UNIT 5

Digital communication & Systems

PAM, PTM, PCM, Comparison of various pulse communication systems (PAM, PTM
and PCM)

Digital communication means communication through 1 and 0 (as in electric

currents being on or off). For example the number 22 becomes 10110. Words
are translated into binary by giving each letter a number in a defined format
(e.g. ASCII code). The word “hat” becomes 01101000 01100001 01110100
It basically
is nothing but communication by decoding information into a binary
code and transmitting it between devices that are able to encode information
from and decode it back into a format the sender and receiver can process,
e.g. words, images, sound.
A digital communication system is designed to transport a message from an
information source through a transmission medium (i.e., channel) to an
information sink. The goal is to accomplish this task such that the information
is efficiently transmitted with a certain degree of reliability.
In this diagram three basic signal processing operations have been included.
They are:
1.Source coding:
In source coding the encoder converts the digital signal generated at the
source output into another signal in digital form.
Different source coding techniques are PCM (Pulse code modulation) DM(Delta
modulation).
2.Channel coding:
Channel encoding is done to minimize the effect of channel noise.
This will reduce the number of errors in the received data and will make the
system more reliable.
3.Modulation:
Modulation is used for providing an efficient transmission of the signal over
the channel.
The detector is used for demodulation channel decoder and source decoder
has exactly the opposite roles to play as compared to the channel encoder and
source encoder respectively.
If the information rate is maximum Digital modulation technique can be used
because due to the digital nature of the signal, it is possible to use the
advanced processing techniques such as digital signal processing, image
processing, and data compression
Communication systems that first convert the source output into a binary
sequence and then convert that binary sequence into a form suitable for
transmission over particular physical media such as cable, twisted wire pair,
optical fiber, or electromagnetic radiation through space.

Digital communication systems, by definition,are communication systems that

use such a digital sequence as an interface between the source and the
channel input (and similarly between the channel output and final destination).

Figure 2.1 in the basic digital communication model the first three blocks of
the diagram (source encoder, channel encoder, and modulator) together
comprise the transmitter .The source represents the message to be
transmitted which includes speech, video, image, or text data among others. If
the information has been acquired in analog form, it must be converted into
digitized form to make our communication easier.

This analog to digital conversion (ADC) is accomplished in the source encoder

block. Placing a binary interface between source and channel. The source
encoder converts the source output to a binary sequence and the channel
encoder (often called a modulator) processes the binary sequence for
transmission over the channel.

The last three blocks consisting of detector/demodulator, channel decoder,

and source decoder form the receiver.

The destination represents the client waiting for the information. This might
include a human or a storage device or another processing station.

This transformation includes digital to analog conversion (DAC) if the

destination is a human waiting to hem or view the information or if it is an
analog storage device. If the destination is a digital storage device, the
information will be kept in its digital state without DAC.

The channel decoder (demodulator) recreates the incoming binary sequence

(hopefully reliably), and the source decoder recreates the source output

Pulse modulation is a type of modulation in which the signal is transmitted in

the form of pulses. It can be used to transmit analogue information. In pulse
modulation, continuous signals are sampled at regular intervals.

Pulse-amplitude modulation (PAM)

 It is the simplest form of Pulse Modulation.

 In this type of modulation, each sample is made proportional to the
amplitude of the signal at the instant of sampling.
 The PAM signal follows the amplitude of the original signal, as the
signal traces out the path of the whole wave.
 Here a signal which is sampled at Nyquist rate can be reconstructed by
passing it through an efficient Low Pass Filter (LPF) with exact cutoff
frequency.
 It is very easy to generate and demodulate PAM.
 This technique transmits the data by encoding in the amplitude of a
series of signal pulses.
There are two types of PAM.

1. Single Polarity PAM: A fixed DC level is added to the signal so that the
signal is always positive.
2. Double Polarity PAM: Here the pulses are both positive and negative.
The figure shown above that the amplitude of the pulses is varying with
respect to the amplitude of analog modulating signal, like in case of
amplitude modulation. But the major difference is that unlike AM, here the
carrier wave is a pulse train rather than continuous wave signal

Advantages of PAM
 Both Modulation and demodulation are simple.
 Easy construction of transmitter and receiver circuits.
Disadvantages of PAM
 Large bandwidth is required for transmission.
 More noise.
 Here the amplitude is varying. Therefore, the power required will be
more.
Applications of PAM
 Mainly used in Ethernet communication.
 Many microcontrollers use this technique in order to generate control
signals.
  It acts as an electronic driver for LED circuits
Pulse Time Modulation (PTM)
 Here the pulses will have the same amplitude.
 However, one of their timing characteristics is made proportional to the
amplitude of the sampled signal.
 This variable characteristic can be either frequency, position or width.
Advantages of PTM
 Low power consumption.
 It has an efficiency of about 90 per cent.
 Noise interference is less.
 High power handling capacity.
Disadvantages of PTM
 The circuit is more complex.
 Voltage spikes can be seen.
 The system is expensive as it uses semiconductor devices.
 Switching losses will be more due to high PWM frequency.
Applications of PTM
 Used in encoding purposes in the telecommunication system.
 Used to control brightness in a smart lighting system.
 Helps to prevent overheating in LED’s while maintaining it’s brightness.
 Used in audio and video amplifiers.
 This way pulse time modulation can be classified into three types.

PAM is basically a sampling technique which converts an analog signal into a

discrete signal (signal that is continuous in amplitude but discrete in time)
whereas PCM is an analog-to-digital conversion technique which is sampling
+quantization.
Pulse Code Modulation (PCM)
 This type of modulation is different from all modulations learnt so far.
 It is clear from the block diagram given at the top that it is a type of
digital modulation.
 That is the signals here are sampled and sent in pulse form.
 A common feature among other techniques is that pulse code
modulation also uses sampling technique
 . In this case, instead of sending a pulse train which is capable of
continuously varying parameters, this type of generator produces a
series of numbers or digits.
 Each digit in it represents the appropriate length of the sample at a
particular instant.
Basic block diagram of its realisation is given below:
Advantages of PCM
 It is mainly used in long distant communication.
 Transmitter efficiency is more.
 It has higher noise immunity when compared to other methods.
Disadvantages of PCM
 More bandwidth is required when compared to analogue systems.
 In this method encoding, decoding and quantisation of the circuit have
to be done. This makes it more complex.
Applications of PCM
 It is used in the satellite transmission system.
 It is also used in space communication.
 Used in Telephony.
 One of the recent applications is the compact disc.
Communication Systems: Radio, TV, Satellite and Optical fibre
Communication (Block diagram approach only)

In radio communication systems, information is carried across space using radio

waves. At the sending end, the information to be sent is converted by some type of
transducer to a time-varying electrical signal called the modulation signal
 The transmission section consists of a baseband processor, which does the
necessary filtering of the input signal by limiting the input signal bandwidth to
the required value

 If the radio communication system is a digital system, necessary source

coding is also done by the baseband processor. This signal is modulated
using analog or digital modulation techniques.

 The baseband signal is converted into the radio frequency in two stages. In
the first stage, called the intermediate frequency (IF) stage, the signal is
translated to an intermediate frequency. The most widely used standard IFs
are 455kHz, 10.7MHz, and 70MHz.

 In the second stage, the signal is translated to the required radio frequency
using an up-converter
 The up-converted signal is given to a power amplifier that pumps out the
modulated radio waves with the desired power level through the antenna

 At the receiving end, the signal is received by the antenna, down-converted to

the IF frequency, demodulated, and filtered, and the original signal is
obtained. The baseband processor in the receiving section carries out the
necessary decoding.

Communication Systems: TV
The block diagram can be broadly divided into two separate section, viz., one
that - Generates an electronic signal (called video signal) corresponding to the
actual picture and then uses this video signal to modulate an R-F carrier so as to
be applied to the transmitting antenna for transmission,

other that generates an electronic signal (called audio signal) containing sound
information and then uses this signal to modulate another RF carrier and then
applied to the transmitting antenna for transmission.

However only one antenna is used for transmission of the video as well as audio
signals.

Thus these modulated signals have to be combined together in some appropriate

network.

In addition there are other accessories also. For instance, video as well as audio
signals have to be amplified to the desired degree before they modulate their
respective RF carriers.
This function is performed by video and audio amplifiers.

The block picture signal transmitter and audio signal transmitter shown in
figure (a) may consist of modulators as the essential component;

Video signal transmitter employs an AM transmitter as amplitude-modulation is

used for video signals whereas audio signal transmitter employs

FM modulator as frequency modulation is used for sound information. Scanning

circuits are used to mike the electron beam scan the actual picture to produce
the corresponding video signal.

The scanning by electron beam is in the receiver too.

The beam scans the picture tube to reproduce the original picture from the
video signal and this scanning at the receiver must be matched properly to the
scanning at the transmitter.

It is for this reason that synchronizing Circuits are used at the transmitter as well
as receiver.

Television Receiver

A radio receiver designed to amplify and convert the video and audio radio-
frequency signals of a television broadcast that have been picked up by a
television antenna;
the receiver reproduces the visual image broadcast and the accompanying
sound.

Television receivers are designed for colour or black-and- white operation; both
non portable and portable models are produced.

Basic block diagram of optical fibre communication system consists of following

important blocks.
1. Transmitter
2. Information channel
3. Receiver.
       Fig. 1 shows block diagram of OFC system.
Modulator:
. The modulator has two main functions.
1) It converts the electrical message into proper format.
2) It impresses this signal onto the wave generated by the carrier source.
Two distinct categories of modulation are used i.e. analog modulation and
digital modulation.
Carrier source:
. Carrier source generates the wave on which the information is transmitted.
This wave is called the carrier. For fibre optic system, a laser diode (LD) or a
light emitting diode (LED) is used. They can be called as optic oscillators, they
provide stable, single frequency waves with sufficient power for long distance
propagation.
Channel coupler:
. Coupler feeds the power into information channel. For an atmospheric optic
system, the channel coupler is a lens used for collimating the light emitted by
the source and directing this light towards the receiver. The coupler must
efficiently transfer the modulated light beam from the source to the optic fibre.
The channel coupler design is an important part of fibre system because of
possibility of high losses
Information channel:
The information channel is the path between the transmitter and receiver. In
fiber optic communications, a glass or plastic fibre is the channel. Desirable
characteristics of the information channel include low attenuation and large
light acceptance cone angle. Optical amplifiers boost the power levels of weak
signals. Amplifiers are needed in very long links to provide sufficient power to
the receiver. Repeaters can be used only for digital systems. They convert
weak and distorted optical signals to electrical ones and then regenerate the
original digital pulse trains for further transmission.
. Another important property of the information channel is the propagation time
of the waves travelling along it. A signal propagating along a fibre normally
contains a range of fibre optic frequencies and divides its power along several
ray paths. This results in a distortion of the propagation signal. In a digital
system, this distortion appears as a spreading and deforming of the pulses.
The spreading is so great that adjacent pulses begin to overlap and become
unrecognizable as separate bits of information.
Optical detector:
. The information begins transmitted is detected by detector. In the fiber
system the optic wave is converted into an electric current by a photodetector.
The current developed by the detector is proportional to the power in the
incident optic wave. Detector output current contains the transmitted
information. This detector output is then filtered to remove the constant bias
and then amplified.
. The important properties of photodetectors are small size, economy, long
life, low power consumption, high sensitivity to optic signals and fast response
to quick variations in the optic power.
. Signal processing includes filtering, amplification. Proper filtering maximizes
the ratio of signal to unwanted power. For a digital syst5em decision circuit is
an additional block. The bit error rate (BER) should be very small for quality
communications.
Signal processing:
. Signal processing includes filtering, amplification. Proper filtering maximizes
the ratio of signal to unwanted power. For a digital syst5em decision circuit is
an additional block. The bit error rate (BER) should be very small for quality
communications.
Message output:
. The electrical form of the message emerging from the signal processor are
transformed into a sound wave or visual image. Sometimes these signals are
directly usable when computers or other machines are connected through a
fibre system.