Source of information

Source of information.

As a matter of fact, The telecommunication environment is dominated by the following four important sources of information.

(i)                 Speech

(ii)               Television

(iii)             Facsimile and

(iv)              Personal computer

A source of nformation may be characterized in term of signal which carrier the information. Further, a signal is defined as a signal valued function of the time that play the role of the depedent variable. At every instant of time, the function has a unique value.

Speech

(a)   Production: an intended message in the speaker mind is represented by the speech signal that consist of sounds generated inside the speaker mouth and whose arrangement is governed by the rule of language.

(b)   Propagation: The siund waves propagates through the air, reaching the listener’s ears.

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

Television

The second source of information, television refers to the transmission of picture in motion by means of electrical signal. To accomplished this transmission, each complete picture has to be sequentially scanned. The scanning process is carried out in a TV camera. In a black and white TV, the camera contains optics designed to focus an image on a photocathode consisting of a large number of photosensitive elements.

The charge pattern so generated on the photosensitive surface is scanned by an electron beam, and thus producing an output current which vaies temporally in accordance with the way in which the brightness of the original picture varies spatially form one point to another. This resulting output current is called a video signal.

The type of scanning used in television is a form spatial sampling called raster scanning. The purpose of raster scanning is basically to convert a two dimensional image intensity into a one dimension waveform.

Facsimile

The purpose of the third source of information, facsimile machine, is to transmit still picture over a communication channel a telephone. Such a machine provide 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 transmitted a picture.

The basic principal employed fo signal generation in a facsimile machine is to scan an original document and use an image sensor to convert the light to an electrical signal.

Personal Computer

As a matter of fact, personal computer are becoming increasingly an important part of our daily lives. We use them for electronic mail, exchange of software, and sharing of resource. It is estimated that over 30 percent of the personal computer in use today are already networked and the number is increasing rapidely. The text transmitted by a PC is usually encoded using the American standard code for information interchange, ASCII which is the first code developed specifically for computer communication.

Each character in ASCII is represented by seven data bits constituting a unique binary pattern made up of 0s and 1s. hence a total of seven power of two = 128 different characters can be represented in ASCII. The character symbols are various lowercase and uppercase letters, numbers, special control symbols, and punctuation symbols commonly used such as @,$ and %. Some of the special “control” symbols such as BS ( back space ) and CR (carriage return), are used to control the printing of characters on a page Other symbols, such as ENQ (enquiry) and ETB (end of transmission block), are used for communication purpose.

The even data bits are ordered starting with the most significant bit b7 down to the least significant bit b1.

At the end of the data bits, an extra bit b8 is appended for the purpose of error detection. This error detected bit is called a parity bit.