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Digital and Analog Communication Systems:

The advantages associate with digital communication systems include

(i) A common format for encoding different kinds of message signals (e.g., speech signal, video signal, computer data, etc.) for the purpose of transmission.

(ii) An improved security of message.

(iii) Increased immunity to noise and external interference.

(iv) Flexibility in configuring digital communication systems.

Disadvantages

The disadvantages associate with digital communication systems include

(i) Increased transmission

(ii) Increased system complexity

Although digital communication is becoming increasingly attractive for practical use, it is a fact that analog communication systems are still in existence. Most of the broadcasting systems and a large part of telephone networks in use today are analog in nature.

There are numerous ways of classifying signals depending on the requirement of study. A signal is usually described by a function of one or more independent variable. The value of the signal may be real or complex. We would consider the signal to be a function of a signal independent variable i.e. time t. Analog signals or continuous-time signals are defined for every value of time and they take on continuous values in a given time interval.

Mathematically these signals can be described by function of a continuous variable. For example, the signals.

s1 (t) = cos (500 pt)

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are analog or continuous-time signals.

Most of the naturally occurring signals are also analog in nature. Speech, electrocardiogram(ECG), electroencephalogram(EEG) signals are some of the examples of naturally occurring analog signals.

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Sine wave; example of an analog signal

An analog signal is best fitted illustrated by a sine wave as shown in above figure. Notice that the sine wave is continuous. The value of the analog signal at any given instant can be anywhere within the range of the signal's extremes.

Digital signals or Discrete-time signals are defined at discrete values of time.

These time instants need not be equidistant. In practice, however, for mathematical convenience these time intervals are taken to be equispaced. The signal x(tn) = exp (-|tn|), where n = 0 ±  1, ± 2, ...............is an example of a discreate - time signal.


Introduction to digital communication:

            A digital communication system offers many advantages to the user that cannot be achieved with an analog system. Digital communications may make use of analog link and concepts.A digital system is a more general case of a binary system. In binary system, only two signal values can exist. They are often called 0 and 1, but these names specific some voltages.

Digital Communication:

           The term data is generally used in digital communication systems. Data is any form of information that has been put into digital form, so that it can be handled by a digital system. The data itself is measured as bit. (Bit is a contraction of the term ‘binary digit’)

           The advantages of the binary signals are easy to generate and process with digital circuits. These digital circuits are available in the IC form and can generate process digital at high speeds.

Advantages of Digital Communication:

Let us see some advantages below:

1. Digital communication is fast and easier.

2. The transmission quality is very high and almost independent of the distance among the terminals.

3. We can increase the capacity for transmission system.

4. The newer types of transmission media such as light beams optical fibers and wave guides operating in the microwave frequency extensively use digital communication.

5. Consistent communication: Very less sensitivity to changes in environmental conditions (temperature and so on.)

6. Then signaling also very easy.

  •          Clip status, address digits and call progress information.

 

7. Audio and data integration.

8. Easy dealing out like encryption and compression.

9. Easy system performance monitoring.

  •          QOS monitoring.

 

10. Incorporation of transmission and switching.

11. Signal regeneration, operation at low SNR, better performance Integration of services important to ISDN.

12. We can store the messages in longer times, without any damages, unlike paper files which are easily get damages or attacked by insects.

13. Digital communication possible to done over large area through internet and other things.

14. Digital communication is comparatively cheaper and the work that needs a lot of people can be done simply by one person as folders and other such amenities can be maintained.

15. It avoids semantic barriers because the written data can be easily changed to any other languages by using computer language.

16. It provides some advance advantages like video conferencing that save a lot of time, money and effort.

Types of Modulation:

Modulation  is the process of superimposing the information contents of a modulating signal on a carrier signal (which is of high frequency)  by varying the characteristic of carrier signal according  to the modulating signal.

Introduction to Types of Modulation

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We can divide modulation in two parts  :-

(1)    Analog modulation.

(2)    Digital modulation.

Types of Modulation : Analog Modulation

Analog modulation can further divide in three parts :

(i)Amplitude modulation :- In amplitude modulation the amplitude of high frequency carrier signal is varied in accordance with the instantaneous value of baseband modulating signal keeping frequency and phase of carrier signal constant. The frequency domain representation of the resultant modulated signal contains carrier signal plus two sidebands of modulating signal (information signal)

Amplitude modulation can be of three types :-

(1)    Full AM (In this modulated signal contains carrier signal & two side bands of modulating signal.)

(2)    DSB-SC (Double sideband suppressed carrier ) In this carrier signal is suppressed.

(3)    SSB-SC (Single sideband suppressed carrier) In this carrier signal plus one sideband is suppressed.

(ii)                Frequency modulation :- In frequency modulation, the frequency of high frequency carrier signal is varied in accordance with instantaneous value of modulating signal  keeping amplitude constant.

(iii)               Phase modulation :- In phase modulation, the phase of high frequency carrier signal is varied directly in accordance with instantaneous value of modulating signal keeping amplitude constant .

Types of Modulation : Digital Modulation

Digital modulation can be further divide into four parts :

(i)                  Pulse code modulation :- In this technique the modulating signal is first passed through a sampler to which take samples of signal at discrete time intervals, then signal is passed through the quantizer  its function is to convert signal into digital signal.Then signal is passed through a encoder which assign n number of bits to each samples, and the signal is transmitted over the channel.

(ii)                Differential pulse code modulation :- The signal does not change rapidly from one sample to next, so instead of transmitting samples at each sampling instant , the difference between sample and its predicted value is transmitted in this technique. Thus due to this system efficiency is increased.

(iii)               Delta modulation :- Delta modulation is a one-bit pulse code modulation system in which staircase approximation of original signal is generated by oversampling the signal. The original signal can be obtained by passing the approximation signal through a low pass filter at  the receiving end.

(iv)              Adaptive delta modulation :- The basic principal of ADM is to vary the step size in accordance with slope of input signal i.e. when slope of input is steep, step size must be increased and if input signal slope is small step size is reduced. This technique is more accurate then above method.


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