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elements of digital communication system , what are the SOURCES 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 computers
A source of information may be characterized in terms of the signal which carries the information. Further, a signal is defined as a single-valued function of time that plays the role of the dependent variable. At every instant of time, the function has a unique value.
(i)      Speech
          We know that speech 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:
(a) Production: An intended message in the speaker’s mind is represented by a speech signal that consists of sounds (i.e., pressure waves) generated inside the speaker’s mouth and whose arrangement is governed by the rules of language.
(b) Propagation: The sound waves propagate 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.
In fact, the speech-production process may be viewed as a form of filtering, in which a sound source excites a vocal tract filter. The vocal tract consists of a tube of non-uniform cross-sectional are, beginning at the glottis (i.e., the opening between the vocal cords) and ending at the lips. Now. as the sound propagates along the vocal tract, the spectrum (i.e., the frequency content) is shaped by the frequency selectivity of the vocal tract. This effect is quite similar to the resonance phenomenon observed in organ pipes. At this stages, it may be noted that the hearing mechanism is highly sensitive to frequency. In addition to this, the type of communication system being considered has an important bearing on the bond of frequencies considered to be essential for the communication process. As an example, a frequency band from 300 to 3100 Hz is considered adequate for telephonic communication on a commercial basis.
(ii)     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 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 varies
temporally in accordance with the way in which the brightness of the original picture varies spatially from one point to another. This resulting output current is called a video signal.
          The type of scanning used in television is a form of spatial sampling called raster scanning. The purpose of raster scanning is basically to convert a two-dimensional image intensity into a one-dimensional waveform.*
Further, 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. For simplicity of presentation, we can refer to the two field I and II. Figure 1.2 illustrates the scanning procedure.
FIGURE 1.2 Illustration of interlaced raster scan
The lines of field I are depicted as solid lines, and the lines of field II are depicted as dashed lines. The figure also includes the start and end of each field. Field I is scanned first. The scanning spot of the TV camera moves with constant velocity across each line of the field from left to right. The image intensity at the center of the spot is measured. Now, when the end of a particular line is reached, the scanning spot quickly flies back (in a horizontal direction) to the start of the next line down in the field. This flyback is known as the horizontal retrace. The scanning process is continued until the whole field has been covered. When this condition is reached, the scanning spot moves quickly (in a vertical direction) from the end of field I to the start of field II.
This second flyback is known as the vertical retrace. Field II is treated in the same fashion as field I. The time taken for each field to be scanned is 1/60 second.
Correspondingly, the time taken for a frame or a complete picture to be scanned is 1/30 second. With 525 lines in a frame, the line-scanning frequency equals 15.75 kHz. Hence, by flashing 30 still pictures per second on the display tube of the TV receiver, the human eye perceives them to be moving pictures. This effect is due to a phenomenon known as the persistence of vision.
* In fact, it is quite similar to the manner in which we read a printed paper in that the scanning is performed from left to right on line-by-line basis.
Also the reproduction quality of a TV picture is limited by the following two basic factors:
(a)     the number of lines available in a raster scan, which limits resolution of the picture In the vertical direction.
(b)     the channel bandwidth available for transmitting the video signal which limits resolution, the picture in the horizontal direction.
(iii) Facsimile
          The purpose of the third source of information, facsimile (fax) machine, is to transmit still picture; over a communication channel (most notably, a telephone 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 (i.e., picture) and use an image sensor to convert the light to an electrical signal.
(iv)     Personal Computers
          As a matter of fact, personal computers (PCs) are becoming increasingly an important part of our daily lives. We use them for electronic mail, exchange of software, and sharing of resources. It is estimated that over 30 per cent 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 is the first code developed specifically for computer communications.
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 27 = 128 different characters can be represented in ASCII. The characters 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 purposes.

An unexpected development is the role that computers play in communications. E-mail now allows individuals with PCs to communicate with one another within and between messages transmitted over networks with office buildings, nation wide or worldwide. And interestingly, e-mail has not replaced the telephone or the fax. It has simply added a new way for people to interact with one another.

FIGURE 1.3 As ASCII format
At the end of the data bits, an extra bit b8 is appended for the purpose of error detection. This error-detection bit is called a parity bit. A sequence of eight bits is referred to as a byte or an octet. The parity bit is set in such a way that the total number of is in each byte is odd for odd parity, even for even parity.
Personal computers are often connected via their RS (recommended standard)-232 ports. When ASCII data are transmitted through these Ports, a start bit, set to 0, and one or more stop bits, set to 1, as depicted in figure 1.3, 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 alive.
Note: Computer-generated data and television signals are both wideband signals in that the power content occupies a wide range of frequencies. Another important characteristics of data communication between personal computers is burstiness, which means that information is usually transmitted from one terminal to another in bursts with silent periods between bursts.