radio transmission spectrum range

Dept of Electrical & Electronic Engineering

Course # EEE-307
Experiment # 01
DSBSC Modulation

Preliminary discussion

DSBSC is a modulation system similar but different to AM (which was explored in

Experiment 4).

Like AM, DSBSC uses a microphone or some other transducer to convert speech and music
to an electrical signal called the message or baseband signal. The message signal is then used
to electrically vary the amplitude of a pure sinewave called the carrier. And like AM, the
carrier usually has a frequency that is much higher than the message’s frequency.

Figure 1 below shows a simple message signal and an unmodulated carrier. It also shows the
result of modulating the carrier with the message using DSBSC.

Figure 1: Message Signal, Carrier Signal, SDBSC Signal

So far, there doesn’t appear to be much difference between AM and DSBSC. However,
consider Figure 2 below. It is the DSBSC signal at the bottom of Figure 1 but with dotted
lines added to track the signal’s envelopes (that is, its positive peaks and negative peaks). If
you look at the envelopes closely, you’ll notice that they’re not the same shape as the
message as is the case with AM.

Figure 2: Envelope of the Modulated Signal

Instead, alternating halves of the envelopes form the same shape as the message as shown in
Figure 3 below.

Figure 3: Half of the Envelope of Modulated Signal

Another way that DSBSC is different to AM can be understood by considering the
mathematical model that defines the DSBSC signal:

DSBSC = the message × the carrier

Do you see the difference between the equations for AM and DSBSC?

When the message is a simple sinewave (like in Figure 1) the equation’s solution (which
necessarily involves some trigonometry) tells us that the DSBSC signal consists of two
sinewaves:

▪ One with a frequency equal to the sum of the carrier and message frequencies
▪ One with a frequency equal to the difference between the carrier and message frequencies

Importantly, the DSBSC signal doesn’t contain a sinewave at the carrier frequency. This is an
important difference between DSBSC and AM.

That said, as the solution to the equation shows, DSBSC is the same as AM in that a pair of
sinewaves is generated for every sinewave in the message. And, like AM, one is higher than
the unmodulated carrier’s frequency and the other is lower. As message signals such as
speech and music are made up of thousands of sinewaves, thousands of pairs of sinewaves
are generated in the DSBSC signal that sit on either side of the carrier frequency. These two
groups are called the sidebands.

So, the presence of both sidebands but the absence of the carrier gives us the name of this
modulation method - double-sideband, suppressed carrier (DSBSC).

The carrier in AM makes up at least 66% of the signal’s power but it doesn’t contain any part
of the original message and is only needed for tuning. So, by not sending the carrier, DSBSC
offers a substantial power saving over AM and is its main advantage.

The experiment
In this experiment you’ll use the MATLAB to generate a DSBSC signal by implementing its
mathematical model. This means that you’ll take a pure sinewave (the message) that contains
absolutely no DC and multiply it with another sinewave at a higher frequency (the carrier).
You’ll examine the DSBSC signal using the scope and compare it to the original message.
You’ll do the same with speech for the message instead of a simple sinewave.

Following this, you’ll vary the message signal’s amplitude and observe how it affects the
carrier’s depth of modulation. You’ll also observe the effects of modulating the carrier too
much.

It should take you about 50 minutes to complete this experiment.

Pre-lab:
Single tone message signal with amplitude am = (Ac/12)& fm = last 2digits of your
ID*100 Hz. Carrier signal amplitude Ac = last 2digits of your ID and frequency, fc = last
2digits of your ID in KHz.

1. Write down the analytical expression for M(f),C(f) and U(f) along with m(t),c(t) and
u(t);
2. Write a code in MATLAB for a DSB-SC AM signal generation; Plot the message
signal, carrier signal and modulated signal in time domain.

Procedure

Part A - Generating a DSBSC signal using a simple message

(i) Using MATLAB, generate a sinusoidal signal having frequency of 2kHz. This is the
message signal. Take the variable t as time so that 2/3 cycles of the message signal are
in the variable. Here, use magnitude of 1.

(ii) Now, take another signal having frequency of 10kHz. The final time of both the
signals should be same. This is the carrier signal. Use magnitude of 1.

(iii) Multiply both the arrays in MATLAB. A new signal will be obtained, this is the
DSBSC signal.

(iv) Plot the message signal, the carrier signal and the DSCSC signal. They should look
like the following.
The set-up in Figure 4 can be represented by the block diagram in Figure 5 below. It
implements the entire equation: DSBSC = the message × the carrier.

Figure 4: Diagram representing the signals and Modulation

Question 1
What feature of the Multiplier module’s output suggests that it’s a DSBSC signal? Tip: If
you’re not sure about the answer to the questions, see the preliminary discussion.

Question 2
The DSBSC signal is a complex waveform consisting of more than one signal. Is one of the
signals a 2kHz sinewave? Explain your answer. Think carefully about it.

Question 3
For the given inputs to the Multiplier module, how many sinewaves does the DSBSC signal
consist of, and what are their frequencies?

Question 4
Why does this make DSBSC signals better for transmission than AM signals?
Part B - Generating a DSBSC signal using speech
This experiment has generated a DSBSC signal using a sinewave for the message. However,
the message in commercial communications systems is much more likely to be speech and
music. The next part of the experiment lets you see what a DSBSC signal looks like when
modulated by speech.

(i) Download a voice file (mp3 or other) from the internet. Make sure it is not too short
or too long. 10-30second is good enough. Plot the signal taking 5000 values from the
middle.
Use the link: https://eslyes.com/easydialogs/ec/dailylife057.htm

Matlab code to import file:

[y,Fs] = audioread('voice.mp3');
t = 0:1/Fs:1000/Fs;
m = y(10000:11000);

Here, I have renamed the downloaded file as voice.mp3. You can rename as you want
and use that name. For m, 1001 sample is taken. You can take 1000 sample as you
want like this, m = y(x : (x+1000)). I have put 10000 in place of x. You should use a
different value. Don’t use value less than 8000.

(ii) Generate a carrier frequency of 100kHz. Use amplitude 1. Remember that the carrier
signal must have 1001samples, not more, not less. Since we will multiply two signals,
their length must be same.

(iii) Multiply both signals and generate the DSBSC signal. Now plot the message signal
(m), carrier signal and DSBSC signal.
Lab Performance Task:

1. Create a MATLAB function named “dsbsc” that will take 3 inputs. Inputs are, first the
frequency of the message signal, second the frequency of the carrier signal and finally
the number of cycles you want to see.
2. The function will have 4 outputs, first output is the DSBSC modulated signal, second
one is the message signal, third is the carrier signal and finally the last one is the time
array.
Report:
Show all MATLAB Codes and the output figures. Also answer the following
question.

1. Why this modulation technique is known as “DSB-SC”?

2. In which cases DSB-SC is advantageous to apply?
3. What is meant by coherent detection? Which one is preferred in communication,
coherent or non-coherent?
4. From the Lab performance Task, edit the function such that now it can take input
the array obtained from a sound file in stead of the message signal frequency. Other
things should remain same. You should select about 1000-2000 samples from the
middle part of the array and give output that part in stead of message signal. You
can assume that input signal frequency is 3kHz and also assume that the value of Fs
will not change.
5. Discuss the limitations of this experiment.
*All diagrams must have a title and all wave shapes/ spectrum should be
identified appropriately. If required, provide necessary diagram along with the
answer.
Appendix: