SOURCES OF COMMUNICATION TECHNOLOGY

A source of information may be characterized in terms of the signal which carries the information. Further, a signal is defined as a signal- valued function of time that plays the role of the dependent variable. At every instant of time, the function has a unique value.

Speech:

It is the primary method for human communication. Basically, the speech communication process involves the transfer of information from a speaker to a listener, which takes place in three successive stages as under:

1.      Production: An intended message in the speaker’s mind is represented by a speech signal that consists of sounds generated inside the speaker’s mouth and whose arrangement is governed by the rules of language.

2.      Propagation: The sound waves propagated through the air reaching the listener’s ears.

3.      Perception: The incoming sounds are deciphered by the listener into a received message, and thus completing the chain of events that result in the transfer of information from the speaker to the listener.

Television:

The second source of information, television (TV), refers to the transmission of pictures in motion by means of electrical signals. To accomplish this transmission, each complete picture has to be sequentially scanned. The scanning process is carried out in a TV camera. In a back –and-white TV, the camera contains optics designed to focus an image on a photo cathode consisting of a large number of photo sensitive elements. The charge pattern so generated on the photo sensitive surface is scanned by an electron beam, and thus producing an output current which varies temporally in accordance with the way in which the brightness of the original picture varies spatially from one point to another. In television, a picture is divided into 525 lines, which constitute a frame. Now each frame is decomposed into two interlaced fields, each one of which consists of 262.5 lines.

Facsimile:

The purpose of the third source of information, facsimile (fax) machine, is to transmit still pictures over a communication channel. Such a machine provides a highly popular facility for the transmission of hand written or printed text from one point to another. Further, transmitting text by facsimile is treated simply like transmitting a picture. The basic principle employed for signal generation in a facsimile machine is to scan an original document and used an image sensor to convert the light to an electrical signal.

Personal computer:

It is becoming increasingly an important part of our daily lives. We use them for electronics mail, exchange of software, and sharing of resources. It is estimated that over 30% of the personal computers in use today are already networked and the number is increasing rapidly. The text transmitted by a PC is usually encoded using the American standard code for Information Interchange (ASCII), which the first code developed specifically for computer communications. Personal computers are often connected via their RS-232 ports. When ASCII data are transmitted through these ports, a start bit, set too, and one or more stop bits, set to 1, as depicted are added to provide character framing. When the transmission is idle, a long series of 1s is sent so as to keep the circuit connective a live.

Cellular and mobile communication:

Although the concept of cellular communication was developed in 1947 in AT and T Bell laboratories, U S A but the first tests were conducted to explore the possibility in commercial applications. After that it then took another 8yrs when the Federal communication commission in USA, set aside new radio frequency for “Land mobile communication”. In this particular year, AT and T proposed to establish the very first high capacity cellular telephone system. It was then called as the advanced mobile phone service or Amps.

NOISE IN COMMUNICATION TECHNOLOGY:

As the message signal travels from the transmitter to the receiver through a medium called channel. Now noise is present in every communication system. The channel introduces an additive noise in the message signal and thus the message which is received at the receiver is distorted. Since, the receiver detects both message signal and the noise signal. It will reproduce a message signal which contains noise. The noise calculation in a communication system is carried out in form of a parameter which is known as figure of merit.

      Channel noise is always white and Gaussian we assume that the channel noise n(t) is always a white noise. This means that it is uniformly distributed over the entire band of frequencies under consideration. Hence, the power spectrum density of channel noise will be uniform over the frequency range under consideration. Thus the total noise power N may be obtained as:

      N=n/2*bandwidth.