What is the space communication ?

Introduction 

Communication of messagefrom one place to another has alwaysbeen an important feature of our everyday life. Earlier, communication was done by beating of drums, by smoke signals or through pigeons. With the development of science, a necessity was felt for faster communications. Now we are relying on communication through electrical waves, which travel with velocity of light  ( 3× 10^8 ms^-1) .

    The basic idea is to convert our primary information into electrical waveform (signals) which propagates through a channel. As the signal propagates, it becomes weaker due to resistance and some noise also creep in. 

The speed of communication depends upon the number of signals transmitted in one second i.e. the frequency of signals. If we want to increase the speed of communication, we shall have to compress the waveform in a certain time so that greater number of signals are transmitted in one second. This necessitates a greater band-width of the channel. S/N ratio and the band-width B are connected to each other by Shannon-Hartley law

C = B log (1+S/N)

Where 'C' is called the channel capacity or rate of message transmission. According to this relation, if we intend to maintain a higher S/N ratio, a narrow band-width can be used while a broader band-width will be required if we have smaller value of S/N .

SOME TERMS CONNECTED WITH A COMMUNICATING SYSTEM 

Following terms are, often, used in connection with a communication network. 

(i) Source.  Our main purpose is to transform data from one place to another. The agent responsible for production of that data is called Source. 

(ii) Information.  The sound produced by the man, the text on the sheet or the photograph is the information. 

(iii) Signal.  In order to achieve speed in communication we have have to depend on electric waveform, since they propagate with the speed of light. The information is first converted into electrical waveform by suitable circuits. The instrument which converts information into a signal is called 'transducer'.

(iv) Channel.  A channel is the medium through which signal propagates from sending station to receiving station. It may be a wire link, a co-axial cable, an optical fibre or a radio link .

For example, the band-width of a telephone wire is 4 kilohertz, allowing it to carry about 28,000 bits per second of information. 

(v) Noise.   As the signal propagates through the medium, it becomes weak due to the resistance of the medium. At the same time it gets distorted also due to some factors external as well as internal. The presence of distortion in the signal is termed as Noise. 

ANALOG AND DIGITAL SIGNALS :

Analog signal. -  When speech is converted into electrical oscillations using a microphone, the electrical signal is sinusoidal in nature. It is a continuously varying signal. 

The term 'analog' is derived from the word analogous which means "similar to" or "compatible with". It refers to the relationship between an original and a representative of that original. Analog specifies that there is a one to one relationship between the parts of an original and the parts of its representative. Human beings are capable of producing oscillations of 50 Hz to 5000Hz. It has been established that the bulk of the energy of speaker lies in the range of about 4000 Hz .

Digital signal :  It is a signal which is discrete in nature and is represented by discontinuous curves diagrammatically . The concept of digital signal transmission is not a new one. A digital signal is one which consists of a sequence of symbols taken from a finite set. The text of this book can be considered to be an example of digital signal. This text consists of a number discrete letters along with punctuation signs and numerical figures. These binary digits are called bits. Fig. 12.1 shows two types of binary representations. 👇

(i) Positive logic :  In this case the voltage used to represent binary 1 is higher than that used for binary 0 [ fig. 12.1 (a)] .

(ii) Negative logic :  In this case the voltage used to represent binary 1 is lower than that used for binary 0 [ fig. 12.1 (b)] .
There are other ways, also, of representing these binary digits. Fig.12.2(a) represents two different binary slabs by a change in frequency while Fig.12.2 (b) shows two binary slabs by turning on and off a single frequency.

-: MODULATION

While sitting at home, we watch a cricket match being played thousands of kilometer away. To broadcast a programme over such large ranges, we need modulation. Modulation is the process of superimposing the signal to be transmitted on other wave of high frequency called carrier wave so that is can be transmitted to larger distances. 

A carrier wave, in general, is represented by the equation, 

Vc = Vcm sin(₩c+ø) .

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