Modulation is a technique in which message signal is transmitted to the
receiver with the help of carrier signal. Here in modulation, we combine both
carrier signal and message signal. You may get the doubt that what is the need
of modulation. Just imagine that you have a paper which contains the message
and you would like to send it to your friend standing 40 feet from your place.
You can’t just through the paper to your friend because paper will not travel
that much distance but if you take small stone and cover the paper with it and
through it to your friend, it will definitely reach the target. In the same
way, we need a carrier signal to transmit our message. Sometimes, message
signal is also called as modulating signal. The exact definition of modulation
is given below:
“Modulation is a process of message signal and modulating is varied
according to the carrier signal for transmission purpose. The message signal
can vary in accordance to the carrier signal that is in terms of angular or
amplitude. So we are modulating the signal.”
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Advantages of Modulation:
·
With the help of modulation, we can
increase the quality of reception.
·
We can also decrease the height of
the antenna.
·
Avoid mixing of different frequency
signals and increase the range of communication i.e. without modulation, we can
transmit the message up to 100 meters and with modulation, we can transmit the
message up to 150 meters.
·
Allow the flexibility for adjusting
the bandwidth.
In the definition, we have seen that message signal can be varied
according to the angular or amplitude of the carrier signal. What happens if
message signal is varied in accordance to angular of carrier signal and what
happens if message signal is varied according to the amplitude of carrier
signal?
Angle Modulation:
In the angle modulation, again there are two different types of
modulations.
1.
Frequency modulation
2.
Phase modulation.
1. Frequency Modulation:
The process of carrier signal frequency is varied according to the message
signal or modulation signal frequency by keeping the amplitude constant is
called frequency modulation.
Advantages of Frequency Modulation:
·
Frequency modulation has more noise
resistivity when compared to other modulation techniques. That’s why they are
mainly used in broadcasting and radio communications.
And we are all well aware that radio communication use mainly frequency modulation for transmission. We know that noise will occur mainly to the amplitude of the signal. In frequency modulation, amplitude is made constant and only frequency is varied, so we can easily find out the noise in the amplitude by using a limiter.
And we are all well aware that radio communication use mainly frequency modulation for transmission. We know that noise will occur mainly to the amplitude of the signal. In frequency modulation, amplitude is made constant and only frequency is varied, so we can easily find out the noise in the amplitude by using a limiter.
·
The frequency modulation is having
greater resistance to rapid signal strength variation, which we will use in FM
radios even while we are travelling and frequency modulation is also mainly
used in mobile communication purposes.
·
For transmitting messages in
frequency modulation, it does not require special equipment like linear amplifiers
or repeaters and transmission levels or higher when compared to other
modulation techniques. It does not require any class C or B amplifiers for
increasing the efficiency.
·
Transmission rate is good for
frequency modulation when compared to other modulation that is frequency
modulation can transmit around 1200 to 2400 bits per second.
·
Frequency modulation has a special
effect called capture effect in which high frequency signal will capture the
channel and discard the low frequency or weak signals from interference.
Disadvantages of Frequency
Modulation:
·
In the transmission section, we
don’t need any special equipment but in the reception, we need more complicated
demodulators for demodulating the carrier signal from message or modulating
signal.
·
Frequency modulation cannot be used
to find out the speed and velocity of a moving object. Static interferences are
more when compared to phase modulation. Outside interference is one of the
biggest disadvantages in the frequency modulation. There may be mixing because
of nearby radio stations, pagers, construction walkie-talkies etc.
·
To limit the bandwidth in the
frequency modulation, we use some filter which will again introduce some
distortions in the signal.
·
Transmitters and receiver should be
in same channel and one free channel must be there between the systems.
·
Spectrum space is limit for the
frequency modulation and careful controlling the deviation ration.
Applications of Frequency Modulation
(FM):
·
Frequency modulation is used in
radio’s which is very common in our daily life.
·
Frequency modulation is used in
audio frequencies to synthesize sound.
·
For recording the video signals by
VCR systems, frequency modulation is used for intermediate frequencies.
·
Used in applications of magnetic
tape storage.
2. Phase Modulation:
In the phase modulation, we vary the carrier signal in accordance with the
phase of the modulating signal or message signal by keeping the frequency
constant. If the amplitude of message or modulating signal is huge then the
phase shift will also be greater.
Advantages and Disadvantages of
Phase Modulation:
·
The main advantage of phase
modulation is that it has less interference from static, which is why we use
this type of modulation in finding out the speed or velocity of a moving object.
In frequency modulation, we cannot find out the velocity of moving object.
·
The main disadvantage is phase
ambiguity comes if we increase the phase modulation index, and data loss is
more and we need special equipment like frequency multiplier for increasing the
phase modulation index.
Applications of Phase Modulation:
·
Phase modulation application is not
different from frequency modulation. Phase modulation is also used in
communication systems.
·
It may be used in binary phase shift
keying.
Amplitude Modulation:
In the amplitude modulation, amplitude of carrier signal wave is varied in
accordance with the modulating or message signal by keeping the phase and
frequency of the signals constant. The carrier signal frequency would be
greater than the modulating signal frequency. Amplitude modulation is first
type of modulation used for transmitting messages for long distances by the
mankind. The AM radio ranges in between 535 to 1705 kHz which is great. But
when compared to frequency modulation, the Amplitude modulation is weak, but
still it is used for transmitting messages. Bandwidth of amplitude modulation
should be twice the frequency of modulating signal or message signal. If the
modulating signal frequency is 10 kHz then the Amplitude modulation frequency
should be around 20 kHz. In AM radio broadcasting, the modulating signal or
message signal is 15 kHz. Hence the AM modulated signal which is used for
broadcasting should be 30 kHz.
Advantages of Amplitude Modulation:
·
Because of amplitude modulation
wavelength, AM signals can propagate longer distances.
·
For amplitude modulation, we use
simple and low cost circuit; we don’t need any special equipment and complex
circuits that are used in frequency modulation.
·
The Amplitude modulation receiver
will be wider when compared to the FM receiver. Because, atmospheric
propagation is good for amplitude modulated signals.
·
Bandwidths limit is also big
advantage for Amplitude modulation, which doesn’t have in frequency modulation.
·
Transmitter and receiver are simple
in Amplitude modulation. When we take a demodulation unit of AM receiver, it
consists of RC filter and a diode which will demodulate the message signal or
modulating signal from modulated AM signal, which is unlike in Frequency
modulation.
·
Zero crossing in Amplitude
modulation is equidistant.
Disadvantages of Amplitude
Modulation:
·
Adding of noise for amplitude
modulated signal will be more when compared to frequency modulated signals.
Data loss is also more in amplitude modulation due to noise addition.
Demodulators cannot reproduce the exact message signal or modulating signal due
to noise.
·
More power is required during
modulation because Amplitude modulated signal frequency should be double than
modulating signal or message signal frequency. Due to this reason more power is
required for amplitude modulation.
·
Sidebands are also transmitted
during the transmission of carrier signal. More chances of getting different
signal interfaces and adding of noise is more when compared to frequency
modulation. Noise addition and signal interferences are less for frequency
modulation. That is why Amplitude modulation is not used for broadcasting songs
or music.
Applications of Amplitude
Modulation:
·
Used to carry message signals in
early telephone lines.
·
Used to transmit Morse code using
radio and other communication systems.
·
Used in Navy and Aviation for
communications as AM signals can travel longer distances.
·
Widely used in amateur radio.
All the above modulation will come under continuous wave modulation, where
we will use a sine wave as carrier signal. Continuous wave modulation can be
used in for both digital and analog communications.
When we take the pulse digital modulation, we use a periodic sequence of
rectangular pulses as the carrier signal. Pulse digital modulation is also used
in both analog and digital communications.
Types of Modulation:
In pulse width modulation, there are different types of modulation for
analog and digital as shown below:
·
PCM: Pulse Code
Modulation for Analog Modulation.
·
PPM: Pulse Position
Modulation for Digital Modulation
·
PDM: Pulse Duration
Modulation for Digital Modulation.
·
PAM: Pulse Amplitude
Modulation for Digital Modulation.
Types of Modulation – Tree Diagram:
1. Pulse Code Modulation (PCM):
Pulse Code Modulation is first introduced by Alec Reeves in the year 1937. In
the pulse code modulation, Analog Signal is reconstructed to digital signal for
ease of transmission by using the analog signal samples. In technical terms,
PCM will transmit the analog in a digital from, whose signal is sampled at
regular intervals of time and quantized at same quantum levels to digital code.
We know that digital code is nothing but binary code which consists of 1’s and
0’s that is logic1 and logic0. So we will transmit the digital data in the form
of 1’s and 0’s. When the signal is received by the receiver, demodulator in the
receiver will demodulate the binary signal back into pulses with same quantum
levels like in modulator and these pulses are again used for regenerating the
required analog signal.
Advantages of Pulse Code Modulation:
·
Pulse code modulation will have low
noise addition and data loss is also very low.
·
We can repeat the exact transmitted
signal at the receiver. This is called repeatability. And we can retransmit the
signal with any distortion loss also.
·
Pulse code modulation is used in
music play back CD’s and also used in DVD for data storing whose sampling rate
is bit higher.
·
Pulse code modulation can be used in
storing the data.
·
PCM can encode the data also.
·
Multiplexing of signals can also be
done using pulse code modulation. Multiplexing is nothing for adding the
different signals and transmitting the signal at same time.
·
Pulse code modulation requires large
bandwidth
·
Pulse code modulation permits the
use of pulse regeneration.
Disadvantages of Pulse Code
Modulation:
·
Specialized circuitry is required
for transmitting and also for quantizing the samples at same quantized levels.
We can do encoding using pulse code modulation but we need to have complex and
special circuitry.
·
Pulse code modulation receivers are
cost effective when we compared to other modulation receivers.
·
Developing pulse code modulation is
bit complicated and checking the transmission quality is also difficult and
takes more time.
·
Large bandwidth is required for
pulse code modulation when compared to bandwidth used by the normal analog
signals to transmit message.
·
Channel bandwidth should be more for
digital encoding.
·
PCM systems are complicated when
compared to analog modulation methods and other systems.
·
Decoding also needs special
equipment’s and they are also too complex.
Applications of Pulse Code
Modulation (PCM):
·
Pulse code modulation is used in
telecommunication systems, air traffic control systems etc.
·
Pulse code modulation is used in compressing
the data that is why it is used in storing data in optical disks like DVD, CDs
etc. PCM is even used in the database management systems.
·
Pulse code modulation is used in
mobile phones, normal telephones etc.
·
Remote controlled cars, planes,
trains use pulse code modulations.
2. Pulse Amplitude Modulation (PAM):
In pulse amplitude modulation, the amplitude of regular interval of
periodic pulses or electromagnetic pulses is varied in proposition to the
sample of modulating signal or message signal. This is an analog type of
modulation. In the pulse amplitude modulation, the message signal is sampled at
regular periodic or time intervals and this each sample is made proportional to
the magnitude of the message signal. These sample pulses can be transmitted
directly using wired media or we can use a carrier signal for transmitting
through wireless. There are two types of sampling techniques for transmitting
messages using pulse amplitude modulation, they are
·
FLAT TOP PAM: The amplitude
of each pulse is directly proportional to instantaneous modulating signal
amplitude at the time of pulse occurrence and then keeps the amplitude of the
pulse for the rest of the half cycle.
·
Natural PAM: The amplitude
of each pulse is directly proportional to the instantaneous modulating signal
amplitude at the time of pulse occurrence and then follows the amplitude of the
modulating signal for the rest of the half cycle.
Flat top PAM is the best for transmission because we can easily remove the
noise and we can also easily recognize the noise. When we compare the
difference between the flat top PAM and natural PAM, flat top PAM principle of
sampling uses sample and hold circuit. In natural principle of sampling, noise
interference is minimum. But in flat top PAM noise interference maximum. Flat
top PAM and natural PAM are practical and sampling rate satisfies the sampling
criteria.
There are two types of pulse amplitude modulation based on signal polarity
1.
Single polarity pulse amplitude
modulation
2.
Double polarity pulse amplitude
modulation
In single polarity pulse amplitude modulation, there is fixed level of DC
bias added to the message signal or modulating signal, so the output of
modulating signal is always positive. In the double polarity pulse amplitude
modulation, the output of modulating signal will have both positive and
negative ends.
Advantages of Pulse Amplitude
Modulation (PAM):
·
It is the base for all digital
modulation techniques and it is simple process for both modulation and
demodulation technique.
·
No complex circuitry is required for
both transmission and reception. Transmitter and receiver circuitry is simple
and easy to construct.
·
PAM can generate other pulse
modulation signals and can carry the message or information at same time.
Disadvantages of Pulse Amplitude
Modulation (PAM):
·
Bandwidth should be large for
transmitting the pulse amplitude modulation signal. Due to NY Quist criteria
also high bandwidth is required.
·
The frequency varies according to
the modulating signal or message signal. Due to these variations in the signal
frequency, interferences will be there. So noise will be great. For PAM, noise
immunity is less when compared to other modulation techniques. It is almost
equal to amplitude modulation.
·
Pulse amplitude signal varies, so
power required for transmission will be more, peak power is also, even at
receiving more power is required to receive the pulse amplitude signal.
Applications of Pulse Amplitude
Modulation (PAM):
·
It is mainly used in Ethernet which
is type of computer network communication, we know that we can use Ethernet for
connecting two systems and transfer data between the systems. Pulse amplitude
modulation is used for Ethernet communications.
·
It is also used for photo biology
which is a study of photosynthesis.
·
Used as electronic driver for LED
lighting.
·
Used in many micro controllers for
generating the control signals etc.
3. Pulse Position Modulation (PPM):
In the pulse position modulation, the position of each pulse in a signal
by taking the reference signal is varied according to the sample value of
message or modulating signal instantaneously. In the pulse position modulation,
width and amplitude is kept constant. It is a technique that uses pulses of the
same breath and height but is displaced in time from some base position
according to the amplitude of the signal at the time of sampling. The position
of the pulse is 1:1 which is propositional to the width of the pulse and also
propositional to the instantaneous amplitude of sampled modulating signal. The
position of pulse position modulation is easy when compared to other
modulation. It requires pulse width generator and monostable multivibrator.
Pulse width generator is used for generating pulse width modulation signal
which will help to trigger the monostable multivibrator, here trial edge of the
PWM signal is used for triggering the monostable multivibrator. After
triggering the monostable multivibrator, PWM signal is converted into pulse
position modulation signal. For demodulation, it requires reference pulse
generator, flip-flop and pulse width modulation demodulator.
Advantages of Pulse Position
Modulation (PPM):
·
Pulse position modulation has low
noise interference when compared to PAM because amplitude and width of the
pulses are made constant during modulation.
·
Noise removal and separation is very
easy in pulse position modulation.
·
Power usage is also very low when
compared to other modulations due to constant pulse amplitude and width.
Disadvantages of Pulse Position
Modulation (PPM):
·
The synchronization between
transmitter and receiver is required, which is not possible for every time and
we need dedicated channel for it.
·
Large bandwidth is required for
transmission same as pulse amplitude modulation.
·
Special equipment are required in
this type of modulations.
Applications of Pulse Position
Modulation (PPM):
·
Used in non-coherent detection where
a receiver does not need any Phase lock loop for tracking the phase of the
carrier.
·
Used in radio frequency (RF)
communication.
·
Also used in contactless smart card,
high frequency, RFID (radio frequency ID) tags and etc.
4. Pulse Duration Modulation (PDM)
or Pulse Width Modulation (PWM):
It is a type of analog modulation. In pulse width modulation or pulse
duration modulation, the width of the pulse carrier is varied in accordance
with the sample values of message signal or modulating signal or modulating
voltage. In pulse width modulation, the amplitude is made constant and width of
pulse and position of pulse is made proportional to the amplitude of the
signal. We can vary the pulse width in three ways
1.
By keeping the leading edge constant
and vary the pulse width with respect to leading edge
2.
By keeping the tailing constant.
3.
By keeping the center of the pulse
constant.
We can generate pulse width using different circuitry. In practical, we
use 555 Timer which is the best way for generating the pulse width modulation
signals. By configuring the 555 timer as monostable or astable multivibrator,
we can generate the PWM signals. We can use PIC, 8051, AVR, ARM, etc.
microcontrollers to generate the PWM signals. PWM signal generation has n
number of ways. In demodulation, we need PWM detector and its related circuitry
for demodulating the PWM signal.
Advantages of Pulse Width Modulation
(PWM):
·
As like pulse position modulation,
noise interference is less due to amplitude has been made constant.
·
Signal can be separated very easily
at demodulation and noise can also be separated easily.
·
Synchronization between transmitter
and receiver is not required unlike pulse position modulation.
Disadvantages of Pulse Width
Modulation (PWM):
·
Power will be variable because of
varying in width of pulse. Transmitter can handle the power even for maximum
width of the pulse.
·
Bandwidth should be large to use in
communication, should be huge even when compared to the pulse amplitude
modulation.
Applications of Pulse Width
Modulation (PWM):
·
PWM is used in telecommunication
systems.
·
PWM can be used to control the
amount of power delivered to a load without incurring the losses. So, this can
be used in power delivering systems.
·
Audio effects and amplifications
purposes also used.
·
PWM signals are used to control the
speed of the robot by controlling the motors.
·
PWM is also used in robotics.
·
Embedded applications.
·
Analog and digital applications etc.
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Source: electronicshub
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