A message signal cannot travel a long
distance because of its low signal strength. In addition to this, physical
surroundings, the addition of external noise and travel distance will further
reduce the signal strength of a message signal. So in order to send the message
signal to a long distance, we need to increase the signal strength of a message
signal. This can be achieved by using a high frequency or high energy signal
called carrier signal. A high energy signal can travel to a larger distance
without getting affected by external disturbances. We take the help of such
high energy signal to transmit the message signal. This high energy or high
frequency signal is known as carrier signal.
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The low energy message signal is mixed with
the high energy or high frequency carrier signal to produce a new high energy
signal which carries information to a larger distance.
The question arises how the message signal
should be added to the carrier signal. The solution lies in changing some
characteristics (amplitude, frequency or phase) of a carrier signal in
accordance with the amplitude of the message signal. This process is called
modulation. Modulation means to “change”.
The Message signal contains information
whereas the carrier signal contains no information. Carrier signal is used just
to transmit the information to a long distance. At the destination, the message
signal is consumed whereas the carrier signal is wasted.
In modulation process, the characteristics of
the carrier signal is changed but the message signal characteristics will not
be changed. The carrier signal does not contain any information so even if we
change the characteristics of the carrier signal, the information contained in
it will not be changed. However, the message signal contains information so if
we change the characteristics of the message signal, the information contained
in it will also changes. Therefore, we always changes the characteristics of
the carrier signal but not the message signal.
Modulation allows the transmission to occur at
high frequency while it simultaneously allows the carrying of the message
signal.
Modulation Definition
Modulation is the process of mixing a low
energy message signal with the high energy carrier signal to produce a new high
energy signal which carries information to a long distance.
Or
Modulation is the process of changing the
characteristics (amplitude, frequency or phase) of the carrier signal, in
accordance with the amplitude of the message signal.
A device that performs modulation is called
modulator.
Example:
The modulation process can be understood with
a simple example. The below figure shows the amplitude modulation.
The first figure shows the modulating signal
or message signal which contains information, the second figure shows the high
frequency carrier signal which contains no information and the last figure
shows the resultant amplitude modulated signal.
From the above three figures, it can be
observed that the amplitude of the carrier signal is varied in accordance with
the instant amplitude of the message signal.
Types of Signals in Modulation
In modulation process, three types of signals
are used to transmit information from source to destination. They are:
1) Message signal
2) Carrier signal
3) Modulated signal
1) Message signal
The signal which contains a message to be
transmitted to the destination is called a message signal. The message signal
is also known as a modulating signal or baseband signal.
The original frequency range of a
transmission signal is called baseband signal. The message signal or baseband
signal undergoes a process called modulation before it gets transmitted over
the communication channel. Hence, the message signal is also known as the
modulating signal.
2) Carrier signal
The high energy or high frequency signal
which has characteristics like amplitude, frequency, and phase but contains no
information is called a carrier signal. It is also simply referred to as a
carrier. Carrier signal is used to carry the message signal from transmitter to
receiver. The carrier signal is also sometimes referred to as an empty signal.
3) Modulated signal
When the message signal is mixed with the
carrier signal, a new signal is produced. This new signal is known as a
modulated signal. The modulated signal is the combination of the carrier signal
and modulating signal.
Need for Modulation
Modulation is extremely necessary in
communication system because of the following reasons:
1) Avoids mixing of signals
2) Increase the range of communication
3) Wireless communication
4) Reduces the effect of noise
5) Reduces height of antenna
1) Avoids mixing of signals
One of the basic challenges facing by the
communication engineering is transmitting individual messages simultaneously
over a single communication channel. A method by which many signals or multiple
signals can be combined into one signal and transmitted over a single
communication channel is called multiplexing.
We know that the sound frequency range is 20
Hz to 20 KHz. If the multiple baseband sound signals of same frequency range
(I.e. 20 Hz to 20 KHz) are combined into one signal and transmitted over a
single communication channel without doing modulation, then all the signals get
mixed together and the receiver cannot separate them from each other. We can
easily overcome this problem by using the modulation technique.
By using modulation, the baseband sound
signals of same frequency range (I.e. 20 Hz to 20 KHz) are shifted to
different frequency ranges. Therefore, now each signal has its own frequency
range within the total bandwidth.
After modulation, the multiple signals having
different frequency ranges can be easily transmitted over a single
communication channel without any mixing and at the receiver side, they can be
easily separated.
2) Increase the range of communication
The energy of a wave depends upon its
frequency. The greater the frequency of the wave, the greater the energy
possessed by it. The baseband audio signals frequency is very low so they
cannot be transmitted over large distances. On the other hand, the carrier
signal has a high frequency or high energy. Therefore, the carrier signal can
travel large distances if radiated directly into space.
The only practical solution to transmit the
baseband signal to a large distance is by mixing the low energy baseband signal
with the high energy carrier signal. When the low frequency or low energy
baseband signal is mixed with the high frequency or high energy carrier signal,
the resultant signal frequency will be shifted from low frequency to high
frequency. Hence, it becomes possible to transmit information over large
distances. Therefore, the range of communication is increased.
3) Wireless communication
In radio communication, the signal is
radiated directly into space. The baseband signals have very low frequency range (I.e. 20
Hz to 20 KHz). So it is not possible to radiate baseband signals directly into
space because of its poor signal strength. However, by using the modulation
technique, the frequency of the baseband signal is shifted from low frequency
to high frequency. Therefore, after modulation, the signal can be directly
radiated into space.
4) Reduces the effect of noise
Noise is an unwanted signal that enters the
communication system via the communication channel and interferes with the
transmitted signal.
A message signal cannot travel for a long
distance because of its low signal strength. Addition of external noise will
further reduce the signal strength of a message signal. So in order to send the
message signal to a long distance, we need to increase the signal strength of
the message signal. This can be achieved by using a technique called
modulation.
In modulation technique, a low energy or low
frequency message signal is mixed with the high energy or high frequency
carrier signal to produce a new high energy signal which carries information to
a long distance without getting affected by the external noise.
5) Reduces height of antenna
When the transmission of a signal occurs over
free space, the transmitting antenna radiates the signal out and receiving
antenna receives it. In order to effectively transmit and receive the signal,
the antenna height should be approximately equal to the wavelength of the
signal to be transmitted.
Now,
The audio signal has a very low frequency
(I.e. 20 Hz to 20 kHz) and longer wavelength, so if the signal is transmitted
directly into space, the length of the transmitting antenna required would be
extremely large.
For instance, to radiate an audio signal
frequency of 20 kHz directly into space, we would need an antenna height of
15,000 meters.
The antenna of this height is practically
impossible to construct.
On the other hand, if the audio signal (20
Hz) has been modulated by a carrier wave of 200 MHz. Then, we would need an
antenna height of 1.5 meters.
The antenna of this height is easy to construct.
Types of Modulation
Basically, the modulation is classified into
two types: analog modulation and digital modulation.
In analog modulation, the analog signal
(sinusoidal signal) is used as a carrier signal that modulates the analog
message signal. In analog modulation, the characteristics (amplitude, frequency
or phase) of the carrier signal is varied in accordance with the amplitude of
the message signal.
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There are four basic
types of analog modulation:
1) Amplitude modulation
2) Frequency modulation
3) Phase modulation
4) Analog pulse modulation
1) Amplitude modulation
Amplitude modulation is a type of modulation
where the amplitude of the carrier signal is varied (changed) in accordance
with the amplitude of the message signal while the frequency and phase of
carrier signal remain constant.
The above figure shows the amplitude
modulation.
The first figure shows the modulating signal
or message signal which contains information, the second figure shows the high
frequency carrier signal which contains no information and the last figure
shows the resultant amplitude modulated signal.
From the above three figures, it can be
observed that the amplitude of the carrier signal is varied in accordance with
the instant amplitude of the message signal.
2) Frequency modulation
Frequency modulation is a type of modulation
where the frequency of the carrier signal is varied (changed) in accordance
with the amplitude of the message signal while the amplitude and phase of
carrier signal remain constant.
The above figure shows the frequency
modulation.
The first figure shows the modulating signal
or message signal, the second figure shows the high frequency carrier signal
which contains no information and the last figure shows the resultant frequency
modulated signal.
From the above three figures, it can be
observed that the frequency of the carrier signal is varied in accordance with
the instant amplitude of the message signal.
3) Phase modulation
Phase modulation is a type of modulation
where the phase of the carrier signal is varied (changed) in accordance with
the amplitude of the message signal while the amplitude of carrier signal
remains constant.
The above figure shows the phase modulation.
The first figure shows the modulating signal
or message signal, the second figure shows the high frequency carrier signal
which contains no information and the last figure shows the resultant phase
modulated signal.
From the above three figures, it can be
observed that the phase of the carrier signal is varied in accordance with the
instant amplitude of the message signal.
In this type of modulation, when the phase is
changed it also affects the frequency so this modulation also comes under
frequency modulation.
The frequency and phase modulation comes
under angle modulation. When the frequency or phase of the carrier signal is
varied (changed) in accordance with the amplitude of the message signal, then
it is called angle modulation.
4) Analog pulse modulation
In amplitude, frequency and phase modulation
techniques, the carrier and message signals are continuous signals (sinusoidal
signals). However, in analog pulse modulation, the carrier signal is a
discontinuous signal (series of pulses) and message signal is a continuous
signal (sinusoidal signal).
Analog pulse modulation is the process of
changing the characteristics (pulse amplitude, pulse width or pulse position)
of the carrier pulse, in accordance with the amplitude of the message signal.
The analog pulse modulation is again
classified as,
1. Pulse amplitude modulation
2. Pulse width modulation
3. Pulse position modulation
Example:
The analog pulse modulation can be understood
with a simple example. The below figure shows the pulse amplitude modulation.
The first figure shows the message signal
(continuous signal), the second figure shows the carrier pulse train (series of
pulses) which contains no information and the last figure shows the resultant
pulse amplitude modulated (PAM) signal.
From the above three figures, it can be
observed that the amplitude of the series of carrier pulses are varied
(changed) in accordance with the instant amplitude of the message signal while
the width and positions of the carrier pulses remain constant.
Analog modulation is more sensitive to noise.
Noise is an unwanted signal that enters the communication system via the
communication channel and interferes with the transmitted signal. The noise
signal degrades the transmitted signal (signal containing information).
Therefore, this drawback can be overcome by the digital modulation technique.
The digital modulation technique is employed
for efficient communication. The main advantage of the digital modulation over
analog modulation include high noise immunity, available bandwidth, and
permissible power. In digital modulation, the modulating signal or message
signal is converted from analog to digital.
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Pulse Code Modulation (PCM)
In digital modulation, the modulation
technique used is Pulse Code Modulation (PCM). The pulse code modulation is the
method of converting an analog signal into a digital signal I.e. 1s and 0s. As
the resultant signal is a coded pulse train, this is called as pulse code
modulation.
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Source：physics-and-radio-electronics
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