Multimedia Presentation
Pulse Modulation
Presented byKetki Kulkarni-3764.Savitri Naik-3778.Dipali Patil-3792.Shweta Patil-3796.Ankita Sawant-3819.
IntroductionModulation is the process of frequency translation in which any
one parameter(Amplitude, frequency or phase) of high frequency carrier signal is varied in accordance with instantaneous value of low frequency modulating signal.
Modulation is either analog or digital.A digital signal is superior to an analog signal because it is more
robust to noise and can easily be recovered, corrected and amplified. For this reason, the tendency today is to change an analog signal to digital data.
The process of transmitting signals in the form of pulses (discontinuous signals) by using special techniques.
This process is called pulse modulation
Types of Pulse Modulation.
Analog Pulse Modulation
Digital Pulse Modulation
Pulse Modulation
Pulse Amplitude Modulation (PAM)Pulse Width Modulation (PWM)
Pulse Code Modulation (PPM)
Pulse Position Modulation (PPM)
Pulse Density Modulation (PPM)
Types of Pulse Modulation
Pulse Amplitude Modulation Pulse Width Modulation Pulse Position Modulation Pulse Code Modulation Pulse Denstity Modulation
Sampling The signal is sampled at regular intervals such that each
sample is propotional to the amplitude of signal at that instant.This technique is called “sampling”.
Sampling means breaking of continous waves into discrete set of points
Sampling is common in all pulse modulation techniques.Analog signal is sampled every TS secs.Ts is referred to as the sampling interval. fs = 1/Ts is called the sampling rate or sampling
frequency.
Sampling Methods
There are 3 sampling methods: Ideal - an impulse at each sampling instant Natural - a pulse of short width with varying
amplitude Flat top - sample and hold, like natural but with
single amplitude value
Sampling Methods
Pulse Amplitude Modulation
In Pulse-Amplitude Modulation (PAM) a pulse signal is used to sample an analog signal. The result is a train of constant-width pulses.
The amplitude of each pulse is proportional to the amplitude of the message signal at the time of sampling.
Generation of PAM Signal
Modulating Signal
Low Pass Filter
MultiplierPAM signal
Pulse train generator
Generation of PAM Signal
The carrier is in the form of narrow pulses having frequency fs.The uniform sampling takes place in multiplier to generate PAM signal.
The PAM signal can be detected by passing it through a low pass filter.
Generation of PAM Signal
Advantages & Disadvantages
Advantages:-Generation & detection is easy.
Disadvantages:-Added noise cannot be removed easily as it has
impact on amplitude which carries information.Transmission bandwidth is too long.
Pulse Width ModulationIn this type, the amplitude is maintained constant
but the width of each pulse is varied in accordance with instantaneous value of the analog signal.
In pulse width modulation (PWM), the width of each pulse is made directly proportional to the amplitude of the information signal.
Generation of PWM signal
Generation of PWM signalThe block diagram of next slide can be used for
generation of PWM as well as PPM. In this case a sawtooth signal of frequency fs is a sampling signal.
It is applied to inverting terminal of a comparator with modulating signal at non inverting terminal.
O/P remains high as long as modulating signal is higher than that of ramp signal.
Generation of PWM signal
Advantages & DisadvantagesAdvantages:-Very good noise immunity.Its possible to separate out signal from noise.
Disadvantages:-Bandwidth requirement is large as compared to
PAM.
Pulse Position ModulationIn this type, the sampled waveform has fixed
amplitude and width whereas the position of each pulse is varied as per instantaneous value of the analog signal.
PPM signal is further modification of a PWM signal.
Generation of PPM signal
Generation of PPM signalThe PPM signal can be generated from PWM signal.The PWM pulses obtained at the comparator output are
applied to a monostable multivibrator which is –ve edge triggered.
Hence for each trailing edge of PWM signal, the monostable output goes high. It remains high for a fixed time decided by its own RC components. Thus as the trailing edges of the PWM signal keeps shifting in proportion with the modulating signal, the PPM pulses also keep shifting.
Therefore all the PPM pulses have the same amplitude and width. The information is conveyed via changing position of pulses.
Generation of PPM signal
Advantages & Disadvantages
Advantages:-High noise immunity.
Disadvantages:-Generation & detection is complex..
Pulse Code ModulationIn PCM, the available range of signal voltages is
divided into levels and each is assigned a binary number
The number of levels available depends upon the number of bits used to express the sample value
The number of levels is given by: N = 2mPCM consists of three steps to digitize an analog signal:I. SamplingII. QuantizationIII. Binary encoding
Generation of PCM Signal
Generation of PCM SignalSampling: The process of generating pulses of zero
width and of amplitude equal to the instantaneous amplitude of the analog signal. The no. of pulses per second is called “sampling rate”.
Quantization: The process of dividing the maximum value of the analog signal into a fixed no. of levels in order to convert the PAM into a Binary Code. The levels obtained are called “quanization levels”.
Generation of PCM Signal
Time
Voltage
76543210
111 110 101 100 011 010 001 000
Levels
Binary
Codes
Time
Time
Voltage
0 1 0 1 0 1 1 1 0 1 1 1 1 1 0 1 0 1 0 1 0
Advantages & DisadvantagesAdvantages:-Secured.Encoding is possible.Very high noise immunity.Convenient for long distance communication.Good signal to noise ratio.Disadvantages:-Complex circuitry.Requires large bandwidth.Synchronization is required between Transmitter &
Reciever
Pulse Density ModulationPulse Density Modulation (PDM) involves changing
the density with which adjacent pulses are packed together.
It is a one-bit, high rate data stream that conveys a signal by modulating the density of the pulses
Generation of PDM SignalA PDM bitstream is encoded from an analog signal
through the process of delta-sigma modulation. This process uses a one bit quantizer that produces either
a 1 or 0 depending on the amplitude of the analog signal. A 1 or 0 corresponds to a signal that is all the way up or
all the way down, respectively
Generation of PDM Signal
Advantages & DisadvantagesAdvantages:-One bit code word for output.Low signaling rate.Low channel bandwidth.
Disadvantages:-Granular noise present.
Thank You