What Is Digital Communication System?

A general communication system explains communication exchange between communicators. Information is transferred from sender to receiver using a channel or path. A communication system is divided depending upon the mode or the path through which the information is propagated. It can be divided into two categories.

1. Analogue communication system
2. Digital communication system

Analogue communication system- In that communication system information is transmitted in form of symbols which are analogue or continuous in nature such as music, video signals etc. Traditional analogue communication systems use frequency modulation (F.M.), amplitude modulation (A.M.) etc. for information exchange and communication.

Digital communication system- In digital communication information is transmitted in the form of symbols which are digital or discrete in nature such as alpha-numeric codes, binary digits etc. There are different modulation techniques used for digital communication systems such as amplitude shift keying (ASK), frequency shift keying (FSK), phase shift keying (PSK) etc.

Need Of Digital Communication Systems

When electronics communication industries started growing it was gradually replacing analogue systems by digital communication systems. Because the conventional methods of communication had some limitations and digital systems have some excellent features of them. And provides advantages over analogue systems. Digital communication systems have allowed better multiplexing, processing, reliability and it is more accurate and clearer.

Below image shows the basic structure of digital communication systems.

Source- It is the person or entity which generates the information, or the message meant to be transmitted and it is the input signal applied to the digital communication system. Source information can be analogue or digital in nature. But physical quantities which are to be transmitted are analogue in nature like sound, which is continuous and varying in nature. The source signal is called baseband signal.

Transducer- Transducer is a device which converts one form of energy into another form. In digital communication systems it is used to convert physical quantities into digital quantities to make it suitable for transmission. For example, human voice or sound signal meant to be transmitted is analogue in nature. It is converted into digital electrical pulses using a microphone.

Source Encoder- Source encoder used to compress the data. It means converting analogue signal waveforms into digital binary digits by sampling the waveforms with the minimum amount of distortion possible. It removes the unnecessary binary digits for effective bandwidth utilization.

Channel Encoder- Channel encoder is used to recover the signal by adding some redundant binary bits and works as error corrector. Because when information signal is transmitted from the channel it gets altered from the noise and redundancy is created in the binary sequence. Channel encoder improves the quality of the signal.

Modulator- Modulator is used for the modulation of the baseband signals. In the process of modulation base band signal is superimposed on the high frequency carrier signal. Modulation is performed by varying the amplitude, frequency or phase of the high frequency carrier signal according to the low frequency baseband signal.

The modulator next to the channel encoder modulates the encoded binary sequence using various modulation techniques used for digital modulation such as amplitude shift keying (ASK), frequency shift keying (FSK), phase shift keying (PSK) etc.

Channel- It is the physical transmission medium between transmitter and receiver through which the information is travelled from the source to destination. Noise is generated in the transmission medium. The amount of noise or interference generated in the information signal while travelling from source to destination or transmitter to receiver depends upon the channel being used. Channel can be wired (copper wires, coaxial cables, optical cables etc.) or wireless (satellite links or microwave links).

Noise can be internal (shot noise, partition noise, transit time noise, thermal noise, flicker noise.) or external (atmospheric noise, extraterrestrial noise, industrial noise).

Demodulator- Demodulator performs the reverse process of the modulator. Demodulator separates the low frequency baseband signal from the received high frequency carrier signal. Output of the demodulator is bit stream.

Channel Decoder- The channel decoder processes the received encoded sequence and decodes the message bits with the help of redundant data provided by the channel encoder. And adds some redundant bits which get lost during transmission. Error detection and correction is also performed by the channel decoder.

Source Decoder- It reconstructs the original baseband signal. It converts the binary data back to analogue waveforms. The signal reconstructed is the approximation of the original signal.

Transducer- Output transducer converts the electrical signal back into its original physical form. Loudspeaker is the example of that which converts the electrical signal back into sound signal which is physical and analogue in nature.

Output- Output is the sound produced by the speaker at the receiver end.

Advantages of Digital Communication Systems

Less Errors- Noise and interference is very less in digital communication because the distortion must be high enough to change the pulse from one state to another.

Multiplexing and Switching- Digital communication allows multiplexing to combine signals using TDM (time division multiplexing) and switching in the communication at a lower cost than analogue communication. And time division multiplexing is easy and simple in digital communication as compared to FDM (frequency division multiplexing) in analogue communication systems.

Cheap and Reliable Hardware- Digital circuits are made of electronics components and circuits. Electronic components and circuits are cheap because of their large-scale production. These circuits are reliable because they work correctly 99.9% of the time until they get damaged.

Security- Digital communication is secure as well. Digital communication provides security in form encryption at the source. Information at the destination can decrypted easily.

Limitations of Digital Communication

1. Synchronization is required between the transmitter and receiver.

2. Digital transmission systems are totally incompatible with analogue transmission lines.

3. Digital transmission requires a large bandwidth for digitally coded analogue signals as compared to analogue transmission systems.

4. In digital transmission systems analogue signal is first converted into digital before the transmission and at the receiver end signal is again converted into digital from analogue.

5. Digital Communication systems have a higher power loss than analogue communication systems.