Digital Communication -

The communication that occurs in our day-to-day life is in the form of signals. These signals,
such as sound signals, generally, are analog in nature. When the communication needs to be
established over a distance, then the analog signals are sent through wire, using different
techniques for effective transmission.

The Necessity of Digitization

The conventional methods of communication used analog signals for long distance
communications, which suffer from many losses such as distortion, interference, and other
losses including security breach.

In order to overcome these problems, the signals are digitized using different techniques. The
digitized signals allow the communication to be more clear and accurate without losses.

The following figure indicates the difference between analog and digital signals. The digital
signals consist of 1s and 0s which indicate High and Low values respectively.

Advantages of Digital Communication

As the signals are digitized, there are many advantages of digital communication over analog
communication, such as −

• The effect of distortion, noise, and interference is much less in digital signals as they
are less affected.
• Digital circuits are more reliable.
• Digital circuits are easy to design and cheaper than analog circuits.
• The hardware implementation in digital circuits, is more flexible than analog.
• The occurrence of cross-talk is very rare in digital communication.
• The signal is un-altered as the pulse needs a high disturbance to alter its properties,
which is very difficult.
• Signal processing functions such as encryption and compression are employed in
digital circuits to maintain the secrecy of the information.
• The probability of error occurrence is reduced by employing error detecting and error
correcting codes.
• Spread spectrum technique is used to avoid signal jamming.
• Combining digital signals using Time Division Multiplexing TDM is easier than
combining analog signals using Frequency Division Multiplexing FDM
• The configuring process of digital signals is easier than analog signals.
 • The capacity of the channel is effectively utilized by digital signals.

Elements of Digital Communication

The elements which form a digital communication system is represented by the following
block diagram for the ease of understanding.

Following are the sections of the digital communication system.

Source

The source can be an analog signal. Example: A Sound signal

Input Transducer

This is a transducer which takes a physical input and converts it to an electrical signal
(Example: microphone). This block also consists of an analog to digital converter where a
digital signal is needed for further processes. A digital signal is generally represented by a
binary sequence.

Source Encoder

The source encoder compresses the data into minimum number of bits. This process helps in
effective utilization of the bandwidth. It removes the redundant bits
unnecessaryexcessbits,i.e.,zeroes

.Channel Encoder

The channel encoder, does the coding for error correction. During the transmission of the
signal, due to the noise in the channel, the signal may get altered and hence to avoid this, the
channel encoder adds some redundant bits to the transmitted data. These are the error
correcting bits.
Digital Modulator

The signal to be transmitted is modulated here by a carrier. The signal is also converted to
analog from the digital sequence, in order to make it travel through the channel or medium.

Channel

The channel or a medium, allows the analog signal to transmit from the transmitter end to the
receiver end.

Digital Demodulator

This is the first step at the receiver end. The received signal is demodulated as well as
converted again from analog to digital. The signal gets reconstructed here.

Channel Decoder

The channel decoder, after detecting the sequence, does some error corrections. The
distortions which might occur during the transmission, are corrected by adding some
redundant bits. This addition of bits helps in the complete recovery of the original signal.

Source Decoder

The resultant signal is once again digitized by sampling and quantizing so that the pure
digital output is obtained without the loss of information. The source decoder recreates the
source output.

Output Transducer

This is the last block which converts the signal into the original physical form, which was at
the input of the transmitter. It converts the electrical signal into physical output (Example:
loud speaker).

Output Signal

This is the output which is produced after the whole process. Example − The sound signal
received.

This unit has dealt with the introduction, the digitization of signals, the advantages and the
elements of digital communications. In the coming chapters, we will learn about the concepts
of Digital communications, in detail.

The electromagnetic spectrum

The electromagnetic spectrum is the entire range of electromagnetic radiation according to
the wavelength or frequencies. It has a range of frequencies from 1Hz to Hz. The waves in
order of increasing frequencies are radio waves, microwaves, infrared rays, visible light, UV
rays, X-rays and gamma rays.Among these range, the frequencies between Hz to Hz are used
for communication.The following diagram shows the electromagnetic spectrum. The first
figure gives the full range of frequency spectrum while the second figure elaborates that part
of the spectrum which can be used for data communication −

The section of the spectrum in the range of radio waves and microwaves has been divided
into a number of frequency bands as officially referred by ITU (International
Telecommunication Union). These bands are rated from VLF (Very Low Frequency) to EHF
(Extremely High Frequency).

The frequency bands and their uses in communication is tabulated as follows −

Band Frequency
Full Forms Propagation Uses
Name Range
Extremely Low
ELF 30 – 300Hz Ground Power line frequencies
Frequency
Marine Communications,
LF Low Frequency 3 – 300 KHz Ground communication over twisted
pair
Medium 300KHz – AM radio, communication over
MF Sky
Frequency 3MHz coaxial cables
Aircraft and ship
HF High Frequency 3 – 30 MHz Sky
communications
Very High 30 – 300 Sky and Line –
VHF FM radio, TV
Frequency MHz of - Sight
Ultra High 300 MHz –
UHF Line – of - Sight TV, cellular phone
Frequency 3GHz
Super High
SHF 3 – 30 GHz Line – of - Sight Satellite, microwave links
Frequency
Extremely High
EHF 3 – 300GHz Line – of - Sight Radar, satellite
Frequency
300 Ghz –
Infrared Infrared Rays Line – of - Sight Consumer electronic goods
400THz
Visible Visible Light 400 THz –
Line – of - Sight Fiber optic communications
Light rays 900 THz