3 Oct 2017

PULSE MODULATION TECHNIQUES (PAM, PWM, PPM, PCM)

Pulse Modulation Techniques 

In this post, I will discuss different kinds of pulse modulation techniques like Pulse Amplitude Modulation (PAM), Pulse Width Modulation (PWM) or Pulse Duration Modulation (PDM), Pulse Position Modulation (PPM) and Pulse Code Modulation (PCM). Here we will also see Pulse Modulation waveforms. But Before starting the topic, it is important to understand the basic difference between Continuous Wave Modulation and Pulse Modulation.

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Pulse Modulation Techniques Video [HD]

 

Difference between Continuous Wave Modulation and Pulse Modulation

In Continuous Wave Modulation (Amplitude Modulation, Frequency Modulation, Phase Modulation) the carrier wave used is continuous in nature, while in case of pulse modulation, the carrier wave is in the form of pulses.
So now let's discuss different types of Pulse Modulation Techniques in detail.

Classification of Pulse Modulation Techniques

Pulse modulation can be categorized broadly into two types-
#Analog Pulse Modulation and
#Digital Pulse Modulation

The Analog Pulse modulation can again be of two types-
#Pulse Amplitude Modulation (PAM)
#Pulse Time Modulation (PTM)

The Pulse Time Modulation (PTM) can further be classified into two types of modulation-
#Pulse Width Modulation (PWM)/Pulse Duration Modulation (PDM)
#Pulse Position Modulation (PPM)

Pulse Code Modulation (PCM) is a digital pulse modulation technique.

You can see the classification of pulse modulation, in the image given below
(Click on the image to enlarge it)-



Image of Pulse Modulation and Pulse Modulation Techniques, Pulse Modulation Techniques pic, classification of Pulse Modulation Techniques, types of pulse modulation, types of modulation
Classification of Pulse Modulation Techniques

To see the classification of all types of modulations including pulse modulation and continuous wave modulation, look at the image given below (Click to enlarge)-

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COMPARISON OF PAM, PWM, PPM MODULATION TECHNIQUES


types of modulation, classification of modulation, pulse modulation techniques
Classification of all Modulation Techniques

Here we will discuss all types of pulse modulation techniques i.e. Pulse Amplitude Modulation (PAM)Pulse Width Modulation (PWM)/Pulse Duration Modulation (PDM)Pulse Position Modulation (PPM) and also Pulse Code Modulation (PCM).
So let's start with Pulse Amplitude Modulation (PAM)-
Here we will see the definition of pulse amplitude modulation and waveform of pulse amplitude modulation.

Pulse Amplitude Modulation (PAM)

Definition:-
The modulation technique in which the instantaneous amplitude of the pulsed carrier is varied according to the modulating signal (message signal) is called as pulse amplitude modulation.

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Waveform of pulse amplitude modulation

Look at the image given below carefully to understand the formation of pulse amplitude modulated wave. (Click the image to enlarge)-


Pulse amplitude modulation, PAM, waveform of Pulse amplitude modulation, PAM waveform
Pulse Amplitude Modulation Waveform

Here in this image, you can see, three waveforms. The first waveform is of modulating signal (message signal), the second one is of the pulsed career wave and the third waveform is of the pulse amplitude modulated wave (PAM).
The message signal can have multiple frequencies and variable amplitude. Here we have used a sinusoidal message signal. The carrier wave that we have used here, is a train of pulses having high frequency. Since we know that in Pulse modulation, the carrier wave is in the form of pulses.
Now we will understand how can we make pulse amplitude modulated wave (PAM).
As we have discussed that in Pulse amplitude modulation, the instantaneous amplitude of the pulsed carrier is varied according to the modulating signal (message signal). 

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Now look at the image shown above very carefully, observe that, as the amplitude of the modulating signal (message signal or baseband signal) increases, the amplitude of the pulsed carrier wave increases and becomes maximum when the amplitude of the modulating signal reaches at its maximum. Now the amplitude of the carrier wave starts decreasing with decrease in amplitude of the modulating signal. So it is clear from the image that the amplitude of the pulsed carrier wave varies according to the instantaneous value of the message signal.

Important Observations about Pulse Amplitude Modulation (PAM)

#As it is Pulse Amplitude Modulation (PAM), therefore only the amplitude of the pulsed carrier changes. There is no change in the width or position of the pulsed carrier wave. 

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So the width and position of the carrier wave pulse is constant here. Therefore we can say that, all the information of the modulating signal is contained in the amplitude variations of the pulses. No information is present in the width or position of the pulses.

Pulse Time Modulation (PTM) 

In pulse time modulation, the width or position of the pulsed carrier wave is varied (variations on the time axis) as per the changes in the amplitude of the modulating signal.
As we have discussed already that Pulse time modulation is of two types-
Pulse Width Modulation (PWM)/Pulse Duration Modulation (PDM) and
Pulse Position Modulation (PPM)
So first let's start with pulse width modulation.

Pulse Width Modulation (PWM)/Pulse Duration Modulation (PDM)

Definition:-
In Pulse width modulation, width (duration) of the pulses of the carrier wave is varied according to the modulating signal (message signal).
Now we will understand the waveform of pulse width modulation.

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Look at the image given below to see the waveform of pulse width modulated signal. (Click the image to enlarge)-


Pulse Width Modulation, PWM waveform, Pulse Width Modulation Waveform
Pulse Width Modulation Waveform

Here you can see waveforms of modulating signal, pulsed carrier wave and pulse width modulated wave.
As per the definition of pulse width modulation, we know that, in Pulse width modulation, the width of the pulses of carrier wave is varied according to the message signal. So you can see here in the image that as the amplitude of the message signal changes, the width of pulses of the pulsed carrier wave changes accordingly.
You can easily observe here, the width of the pulse is maximum when amplitude of the message signal is at maximum and in the same way, width is minimum when amplitude of the modulating signal is minimum.

Important Observations about Pulse Width Modulation (PWM)

#Since it is pulse width modulation (pulse duration modulation), therefore only the width (duration) of the pulses of the carrier wave changes. No change takes place in amplitude or position of the pulses.
#As the width of pulses changes, according to the modulating signal; information is present only in the width of the pulses. In Pulse width modulation, amplitude or position of the pulses contain no information.

Pulse Position Modulation (PPM)

Definition:-
In Pulse position modulation, the position of the pulsed carrier wave is varied according to the message signal (modulating signal).
Look at the waveforms shown in the image given below, (Click the image to enlarge)-



Pulse Position Modulation, PPM, Pulse Position Modulation Waveform, PPM waveform
Pulse Position Modulation Waveform

This image shows, the waveforms of modulating signal, pulsed carrier wave, pulse width modulated wave (PWM) and pulse position modulated wave (PPM).
Please note that, here we will obtain the pulse position modulated wave from the pulse width modulated wave. 

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Therefore to draw the waveform of PPM we need to draw the waveform of PWM too.
Now observe the waveforms shown in the image.
Now we know how to obtain the waveform of pulse width modulated wave. So we will get the waveform of pulse position modulation from PWM. 
As you can see in the image, that the starting point of each pulse in PPM waveform is obtained from the trailing edge of the PWM pulse. It means, each pulse in PPM starts where the PWM pulse ends (trailing edge of PWM pulse).

Important Observations about Pulse Position Modulation

#It is important to note that in pulse position modulation, only the position of of the pulses changes as per the variations in amplitude of the modulating signal. There is no change in the amplitude or width of the pulses of the carrier wave. So the information about the modulating signal is present in the position variations of the pulses. 
#Also note here that in pulse position modulation, synchronizing pulses need to be sent by the transmitter to keep the transmitter and receiver in synchronism. 

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This kind of synchronization is not required in case of pulse amplitude modulation or in Pulse width modulation.

Pulse Code Modulation (PCM)

Now we will discuss the digital form of pulse modulation technique. This form of pulse modulation technique is known as Pulse Code Modulation (PCM).
Pulse code modulation is a technique to convert analog signals into digital signals.
After converting the signal into digital form, it becomes possible to transmit the digital signal through digital communication network and at the receiving end, it is converted back into it's original analog form.
Pulse code modulation process involves the following three stages -
#Sampling
#Quantization and
#Coding

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Now observe the image given below
(Click the image to enlarge)-


Sampling and Quantization of analog signal, sampling og signal, Quantization of signal
Sampling and Quantization of analog signal

This image shows the following-
#Signal in its original analog form 
#The sampled signal obtained after sampling of this analog signal and 
#Quantized signal obtained after performing the quantization process.
Now we are going to discuss, the  process of sampling and quantization. During the discussion, please see the image carefully -
The first waveform given in the image is of the analog signal, that we want to transmit over the digital communication network. But since it is in analog form, therefore first it needs to be converted into digital form. So to do this job, we take the help of Pulse Code Modulation (PCM).

Sampling of signal

The second part of this image, shows the sampled signal. In the process of sampling of the signal, we convert the continuous time signal into discrete time signal. 

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You can see this conversion in the sampled signal shown in the image. The analog signal was continuous in time, since it had some value at every instant of time. But in the sampled signal, which is discrete in time, the value of the signal is present only at certain instants of time. Accuracy of the sampling increases with increase in frequency of the sampling. But some sampling error is introduced because of this sampling process, since it is not possible to have infinite sampling frequency practically.
Sampling of the analog signal is performed with the help of sampling theorem. So let me define here, what is sampling theorem.

Sampling Theorem

A continuous-time signal can be completely represented in its samples and recovered back into its original form if the sampling frequency is greater than or equal to twice the highest frequency present in the modulating signal (message signal).
It can be represented mathematically as-
fs >= 2fm
Here 'fs' is the sampling frequency and 'fm' is the highest frequency present in the modulating signal.

Quantization of Signal

To understand the process of quantization look at the image given above. In the process of quantization, amplitude of the signal is cut horizontally into a number of fixed levels. 

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Now the value of the signal is rounded-off (approximated) to the nearest level of amplitude. These certain levels of amplitude are shown in the image on the vertical axis.
Here we have divided the amplitude range (peak to peak amplitude) into 'l' number of levels. The magnitude of each level is equal to the peak to peak amplitude range divided by the number of levels. It is important to note here that, quantized signal is just an approximation of the original signal. As the number of levels increases, the accuracy of quantization increases. 
Pulse Code Modulation (PCM) technique  uses the process of quantization to convert analog signals into digital signals. 

Pulse Code Modulation Waveform

The image given below shows pulse code modulated waveform.

Pulse Code Modulation Waveform, Pulse Code Modulation, PCM waveform
Pulse Code Modulation Waveform

You can see in the image that PCM wave contains only two levels amplitude.

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These levels are represented by 0's and 1's. So it is clear that, here we have converted an analog signal having infinite number of levels of amplitude into just two levels of amplitude represented by 0's and 1's. Hence an analog signal has been converted into a digital signal.


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