Digital Communication Systems Lab Report: PCM, ASK, and Demodulation

This lab report details experiments on digital communication systems, focusing on Pulse Code Modulation (PCM) and Amplitude Shift Keying (ASK) techniques. The report covers the implementation of PCM encoding and decoding, including the use of low-pass filters and the generation of sine waves. It also explores ASK modulation and demodulation, comparing signal characteristics and addressing practical considerations such as noise effects. The experiments involve setting up PCM encoders and decoders, generating and demodulating ASK signals, and analyzing the impact of noise on signal transmission. The report answers experiment questions related to the PCM encoder's output, binary number changes, and the challenges of obtaining specific voltage levels. It also discusses methods for obtaining variable DC voltages and their impact on the system. The objective is to develop an understanding of digital communication systems and their practical applications.

DIGITAL COMMUNICATION SYSTEM
BY
AFFILIATE

Contents
I.0 INTRODUCTION....................................................................................................................2
1.1 OBJECTIVE.........................................................................................................................2
2.0 SESSION IMPLEMENTATIONS.........................................................................................2
2.1 PULSE CODE MODULATION.........................................................................................2
2.1.1 Low Pass Filter................................................................................................................3
2.1.2 Decoder...........................................................................................................................3
2.1.3 Setting up PCM encoder..................................................................................................4
2.1.4 Setting up PCM decoding................................................................................................5
2.1.4 PCM Sine Wave.............................................................................................................6
2.1.6 Sound form PCM Decoder...........................................................................................7
2.1.7 The PCM headphone.....................................................................................................8
2.2 THE AMPLITUDE SHIFT KEYING................................................................................8
2.2.1 Demodulating ASK........................................................................................................9
2.2.2 Comparing the signal ASK.........................................................................................10
3.0 EXPERIMENT QUESTIONS..............................................................................................12
3.1 Experiment 12: PCM Encoding.......................................................................................12
3.2 Experiment 13: PCM Decoding.......................................................................................14
3.3 EXPERIMENT 10: AMPLITUDE SHIFT KEYING DEMODULATION..................15
3.3.1 Part A: Generating an ASK signal................................................................................15
3.3.2 PART B: Demodulating an ASK Signal.......................................................................16
3.3.3 PART C: Restoring the Recovered Digital Signal using Comparator..........................16
3.4 Experiment 11: ASK Demodulation Using Product Detection......................................18

4.0 CONCLUSION.....................................................................................................................19
REFERENCES............................................................................................................................20
I.0 INTRODUCTION
Digital communication system is a method of correspondence where the data or the thinking is
encoded carefully as circumspect signs and electronically exchanged to the beneficiaries. Digital
communication system is a standout amongst the most normally utilized method of
communication in the present situation. Associations by and large depend on this mode for all
their business communication.
Currently, the digital communication is taking over from the analog systems of communication ,
a good example of the situation where the digital systems have taken over the analog systems is
the telecommunication channel. It is therefore very important and better to have a good know
how of the digital communication systems. For the communication to be termed complete, there
must pass through a medium. The channel must be there for the communication to be complete,
or even for the communication to take place. This is not limited to the kind of channel the
communication uses, this might be optical fiber cables, free spaces or even the copper wire.
Assuming that the channel is not shared as required, then transmission of information could only
take place in a single one direction creating a lot of shortcomings when it comes to
communication.
The time division multiplexing is one of the method that is used for sharing channel, this method
involves allowing the user to have full access to the channel though the period that the user has
been allowed is very short.

Another method is the frequency division multiplexing, which involves allowing use to have
uninterrupted time with the channel for as long time period.
1.1 OBJECTIVE
The main objective of this session is to develop a digital communication systems that are
effective and efficient enough to completely take over the analog systems of communication.
2.0 SESSION IMPLEMENTATIONS
2.1 PULSE CODE MODULATION
This is a system that is used to encode the analog system to a 0s and 1s stream. This PCM
encoder uses the codec in changing the analog voltage that is in between the -2 and the +2 into
an 8 bit binary numbers.
2.1.1 Low Pass Filter
This kind of channel expels all high recurrence components that are inside the information
simple analog signal which is more noteworthy than the most astounding recurrence of the
message motion, to abstain from associating of the message signal. The figure below shows how
low pass filter works.

2.1.2 Decoder
The decoder circuit unravels the beat coded waveform to imitate the first flag. This circuit goes
about as the demodulator.
To recuperate the first flag from the examined information, a "demodulator" can apply the
technique of regulation in switch. After each testing period, the demodulator peruses the
following worth and movements the yield flag to the new esteem. Because of these changes, the
flag has a lot of high-recurrence vitality caused by associating. To expel these unwanted
frequencies and leave the first flag, the demodulator goes the flag through simple channels that
smother vitality outside the normal recurrence go

2.1.3 Setting up PCM encoder
The contribution to the PCM ENCODER module is a simple message. This should be obliged to
a characterized transfer speed and sufficiency extend.
The most extreme admissible message transmission capacity will rely on the examining rate that
is to be utilized.
The plentifulness extend must be held inside the ± 2.0 volts scope of the TIMS ANALOG
REFERENCE LEVEL. This is with regards to the information abundancy limits that is set for
every unit simple module.
A well ordered depiction of the task of the module takes after:
1. The module is driven by an outer TTL clock.
2. The info simple message is then inspected occasionally. The example rate is then controlled
by the outer clock.
3. The examining is an example and-hold task. This is inward to the module, and it can't be seen
by the client . What is held is abundancy of simple message at inspecting moment.
4. Each example adequacy is contrasted and the limited arrangement of abundancy levels. These
are inside a range of the ± 2.0 volts (the TIMS ANALOG REFERENCE LEVEL). All this are
the framework quantizing levels.

5. Each and every quantizing levels are then allocated value which is a number, beginning from
value zero for the least (generally negative) stage, with the most astounding number being (L1),
and given that L is the accessible number of stages.
6. Each example is allocated a computerized (double) code word speaking to the number related
with the quantizing stage that is nearest to the example sufficiency.
7. The code word is then gathered to the time period together with the different bits as this might
be required (portrayed underneath). In a TIMS PCM ENCODER (and numerous business
frameworks) solitary additional bit is then included, all huge piece position. This is then once
again 1 or 0. All these bits are then utilized by resulting decoders for all outline
synchronization.
8. The casings are all transmitted serially. They are all transmitted at an indistinguishable rate
from the examples are taken. The serial piece stream shows up at yield of module.
9. Additionally accessible from module is synchronizing flag .This flags the end of each and
every datum outline.
2.1.4 Setting up PCM decoding

Decoding the PCM is driven by the outer clock also. The signals of the clock are synchronized
similarly to that of the transmitter.
The following are the steps for decoding the PCM ;
1. The PCM frame synchronization for the signal is first extracted from the information or
taking the information from the transmitter.
2. The binary numbers and values are then extracted , the numbers are the amplitude coded
from that which is taken from its sample which is a frame.
3. The quantization stage that the number stands for is then identified
4. The voltage which is proportional to the amplitude stage is generated.
5. The voltage which is the output is presented.
6. The re-construction of the message might be achieved though with some content break
down and distortion , this can be done and made possible by the low pass filtering. The
module provides a reconstruction which is built in .
2.1.4 PCM Sine Wave
There are extremely just 3 things we must acknowledge to know keeping in mind the end
goal is just to make a sine wave: The recurrence, example rate and the stage. For this
crucial case, how about we make a 100Hz sine wave which utilizes CD quality encoding.
.

A sine wave actually keeps on utilizing the Sin() capacity in order to decide the
recurrence. From the level of trigonometry, we can easily notice and realize that the time
of a sine wave is 360 digress. This simply means that sine wave finishes a whole
transformation in exactly 360 degrees. In form of radians, that can be assumed as 2 *
Pi(the pie). Sin() can shifts from negative one to positive one. We could likewise realize
that the most extreme estimation of the 16 bit recording is exactly 32,767 and its base
esteem is negative 32,768.
2.1.6 Sound form PCM Decoder

2.1.7 The PCM headphone
2.2 THE AMPLITUDE SHIFT KEYING

The amplitude shift keying is a unique type of amplitude modulation that shows all
binary information that is in the form of variation in amplitude of flag.
Any signal that is modulated always has their frequency carrier very high. The signal of
the binary gives out a zero value for all inputs that are low and gives the output of the
carrier for high input when the amplitude shift keying is modulated.
This can also be taken as a type of sufficiency tweak that speaks to advanced information
as varieties in the plentifulness of the transporter wave. In the case of an ASK
framework, the double image 1 is spoken to by transmitting a settled adequacy bearer
wave and settled recurrence for a bit term of T seconds. On the off chance that the flag
esteem is 1 then the bearer flag will be transmitted; something else, a flag estimation of 0
will be transmitted.
Any computerized regulation plan that is utilizing a limited number of unmistakable signs
to speak to the advanced information. Solicit makes use of a limited number from the
amplitudes, each doled out a one of a kind example of twofold digits. Typically, every
abundancy encodes an equivalent number of the bits. Each example of bits frames the
image that is spoken to using the specific sufficiency. The demodulator, that is composed
of particularly for the image set makes use of the modulator, then decides the adequacy
of the got flag and maps it back to the image it speaks to, consequently recouping the first
information. Recurrence and period of the transporter are kept steady.

2.2.1 Demodulating ASK
The diagram that represents the ASK modulator is made up of the a generator carrier
signal. The sequence of the binary which is from the message flag and also band limited
filter. The below diagram shows an ASK modulator
The transporter generator, sends a nonstop high-recurrence bearer. The paired
arrangement from the message flag makes the unipolar contribution to be either High or
Low. The high flag shuts the switch, permitting a bearer wave. Henceforth, the yield will
be the bearer motion at high information. At the point when there is low info, the switch
opens, enabling no voltage to show up. Henceforth, the yield will be low.
The band-restricting channel, shapes the beat contingent on the abundancy and stage
qualities of the band-constraining channel or the beat forming channel.

2.2.2 Comparing the signal ASK
In ASK demodulation, the presence and the absence of the sinusoid only in given
interval time is needed to be calculated. The advantage of the ASK is that of simplicity
though ASK is too susceptible to noise and interference . the effects of the noise in ASK
is that , it affects the amplitude hence the technique that is used for modulation is then
affected by the noise.
ASK functions to transmit digital information on an optical fiber as a channel.
Noise has a great effect in the performance of an ASK modulation. As seen from the
diagram below.

3.0 EXPERIMENT QUESTIONS
3.1 Experiment 12: PCM Encoding
Question 6: Why does the PCM Encoder module output this code for 0V DC and not
0000000?
This is to allow the PCM Encoder module output to encode voltages above and below 0V
as this is a property of analog message signals such as music and speech.

Question 8: In what way does the binary number that the PCM Encoder module outputs
change?
As the input voltage increases positively, the number gets bigger.
Question 9: It’s possible that you were unable to obtain 11111111 on the PCM Encoder
module’s output at Step 26. Explain why.
This is because of the small amounts of DC offset.
Question 10: Devise a metod of obtaining a variable DC voltage that can reach(or exceed)
the upper and lower limits of the PCM Encoder module’s input range. Describe your
solution below.
Variable DC = input voltage + DC offsets.
This method attains the desired limit by varying the input range depending on the DC offsets.
Question 11: What happens to the Binary number on the PCM encoder module’s output as
the size of the negative input voltage increases?
The Binary number gets smaller.
Question 12: Based on the information in Table 1, what is the maximum allowable
amplitude(peak-to-peak) for an AC signal on the PCM Encoder module’s INPUT?
Typically 5Vp-pv : The value in Table 1 should be about ±2.5V.

Question 13: What’s the name for the difference a sampled voltage and its closest
quantisation level?
Quantisation Error.
Question 14: Calculate the difference between the quantisation levels in the PCM Encoder
module by subtracting the value in Table 1 and dividing the number by 256 (the number of
codes).
(2.5V - -2.5V) / 256 = 0.01953125V or 19.53125mV
Question 15: To reduce quantisation error it’s better to have
more quantisation levels between ±2.5V.
Question 16: Why does the PCM DATA change continuously?
This is because every time it samples the input signal, its voltage is different hence
producing different numbers.
3.2 Experiment 13: PCM Decoding.
Question 1: What does the PCM Decoder’s “stepped” output tell about the type of signal
that it is?
PCM Decoder creates an apparent copy of the message using a finite number of
quantisation levels. Thus, it’s not as smooth as the original message. A copy of the sinewave is
always included in the original message.

Question 2: What must be done to the PCM Decoder module’s output to reconstruct the
message properly?
-Turn the Buffer module’s Gain control clockwise to improve audiblility.
-Usage of Low-pass Filter module’s Gain.
-Frame synchronization using the alternating synchronization bit.
Question 3: Even though the two signals look and sound the same, why isn’t the
reconstructed message a perfect copy of the original message.
This is because the reconstructed message is ‘stepped’ and can only get closer to
the original but not become identical. Higher frequencies yield better results.
3.3 EXPERIMENT 10: AMPLITUDE SHIFT KEYING DEMODULATION
3.3.1 Part A: Generating an ASK signal

Figure 11.1 - The digital data
Question 1: What is the Relationship between the Digital Signal and the presence of the
carrier in the ASK signal
The Digital Signal and presence of carrier in the ASK signal are set up to demonstrate
how interruption and Distortion of data due to interference.
Question 2: What is the ASK signal’s voltage when the digital signal is logic-0?

When the original digital data is logic-0, there is nothing being multiplied with the local
carrier so the product detector’s output is null also.
Question 3: What feature of ASK suggests that it’s an AM signal?
ASK is as a result of multplexing digital data using AM signal.
3.3.2 PART B: Demodulating an ASK Signal.
Question 4: Why is the recovered digital signal not a perfect copy of the original?
This is because some of the recovered message’s higher frequency harmonics are
removed by the product detector’s low-pass filter, leading to the distortion in the signal.
Question 5: What can be used to “clean-up” the recovered digital signal?
A Comparator.
3.3.3 PART C: Restoring the Recovered Digital Signal using Comparator
QUESTION 6: How does the comparator turn the slow rising voltages of the recovered digital
signal into sharp transitions?

A comparator normally changes its output state when the voltage between its inputs
crosses through approximately zero volts

3.4 Experiment 11: ASK Demodulation Using Product Detection.
Question 1: Why is the recovered digital signal not a perfect copy of the original?
This is because some of the recovered message’s higher frequency harmonics are
removed by the product detector’s low-pass filter, leading to the distortion in the signal.
Question 2: What can be used to “clean-up” the recovered digital signal?
A Comparator.
Question 3: Given the size of the recovered message’s amplitude, what is the phase difference
between the two carriers?
The two carriers has a 90° phase difference or very close to it. Therefore, there is a total
cancellation between the two messages because of their 180° phase difference.
Question 4: Given the size and shape of the recovered message’s amplitude, what is the phase
difference between the two carriers now?
The two carriers has a 180° phase difference or very close to it.

Question 6: Why cant You eliminate the frequency error between the carriers altogether to
produce a stable copy of the original digital data signal?
Because frequency errors between the multiplier’s two messages, even the tiniest values,
would lead to the recovery of an unstable copy of the original message.
Question 7: What does the recovered message now look like?
The recovered message looks like an ASK signal.
Question 8: Why is there nothing out of the product detector during the time that the original
digital data is logic-0?
When the original digital data is logic-0, there is nothing being multiplied with the local
carrier so the product detector’s output is null also.
4.0 CONCLUSION
Conversion from analog systems of communication to digital systems of communication
has really helped to improve communication means in the world. It is therefore great and
very important to understand how communication is being converted to digital.
Through the introduction of digital communication, we are now able to share anything
around the world since communication devices are now going mobile hence portable.

It is therefore recommended that digital communication system be changed to digital so
that the ways of exchanging ideas among people might improve more and more.
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