digicom 101

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DIgital CommunicationECE 4 L

2013

Pulse Modulation

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Elements of Digital

Communication System

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I n Source and I nput Transducer :

Digital Source:

Source alphabet Symbol rate

Source alphabet

probabilities

Probabilistic dependenceof symbols in a sequence

Analog source:

audio

video signal

Elements of Digital


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Source Encoder use as few binary digits as possible to represent the signal.

This sequence of binary digits is called information sequence.

Source Encoding or Data Compression: the process of efficientlyconverting the

output of wither analog or digital source into a sequence of binary digits

Codeworda group of bits used to represent symbols

Blocksizemaximum number of distinct codewords

Codeword lengthnumber of bits used to represent each codeword

Average data rate

Effeciency of the encoder

Elements of Digital


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Channel Encoder: to introduced, in controlled manner, some redundancy in the binary

information sequence that can be used at the receiver to overcome

the effects of noise and interference encountered in thetransmission on the signal through the channel.

Parameters:

Coding rate that depends upon the number the redundant bit added

Coding method used Coding efficiency

Error control capabilities

Feasibility of the encoder and decoder

Elements of Digital


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Digital Modulator: The binary sequence is passed to digital modulator which in turns

convert the sequence into electric signals so that we can transmit

them on channel

Parameters:

Transmission bandwidth

Probability of symbol

Synchronous or asynchronous method of detection

Complexity of implementation

Elements of Digital


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Channel: is the physical medium that is used for transmitting signals from

transmitter to receiver.

Digital Demodulator: processes the channel corrupted transmitted waveform and reduces

the waveform to the sequence of numbers that represents estimates

of the transmitted data symbols.

Elements of Digital


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Channel Decoder: attempts to reconstruct the original information sequence from the

knowledge of the code used by the channel encoder and the

redundancy contained in the received data

Source Decoder At the end, if an analog signal is desired then source decoder tries

to decode the sequence from the knowledge of the encoding

algorithm. And which results in the approximate replica of the

input at the transmitter end

Elements of Digital


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Merits of Digital

Communication

1. Digital signals are very easy to receive.

2. In digital signals, the original signal can

be reproduced accurately.

3. digital signals can be cleaned up to

restore the quality and amplified by the

regenerators.

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4. The noise may change the shape of the pulses

but not the pattern of the pulses.

5. But digital signals can be coded so that only the

person, who is intended for, can receive them.

6. digital signals can be stored at the receiving

end.

7. The digital signals can be processed

Merits of Digital

Communication

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Analog data

Takes on continuous values. Ex. Voice or video

Digital data

Takes on discrete values. Ex. Text and integers

Analog Signal

Continuously varying electromagnetic wave representing datacarried over a variety of medium

Digital Signal Sequence of voltage pulses representing data transmitted over a

wire medium

Data and Signal

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Analog or Digital Data Can Be Represented By

Either Analog or Digital Signals.

These Signals Can Then Be Propogated (Moved

Along a Medium).

Optical Fiber Only Propogates Analog Signals

Remember!

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Analog Data, Analog Signals

radio

Digital Data, Analog Signals (modem)

broadband & wireless

Analog Data, Digital Signals [codec]

Frequency Division Multiplexing (FDM)

Wave Division Multiplexing (WDM) [fiber]

Time Division Multiplexing (TDM)

Pulse Code Modulation (PCM) Delta Modulation

Digital Data, Digital Signals (baseband)

wired LAN, (e.g., Ethernet)

Data and Signal

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Digital dataDigital Signal

Easy and simple to implement

Analog data Digital Signal

Allows the use of digital transmission and switching equipment

Digital dataAnalog Signal

Allows us of the public telephone system

Allows use of optical fiber

Analog DataAnalog Signal

Easy

Telephone system was primarily analog

Data and Signal

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Short distance transmissions, baseband

modulation is usually used.

Baseband modulation is often called line coding For long distance and wireless transmissions,

bandpass modulation is usually used.

Bandpass modulation is also called carrier

modulation

Remember!

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Consist essentially of sampling analog information signals

Then converting those samples into discrete pulses

Transporting the pulses from a source to destination over aphysical transmission medium.

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SAMPLING

Sampling is the process of taking samples of theanalogue signals at given interval of time. Only samplesare being transmitted.

If sufficient samples are sent and sampling theoremare met the original signal can be reconstructed at thereceiver

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SAMPLING THEOREM

Sampling theorem states that, if the sampling rate inany pulse modulation system exceeds twice themaximum information signal frequency, the original

signal can be reconstructed in the receiver withminimum distortion.

This is called Nyquist Rate, fs 2fmax

fssampling frequency,

fmaxmaximum freq of the modulating signal

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This slides includes:

Pulse AmplitudeModulation

Pulse Width Modulation

Pulse PositionModulation

Pulse Code Modulation

The process of transmitting signals in the form of

pulses by using special techniques.

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Analog Pulse Modulation Digital Pulse Modulation

Pulse Amplitude (PAM)

Pulse Width (PWM)

Pulse Position (PPM)

Pulse Code (PCM)

Delta (DM)

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Analog pulse modulation

A periodic pulse train is used as

the carrier wave

Some characteristic feature of

each pulse is varied in a

continuous manner in

accordance with the

corresponding sample value of

the message signal

Analog pulse-modulation

systems rely on the samplingprocess to maintain continuous

amplitude representation of the

message signal

Digital pulse modulation

The message signal is represented

in a form that is discrete in both

time and amplitude

Its transmission in digital form as

a sequence of coded pulse

Digital pulse-modulation system

use not only the sampling process

but also the quantization process.

Digital modulation makes it

possible to exploit the full powerof digital signal-processing

techniques.

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* amplitude of discrete carrier signal changes in accordance

with the instantaneous amplitude of modulating

signal(message signal) keeping width and position of carrier

constant*The signal is sampled at regular intervals such that each

sample is proportional to the amplitude of the signal at that

sampling instant. This technique is calledsampling.

* For minimum distortion, the sampling rate should be morethan twice the signal frequency.

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AND

Gate

Pulse Shaping

Network

FM

Modulator

Analog

Signal

PAM - FM

Pulses at sampling frequencyHF Carrier Oscillator

PAM

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Analog Signal

Amplitude Modulated

Pulses

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There are 2 types of PAM :

1. Natural sampling

2. Flat top sampling

Types of PAM

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Natural Sampling

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Flat Top Sampling

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Merits

Generation and detection is easy.

Demerits

Added noise cannot be removed easily asit has impact on amplitude which carries

information. Transmission bandwidth is too large.

Merits and Demerits of PAM

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* the amplitude is maintained constantbut the duration or

lengthor widthof each pulse is varied in accordance withinstantaneous value of the analog signal keeping amplitude

and position of carrier constant

* The negative side of the signal is brought to the positive

side by adding a fixed d.c. voltage.


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PWM Waveform

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Merits

Very good noise immunity.

Its possible to separate out signal fromnoise.

Demerits

Bandwidth requirement is large ascompared to PAM.

Merits and Demerits of PWM

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* In this type, the sampled waveform has fixed amplitude and

widthwhereas the positionof each pulse is variedas per

instantaneous value of the analog signal.

* PPM signal is further modification of a PWM signal. It has

positive thin pulses(zero time or width) corresponding to the

starting edgeof a PWM pulse and negative thin pulses

corresponding to the ending edgeof a pulse.

Pulse Position Modulation

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* This wave can be

further amended

by eliminating the

whole positivenarrow pulses.

The remaining

pulse is called

clipped PPM.

PWM

PPM


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The modulation system in which

position of the discrete carrier signal

changes in accordance with theinstantaneous amplitude of modulating

signal(message signal) keeping

amplitude and Width of carrier constantis called as PPM.

Pulse Position Modulation

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PPM Generator

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PPM Waveform

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Merits

High noise immunity.

Demerit Generation and detection is complex.

Merits and Demerits of PPM

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PAM, PWM and PPM at a glance:

Analog Signal

Amplitude Modulated Pulses

Width Modulated Pulses

Position Modulated Pulses

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lets move on to

digital pulse modulation.

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ANALOG-TO-DIGITAL

CONVERSION

A digital signal is superior to an analogsignal because it is more robust to noiseand can easily be recovered, corrected

and amplified.For this reason, the tendency today is tochange an analog signal to digital data.

Generally used two techniques are :

pulse code modulation and

delta modulation.

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It is the type of pulse modulation in

which the group of pulses or codes are

transmitted which represent binarynumbers corresponding to modulating

signal voltage.

They are a primary building block for advanced communication

systems


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PCM is the most commonly used technique in digital communications

Used in many applications:

Telephone systems

Digital audio recording CD laser disks

voice mail

digital video etc.

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Trivia!

44

PCM was invented by the British engineer

Alec Reevesin 1937in France.

It was not until about the middle of 1943that the Bell Labspeople became aware of

the use of PCM binary coding as already

proposed by Alec Reeves.
http://en.wikipedia.org/wiki/Alec_Reeveshttp://en.wikipedia.org/wiki/1937http://en.wikipedia.org/wiki/Bell_Labshttp://en.wikipedia.org/wiki/Bell_Labshttp://en.wikipedia.org/wiki/1937http://en.wikipedia.org/wiki/Alec_Reeves

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PCM Encoder

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PCM consists of three steps to digitize an analog signal:

1. Sampling:

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.

Nyquist theorem


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3 DIFFERENT SAMPLING METHODS

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2. Quantization:

The process of dividing the maximum value of the analog

signal into a fixed no. of levels in order to convert thePAM into a Binary Code.

The levels obtained are called quanization levels.

quantizing process will produce errors called

quantizing errors or quantizing noise


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Two types of quantization.

(a) midtread (b) midrise

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Illustration of the quantization process

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Nonuniform Quantizing

Voice analog signals are more likely to have

amplitude values near zero than at the extreme

peak values allowed.

For signals with nonuniform amplitude distribution,the granular quantizing noise will be a serious

problem if the step size is not reduced for amplitude

values near zero and increased for extremely large

values. This is called nonuniform quantizing since avariable step size is used.

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Nonuniform Quantizing

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Quantization Error and SNQR

When a signal is quantized, we introduce an error -the coded signal is an approximation of the actualamplitude value.

The difference between actual and coded value(midpoint) is referred to as the quantization error.

Signals with lower amplitude values will suffer morefrom quantization error as the error range: /2, isfixed for all signal levels.

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Non linear quantization is used to alleviate thisproblem. Goal is to keep SNQR fixed for all samplevalues.

Two approaches: The quantization levels follow a logarithmic curve.

Smaller s at lower amplitudes and largers athigher amplitudes.

Companding: The sample values are compressedat the sender into logarithmic zones, and thenexpanded at the receiver.


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Qe=Resolution/2

SNQR = minimum voltage / quantization noise voltage

SNQR = 10 log (average signal power/average quantizationnoise power)


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PCM consists of three steps to digitize an analog signal:

3. Binary encoding:

Note:

A digital signalis described by its bit ratewhereas

analog signalis described by its frequency range.

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Encoding

Encoding is the process of representing the sampledvalues as a binary number in the range 0to n.

The value of nis chosen as a power of 2, depending

on the accuracy required.

Increasing nreduces the step size between adjacentQuantization levels and hence reduces theQuantization noise.

The down side of this is that the amount of digitaldata required to represent the analog signalincreases.

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PCM Decoder

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To recover an analog signal from a digitized signal

we follow the following steps:

We use a hold circuit that holds the amplitude value of

a pulse till the next pulse arrives.

We pass this signal through a low pass filter with a

cutoff frequency that is equal to the highest frequency

in the pre-sampled signal.

PCM Decoder

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PCM TRANSMISSION SYSTEM

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Dynamic Range

Resolution

Maximum allowable input amplitude

Coding efficiency

Ratio of the largest possible magnitude to

the smallest possible magnitude that can be

decoded

PCM Parameter

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Dynamic Range, DR

Ratio of the largest possible magnitude to the

smallest possible magnitude that can bedecoded

DR=Vmax/Vmin = Vmax/Resolution

2n1 >=DR

PCM Parameter

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Is the ratio of the strongest possible signal that can betransmitted and the weakest discernible signal

In a linear PCM system, the maximum dynamic range is

found by:

DR = (1.76 + 6.02m) dB

Dynamic Range

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Companding

Sometimes called compassion

used to improve dynamic range

Compression is used on the transmitting end andexpandingis used on the receiving end

Keep the bit rate and bandwidth low

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A LAW & - LAW

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A LAW & - LAW

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Mu Law

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The human auditory system is believed to be alogarithmic process in which high amplitude soundsdo not require the same resolution as low amplitude

sounds. The human ear is more sensitive to quantization

noise in small signals than large signals.

A-law and -law coding apply a logarithmic

quantization function to adjust the data resolution inproportion to the level of the input signal.

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Speech Companding

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quantises the differencebetween the original and the

predicted signals, i.e. the difference between

successive values.

Leads to reduction in the number of bits used persample over that used for PCM. Using DPCM can

reduce the bit rate of voice transmission down to 48

kbps.

Speech Companding

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Inter Symbol Interference

If the system impulse response h(t)extends over

more than 1 symbol period, symbols become

smeared into adjacent symbol periods

Known as inter symbol interference (ISI)

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Time (bit periods)0 2 4 6

amplitud

e

0.5

1.0

Time (bit periods)0 2 4 6

amplitud

e

0.5

1.0

Modulator input Slicer input

Binary 1 Binary 1

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Inter Symbol Interference

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Noise in PCM Systems

The performance of a PCM system is influenced by

two noise sources:

(1) channel noise

introduce bit errors into the received signal. The

presence of this noise can be measured in terms of

probability of symbol erroror bit error rate

(BER).

can be made practically negligible by using highsignal energy-to-noise density ratio through short

spacing between regenerative repeaters.

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Noise in PCM Systems

(2) quantization noise.

can be made negligible by increasing the number

of levelsL

selecting a compressor-expander pair that is

matched to the message signal characteristics.

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Limitations of PCM systems

Choosing a discrete value near the analog

signal for each sample leads to quantization

error

Between samples no measurement of the

signal is made;

Accurate clock is required for accurate

reproduction

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Merits

Secured.

Encoding is possible.

Very high noise immunity.

Convenient for long distance communication.

Good signal to noise ratio.

Merits and Demerits of PCM

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Demerits

Complex circuitry.

Requires large bandwidth. Synchronization is required between

transmitter & receiver.

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only one bit is transmitted per sample

That bit is a oneif the current sample is

more positivethan the previous sample,

and a zeroif it is more negative

Since so little information is transmitted,

delta modulation requires higher

sampling rates than PCM for equal

quality of reproduction

Delta Modulation (DM)

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D lt M d l ti (DM)

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D lt M d l ti (DM)

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sizesteptheisand,ofversionquantizedthe

is,outputquantizertheiswhere

1

)sgn(

1

issignalerrorThe

).(ofsampleais)(andperiodsamplingtheiswhere

,2,1,0,)(Let

ne

nenm

nenmnm

nene

nmnmne

tmnTmT

nnTmnm

qq

qqq

q

q

ss

s


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DM S t

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DM System

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Sl l d di t ti

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Slope overload distortion

And Granular noise

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Merits

One bit code word for output.

Low signaling rate.

Low channel bandwidth.

No ADC is required

Demerits

1. Slope overload present.

2. Granular noise present.

Merits and Demerits of DM

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D lt Si M d l ti

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Delta-Sigma Modulation

Alternatively known as Pulse Density

modulation or Pulse Frequency modulation

Modification of the delta modulation

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D lt Si M d l ti

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Conventionaldelta modulation - Quantizerinput is an approximationof the derivativeof the input message signalm(t).

Results in the accumulation of error (noise)accumulated noise (transmission disturbances)

at the receiver (cumulative error).

Possible solution: integrating the messagebefore delta modulationcalled delta sigmamodulation

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The message signal is defined in itscontinuous formso pulse modulatorcontains a hard limiter and a pulse generator

to produce a 1-bit encoded signal integration at the tx requires differentiation

at the rx side.

But:As in conventional DM the messagehas to be integratedat the final stage thiseliminates the need of differentiation here.


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Delta-Sigma Modulation System

Figure 3.25

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Merits

Low frequency component of input signal

is boosted

Correlation between adjacent samples ofdelta modulator is increased

Simplifies the receiver design

Demerits Requires sampling rate far in excess of

the Nyquist rate

Merits and Demerits of DSM

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Adaptive Delta Modulation

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This is the advanced version of DM.

Avoid the problem on slope over load error

and granular noise problem.

step size is adapted to the slope

(variation) of the message signal.

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If successive errors are of opposite polarity, then

the delta modulator is operating in the granular

mode; in such a case it is advantageous to use

reduced step size. If successive errors are of the same polarity,

then the delta modulator is operating in its slope-

overload mode; in this case, the step size should

be increased.

.

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ADM System

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ADM System

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Differential Pulse Code

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Voice and video signals represented in PCM

exhibit high correlation, which means that

PCM signals contain redundant

information. The result is an inefficient

coding.

By removing the PCM information

redundancy a more efficient coded signal

may be obtained.

Differential Pulse Code

Modulation (DPCM)

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DPCM S

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DPCM System

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DPCM S t

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If the prediction is well performed, then

the variance of e(k) will be much smaller

than the variance of m(k), which resultsinto a smaller number of levels to

quantize e(k).

DPCM can be described as a predictivecoding scheme.

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DPCM System

T f P di t

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Type of Predictor

One-tap predictor

N-tap predictor

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M it d D it f DPCM

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Merits

1. Less signaling rate.

2. Less bandwidth.

3. Requires less quantization levels

Demerits

1. High bit rate.

2. Needs the predictor circuit to be used

which is complex.

Merits and Demerits of DPCM

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Reference

Digital Communication

by Sanjay Sharma

Advance Electronic Communicationby Robert Tomasi

World Wide Web

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digicom 101

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