system, a channel may be in the form of wires or cables. The different channels are: Copper wires, Coaxial cable, Optical fibre, Radio links, Satellite channel or combination of any of these.
Suppose, you are calling a person in USA, the call may go through air, undersea, and space etc. So here the channel is combination of wires, radio connection, satellite and undersea cabling. The important parameters of the channel are Bandwidth and Signal to Noise Ratio (SNR). SNR is the ratio of signal power to nose power. Ideally signal power should be very high and noise power should be zero. Other important characteristics of the channel are BANDWIDTH and POWER.
NOISE
Noise is unwanted signal that disturbs, interferes and affects the transmitted signal. We cannot prevent it but we can minimize it. Noise is a random signal and its behaviour can’t be predicted. The measure of noise is usually expressed in terms of SNR (Signal to Noise ratio). SNR is the ratio of two quantities: Signal power and Noise power. Decibel (dB) unit is used to indicate SNR values. Note that a very high SNR is preferred for best performance.
RECEIVER
The main function of the receiver is to reconstruct the transmitted signal and delivers it to the destination called user of information. It accepts the transmitted message from the channel and converts to a form understandable by humans. Receivers contain amplifiers, oscillators, mixers, tuned circuits, filters and a demodulator (detector). Note that demodulation is the process of removing carrier from transmitted signal. Functions in receiver can be grouped as: decoding, decompressing, error detection and demodulation. The output of a receiver may be a voice signal, video signal or computer data.
TRANSCEIVER
Most electronic communications are two-way. So, both end users must have ability to send and receive messages. As a result, most communication equipment incorporates both transmitter and receiver in a single unit. All the transmitter and receiver circuits are packaged within a single circuit and referred to as transceivers. Telephones, fax machines, cellular phones and computer MODEM are examples of transceivers.
DESTINATION (User of Information)
It accepts the messages from the receiver. The destination may be a simple telephone speaker, computer screen.
