C OMMUNICATION S YSTEMS By now, we have become familiar with some terms like information source, transmitter, receiver, channel, noise, etc. It would be easy to understand the principles underlying any communication, if we get ourselves acquainted with the following basic terminology. (i) Transducer: Any device that converts one form of energy into another can be termed as a transducer. In electronic communication systems, we usually come across devices that have either their inputs or outputs in the electrical form.
An electrical transducer may be defined as a device that converts some physical variable (pressure, displacement, force, temperature, etc) into corresponding variations in the electrical signal at its output. (ii) Signal: Information converted in electrical form and suitable for transmission is called a signal. Signals can be either analog or digital . Analog signals are continuous variations of voltage or current.
They are essentially single-valued functions of time . Sine wave is a fundamental analog signal. All other analog signals can be fully understood in terms of their sine wave components. Sound and picture signals in TV are analog in nature.
Digital signals are those which can take only discrete stepwise values. Binary system that is extensively used in digital electronics employs just two levels of a signal. ‘ ’ corresponds to a low level and ‘ ’ corresponds to a high level of voltage/ current. There are several coding schemes useful for digital communication.
They employ suitable combinations of number systems such as the binary coded decimal (BCD) * . American Standard Code for Information Interchange (ASCII) ** is a universally popular digital code to represent numbers, letters and certain characters. (iii) Noise: Noise refers to the unwanted signals that tend to disturb the transmission and processing of message signals in a communication system. The source generating the noise may be located inside or outside the system.
(iv) Transmitter: A transmitter processes the incoming message signal so as to make it suitable for transmission through a channel and subsequent reception. (v) Receiver: A receiver extracts the desired message signals from the received signals at the channel output. (vi) Attenuation: The loss of strength of a signal while propagating through a medium is known as attenuation. * In BCD, a digit is usually represented by four binary ( or ) bits.
For example the numbers , , , , in the decimal system are written as , , , and . would represent eight. ** It is a character encoding in terms of numbers based on English alphabet since the computer can only understand numbers.