Pulse code modulation- Delta Modulation and Adaptive Delta Modulation

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7. Definition…..
The amplitude and of the width of the pulses
are kept constant, while the position of each
pulse, with the reference to the position of
reference pulse is changed according to the
instantaneous sampled value of the
modulating signal.
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8.  First generate PWM
 To generate PPM , PWM signal as a Trigger
input to one monostable multivibrator.
 The monostable output remains zero untill it
is triggered.
 The monostable is triggered ON the falling
edge of PWM.
 The output of monostable is high for the
fixed period then goes low.
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10.  It consists of Differentiator & Monostable
Multivibrator
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Differentiator
Positive and
Negative Spikes
Monostable
Multivibrator

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14.  AM , FM – Analog modulation (continuous
form of modulation)
 PAM, PWM,PPM – also Analog pulse
modulation, ie., Signal was sampled and sent
in pulse form
 PCM – PCM is also sampling technique but it
differ from the others, in that it is a digital
process
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15.  . i.e., instead of sending a pulse train capable
of continuously varying one of the parameter.
 The PCM generator produce a series of
numbers or digit (digital process).
 Each one of these digit, almost always in
binary code, represent the approximate
amplitude of the signal samples.
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16.  Any pulse can be approximated to one of the
values of quantization levels.
 The Presence of pulse may represented by 1
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Quantization level Binary code
0 0 0 0
1 0 0 1
2 0 1 0
3 0 1 1
4 1 0 0
5 1 0 1
6 1 1 0
7 1 1 1

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18.  If noise occur , it makes no difference.
 The probability of occurrence of such error is
extremely small.
 Practically error-free transmission is possible.
 Quantization error can be minimized by
reducing the step size.
 The only error that is possible is due to
quantization.
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Quantization is a non linear transformation which maps elements
from a continuous set to a finite set. It is also the second step
required by A/D conversion.
Sample Quantize
Analog Signal
- Continuous time
- Continuous value
Digital Signal
- Discrete time
- Discrete value
- Discrete time
- Continuous value

21. Block Diagram of
PCM
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22.  Quantization is defined as the process of
transforming the sample amplitude of a
message signal into discrete amplitudes.
 Quantization Noise:
Difference between input and quantization
output signal is called quantization error or
quantization noise.
◦ Step size of quantization is δ
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23. Quantization
Uniform Quantization/linear Non-
Uniform
Mid tread mid riser biased µ - law A-
law
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24.  The samples of signals are highly correlated
with each other.
 Its value from present sample to next sample
does not differ by large amount.
 The adjacent samples of the signal carry the
same information with little difference.
 When these samples are encoded by PCM, the
resulting encoded signals contains redundant
information
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26. Definition of DPCM….
 If this redundancy is reduced, then overall bit
rate will decrease and no of bits required to
transmit one sample will also be reduced.
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29.  In PCM - it transmits all the bits which are
used to code the sample. Hence signaling
rate and transmission channel BW are large.
 To overcome this problem Delta Modulation
is used.
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30.  Delta Modulation is a technique which
provides a staircase approximation to an
over-sampled version of the message signal
(analog input).
 sampling is at a rate higher than the Nyquist
rate – aims at increasing the correlation
between adjacent samples; simplifies
quantizing of the encoded signal
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32.  It transmits only one bit per sample
 i.e, the present sample value is compared
with the previous sample(whether the
amplitude is increased or decreased).
 Input signal m(t) is approximated to step
signal, this step size is fixed.
 The difference between the x(t) and
approximated signal produces 2 levels ±δ.
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33.  If the difference is +ve  approximated
signal is increased by one step (i.e., δ)
1 is transmitted
 If the difference is –ve  approximated signal
is reduced by one step size.
0 is transmitted
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35.  Slope overload distortion
 Granular Noise
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36.  Staircase cannot approximate it , the step
size δ is too small (because of m(t) is too
large).
 Thus there is large error between the
staircase approximated and original signal
m(t).
 This error is called Slope overload distortion.
 To reduce this error , the step size should be
increased when m(t) is increased.
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37.  Granular noise occur when δ is too large for
small variation in m(t).
 The staircase signal keep on oscillating ±δ
around the signal.
 To solve this problem to make step size small
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38.  1. Large step-size is necessary to
accommodate a wide dynamic range
 2. Small step-size is required for accuracy
with low-level signals
 compromise between slope overhead and
granular noise
 adaptive delta modulation, where the step
size is made to vary with the input signal
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39.  Slope overload distortion – δ is too small
 Granular noise - δ is too large
( since DM has fixed δ size)
Need variable step size…..
ADAPTIVE Delta Modulation (ADM)
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40.  When the input is varying slowly, the step
size is reduced. Then this method is called
ADM.
 It takes continuous changes in step size.
 δ is automatically varied according to
variation in input of ADM
 The receiver must be able to adapt step sizes
in exactly the same manner as the transmitter
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41.  Using logic for step size control.
 The δ is increased or decreased according to
certain rule depending on one bit quantizer
output.
◦ If one bit quantizer output is high (1) then δ may
doubled for next samples.
◦ If one bit quantizer output is low (0) then δ may
decreased by one step.
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