Pcm

Modulation Techniques(Contd..)

Sampling
• The signals we use in the real world, such as our voices, are
called "analog" signals.
• To process these signals in computers, we need to convert
the signals to "digital" form.
• While an analog signal is continuous in both time and
amplitude, a digital signal is discrete in both time and
amplitude.
• To convert a signal from continuous time to discrete time, a
process called sampling is used.
• The value of the signal is measured at certain intervals in
time. Each measurement is referred to as a sample.
•

How many Samples
• How many samples are necessary to ensure we are
preserving the information contained in the signal?
• If the signal contains high frequency components, we will
need to sample at a higher rate to avoid losing information
that is in the signal.
• In general, to preserve the full information in the signal, it is
necessary to sample at twice the maximum frequency of the
signal. This is known as the Nyquist rate.
• The Sampling Theorem states that a signal can be exactly
reproduced if it is sampled at a frequency F, where F is
greater than twice the maximum frequency in the signal.

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t
x(t)
Sampling
Consider the analog Signal x(t).

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n
x[n]
Sampling
The signal is first sampled

• A modulation system in which amplitude of
discrete carrier signal changes in accordance
with the instantaneous amplitude of
modulating signal(message signal) keeping
width and position of carrier constant is called
as PAM.
•
Pulse Amplitude Modulation
(PAM)

PAM
• PAM signal is discrete in time but analog in
amplitude!

• There are 2 types of PAM :
1.Natural sampling
2.Flat top sampling
Types of PAM

• In PAM, the amplitude of regular interval of
periodic 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.
PAM Basics

PAM Basics
• 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
• Natural PAM

• 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.
PAM Basics

• 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.
Advantages

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t
x(t)
Pulse Code Modulation (PCM)
Consider the analog Signal x(t).

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n
x[n]
The signal is first sampled

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n

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Sample n

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3
And Hold n

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3
Assign Closest
Level
n

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3
n
Each quantization level corresponds to a unique combination
of bits. The analog signal is transmitted/ stored as a stream of
bits and reconstructed when required.

0
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3
n
0 0 0 1 1 0 1 1 1 0 0 1 0 0
Each quantization level corresponds to a unique combination
of bits. The analog signal is transmitted/ stored as a stream of
bits and reconstructed when required.

0
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t
x(t) Original Signal

0
1
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3
t
x~(t)
Quantized Signal
It is quite apparent that the quantized signal is not exactly the
same as the original analog signal. There is a fair degree of
quantization error here. However; as the number of quantization
levels is increased the quantization error is reduced and the
quantized signal gets closer and closer to the original signal

0
t
x~(t)
Quantized Signal
It is quite apparent that the quantized signal is not exactly the
same as the original analog signal. There is a fair degree of
quantization error here. However; as the number of quantization
levels is increased the quantization error is reduced and the
quantized signal gets closer and closer to the original signal

PCM System Block Diagram
Sample & Hold Comparator
Ramp Generator
Binary Counter
Parallel to Serial
Converter
All pulses have same height
and width.
f(t)

Delta Modulation
• Delta modulation is a system of digital modulation developed
after pulse code modulation.
• In this system, at each sampling time, say the Kth sampling
time, the difference between the sample value at sampling
time K and the sample value at the previous sampling time (K-
1) is encoded into just a single bit. i.e. at each sampling time
we ask simple question
Has the signal amplitude increased or decreased since the last
sample was taken?

Delta Modulation
• If signal amplitude has increased, then
modulator's output is at logic level 1. If the
signal amplitude has decreased, the
modulator output is at logic level 0. Thus, the
output from the modulator is a series of zeros
and ones to indicate rise and fall of the
waveform since the previous value.

Pcm

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