Communication system

Communication means sharing one’s thoughts with others. It is a bidirectional process. Electronics communication deals with transmission, reception and processing of information by means of electrical signals. Telephones, telex/telegraph systems provide communication of voice and written message from one place to another where it could be stored, processed or retransmitted to other places. The systems that may be thoughts as one way communication system include radio/TV broadcasting systems. The different signals that are used in communication system include analog signals, telephone signals and TV signal, radio broadcast signals, digitals signals etc. The transmission medium may be a twisted pair of wires, coaxial cable or an optical fibre. If the communication is through air medium, it is called wireless communication.

The interaction between communication and computer technology and the application of satellite and the fibre optics in communication systems has brought rapid growth in the communication systems.

Modulation

Modulation is the process of converting the low frequency information signal in to a high frequency signal, so that the information bearing signal can be transmitted over the long distance. In this process the low frequency information signal (carrier signal) such as amplitude frequency and phase angle. The resultant signal is called modulated signal. Modulation reduces the size of antenna. It also enables multiplexing (enabling the transmission of different modulated signals at different carrier frequencies through a common channel) whereas; demodulation is the reverse of the modulation process. It is the process of recovering the signal from the modulated carrier wave.

Need for modulation

The low frequency signal (information/message) to be transmitted cannot be directly transmitted without modulation because

(i)The size of the antenna required for the efficient radiation becomes impractically high as the wavelength of the low frequency signal is very large.

(ii)Mutual interference may take place among the signal with the help of a high and it becomes impossible to separate the various signals.

(iii)The low frequency signal has relatively shorter range.

Thus, it is necessary to modulate the low frequency signal with the help of a high frequency carrier wave, before transmitting it through space for large distance.

Carrier wave

It is a wave whose function is to carry the signal (video/audio) from transmitting station to receiving station. It is a high frequency sinusoidal signal generated by radio frequency oscillator. The carrier wave, as it is does not have any information since its amplitude, frequency and phase parameters are constant. The antenna radiates out high frequency wave in to space. These waves do not produces any sound in the loud speaker of a receiver.

Types of modulation

There are three types of modulations. They are as follows:

Analog modulation =  Analog modulation (AM)

Frequency modulation (FM)

Phase modulation (PM)

Pulse modulation =     pulse amplitude modulation (PAM)

Pulse width modulation (PWM)

Pulse position modulation (PPM)

Digital modulation=    Pulse code modulation (PCM)

Amplitude shift key (ASK)

Frequency shift keying (ASK)

Phase shift keying (ASK)